<SYSTEM>This is the full developer documentation for Svelte and SvelteKit.</SYSTEM>
# Start of Svelte documentation
# Overview
Svelte is a framework for building user interfaces on the web. It uses a compiler to turn declarative components written in HTML, CSS and JavaScript...
```svelte
<!--- file: App.svelte --->
<script>
function greet() {
alert('Welcome to Svelte!');
}
</script>
<button onclick={greet}>click me</button>
<style>
button {
font-size: 2em;
}
</style>
```
...into lean, tightly optimized JavaScript.
You can use it to build anything on the web, from standalone components to ambitious full stack apps (using Svelte's companion application framework, [SvelteKit](../kit)) and everything in between.
These pages serve as reference documentation. If you're new to Svelte, we recommend starting with the [interactive tutorial](/tutorial) and coming back here when you have questions.
You can also try Svelte online in the [playground](/playground) or, if you need a more fully-featured environment, on [StackBlitz](https://sveltekit.new).
# Getting started
We recommend using [SvelteKit](../kit), the official application framework from the Svelte team powered by [Vite](https://vite.dev/):
```bash
npx sv create myapp
cd myapp
npm install
npm run dev
```
Don't worry if you don't know Svelte yet! You can ignore all the nice features SvelteKit brings on top for now and dive into it later.
## Alternatives to SvelteKit
You can also use Svelte directly with Vite by running `npm create vite@latest` and selecting the `svelte` option. With this, `npm run build` will generate HTML, JS and CSS files inside the `dist` directory using [vite-plugin-svelte](https://github.com/sveltejs/vite-plugin-svelte). In most cases, you will probably need to [choose a routing library](faq#Is-there-a-router) as well.
There are also plugins for [Rollup](https://github.com/sveltejs/rollup-plugin-svelte), [Webpack](https://github.com/sveltejs/svelte-loader) [and a few others](https://sveltesociety.dev/packages?category=build-plugins), but we recommend Vite.
## Editor tooling
The Svelte team maintains a [VS Code extension](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode), and there are integrations with various other [editors](https://sveltesociety.dev/resources#editor-support) and tools as well.
You can also check your code from the command line using [sv check](https://github.com/sveltejs/cli).
## Getting help
Don't be shy about asking for help in the [Discord chatroom](/chat)! You can also find answers on [Stack Overflow](https://stackoverflow.com/questions/tagged/svelte).
# .svelte files
Components are the building blocks of Svelte applications. They are written into `.svelte` files, using a superset of HTML.
All three sections — script, styles and markup — are optional.
<!-- prettier-ignore -->
```svelte
/// file: MyComponent.svelte
<script module>
// module-level logic goes here
// (you will rarely use this)
</script>
<script>
// instance-level logic goes here
</script>
<!-- markup (zero or more items) goes here -->
<style>
/* styles go here */
</style>
```
## `<script>`
A `<script>` block contains JavaScript (or TypeScript, when adding the `lang="ts"` attribute) that runs when a component instance is created. Variables declared (or imported) at the top level can be referenced in the component's markup.
In addition to normal JavaScript, you can use _runes_ to declare [component props]($props) and add reactivity to your component. Runes are covered in the next section.
<!-- TODO describe behaviour of `export` -->
## `<script module>`
A `<script>` tag with a `module` attribute runs once when the module first evaluates, rather than for each component instance. Variables declared in this block can be referenced elsewhere in the component, but not vice versa.
```svelte
<script module>
let total = 0;
</script>
<script>
total += 1;
console.log(`instantiated ${total} times`);
</script>
```
You can `export` bindings from this block, and they will become exports of the compiled module. You cannot `export default`, since the default export is the component itself.
> [!NOTE] If you are using TypeScript and import such exports from a `module` block into a `.ts` file, make sure to have your editor setup so that TypeScript knows about them. This is the case for our VS Code extension and the IntelliJ plugin, but in other cases you might need to setup our [TypeScript editor plugin](https://www.npmjs.com/package/typescript-svelte-plugin).
> [!LEGACY]
> In Svelte 4, this script tag was created using `<script context="module">`
## `<style>`
CSS inside a `<style>` block will be scoped to that component.
```svelte
<style>
p {
/* this will only affect <p> elements in this component */
color: burlywood;
}
</style>
```
For more information, head to the section on [styling](scoped-styles).
# .svelte.js and .svelte.ts files
Besides `.svelte` files, Svelte also operates on `.svelte.js` and `.svelte.ts` files.
These behave like any other `.js` or `.ts` module, except that you can use runes. This is useful for creating reusable reactive logic, or sharing reactive state across your app.
> [!LEGACY]
> This is a concept that didn't exist prior to Svelte 5
# Public API of a component
### Public API of a component
Svelte uses the `$props` rune to declare _properties_ or _props_, which means describing the public interface of the component which becomes accessible to consumers of the component.
> [!NOTE] `$props` is one of several runes, which are special hints for Svelte's compiler to make things reactive.
```svelte
<script>
let { foo, bar, baz } = $props();
// Values that are passed in as props
// are immediately available
console.log({ foo, bar, baz });
</script>
```
You can specify a fallback value for a prop. It will be used if the component's consumer doesn't specify the prop on the component when instantiating the component, or if the passed value is `undefined` at some point.
```svelte
<script>
let { foo = 'optional default initial value' } = $props();
</script>
```
To get all properties, use rest syntax:
```svelte
<script>
let { a, b, c, ...everythingElse } = $props();
</script>
```
You can use reserved words as prop names.
```svelte
<script>
// creates a `class` property, even
// though it is a reserved word
let { class: className } = $props();
</script>
```
If you're using TypeScript, you can declare the prop types:
```svelte
<script lang="ts">
interface Props {
required: string;
optional?: number;
[key: string]: unknown;
}
let { required, optional, ...everythingElse }: Props = $props();
</script>
```
If you're using JavaScript, you can declare the prop types using JSDoc:
```svelte
<script>
/** @type {{ x: string }} */
let { x } = $props();
// or use @typedef if you want to document the properties:
/**
* @typedef {Object} MyProps
* @property {string} y Some documentation
*/
/** @type {MyProps} */
let { y } = $props();
</script>
```
If you export a `const`, `class` or `function`, it is readonly from outside the component.
```svelte
<script>
export const thisIs = 'readonly';
export function greet(name) {
alert(`hello ${name}!`);
}
</script>
```
Readonly props can be accessed as properties on the element, tied to the component using [`bind:this` syntax](bindings#bind:this).
### Reactive variables
To change component state and trigger a re-render, just assign to a locally declared variable that was declared using the `$state` rune.
Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.
```svelte
<script>
let count = $state(0);
function handleClick() {
// calling this function will trigger an
// update if the markup references `count`
count = count + 1;
}
</script>
```
Svelte's `<script>` blocks are run only when the component is created, so assignments within a `<script>` block are not automatically run again when a prop updates.
```svelte
<script>
let { person } = $props();
// this will only set `name` on component creation
// it will not update when `person` does
let { name } = person;
</script>
```
If you'd like to react to changes to a prop, use the `$derived` or `$effect` runes instead.
```svelte
<script>
let count = $state(0);
let double = $derived(count * 2);
$effect(() => {
if (count > 10) {
alert('Too high!');
}
});
</script>
```
For more information on reactivity, read the documentation around runes.
# Reactivity fundamentals
Reactivity is at the heart of interactive UIs. When you click a button, you expect some kind of response. It's your job as a developer to make this happen. It's Svelte's job to make your job as intuitive as possible, by providing a good API to express reactive systems.
## Runes
Svelte 5 uses _runes_, a powerful set of primitives for controlling reactivity inside your Svelte components and inside `.svelte.js` and `.svelte.ts` modules.
Runes are function-like symbols that provide instructions to the Svelte compiler. You don't need to import them from anywhere — when you use Svelte, they're part of the language.
The following sections introduce the most important runes for declare state, derived state and side effects at a high level. For more details refer to the later sections on [state](state) and [side effects](side-effects).
## `$state`
Reactive state is declared with the `$state` rune:
```svelte
<script>
let count = $state(0);
</script>
<button onclick={() => count++}>
clicks: {count}
</button>
```
You can also use `$state` in class fields (whether public or private):
```js
// @errors: 7006 2554
class Todo {
done = $state(false);
text = $state();
constructor(text) {
this.text = text;
}
}
```
> [!LEGACY]
> In Svelte 4, state was implicitly reactive if the variable was declared at the top level
>
> ```svelte
> <script>
> let count = 0;
> </script>
>
> <button on:click={() => count++}>
> clicks: {count}
> </button>
> ```
## `$derived`
Derived state is declared with the `$derived` rune:
```svelte
<script>
let count = $state(0);
let doubled = $derived(count * 2);
</script>
<button onclick={() => count++}>
{doubled}
</button>
<p>{count} doubled is {doubled}</p>
```
The expression inside `$derived(...)` should be free of side-effects. Svelte will disallow state changes (e.g. `count++`) inside derived expressions.
As with `$state`, you can mark class fields as `$derived`.
> [!LEGACY]
> In Svelte 4, you could use reactive statements for this.
>
> ```svelte
> <script>
> let count = 0;
> $: doubled = count * 2;
> </script>
>
> <button on:click={() => count++}>
> {doubled}
> </button>
>
> <p>{count} doubled is {doubled}</p>
> ```
>
> This only worked at the top level of a component.
## `$effect`
To run _side-effects_ when the component is mounted to the DOM, and when values change, we can use the `$effect` rune ([demo](/playground/untitled#H4sIAAAAAAAAE31T24rbMBD9lUG7kAQ2sbdlX7xOYNk_aB_rQhRpbAsU2UiTW0P-vbrYubSlYGzmzMzROTPymdVKo2PFjzMzfIusYB99z14YnfoQuD1qQh-7bmdFQEonrOppVZmKNBI49QthCc-OOOH0LZ-9jxnR6c7eUpOnuv6KeT5JFdcqbvbcBcgDz1jXKGg6ncFyBedYR6IzLrAZwiN5vtSxaJA-EzadfJEjKw11C6GR22-BLH8B_wxdByWpvUYtqqal2XB6RVkG1CoHB6U1WJzbnYFDiwb3aGEdDa3Bm1oH12sQLTcNPp7r56m_00mHocSG97_zd7ICUXonA5fwKbPbkE2ZtMJGGVkEdctzQi4QzSwr9prnFYNk5hpmqVuqPQjNnfOJoMF22lUsrq_UfIN6lfSVyvQ7grB3X2mjMZYO3XO9w-U5iLx42qg29md3BP_ni5P4gy9ikTBlHxjLzAtPDlyYZmRdjAbGq7HprEQ7p64v4LU_guu0kvAkhBim3nMplWl8FreQD-CW20aZR0wq12t-KqDWeBywhvexKC3memmDwlHAv9q4Vo2ZK8KtK0CgX7u9J8wXbzdKv-nRnfF_2baTqlYoWUF2h5efl9-n0O6koAMAAA==)):
```svelte
<script>
let size = $state(50);
let color = $state('#ff3e00');
let canvas;
$effect(() => {
const context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
// this will re-run whenever `color` or `size` change
context.fillStyle = color;
context.fillRect(0, 0, size, size);
});
</script>
<canvas bind:this={canvas} width="100" height="100" />
```
The function passed to `$effect` will run when the component mounts, and will re-run after any changes to the values it reads that were declared with `$state` or `$derived` (including those passed in with `$props`). Re-runs are batched (i.e. changing `color` and `size` in the same moment won't cause two separate runs), and happen after any DOM updates have been applied.
> [!LEGACY]
> In Svelte 4, you could use reactive statements for this.
>
> ```svelte
> <script>
> let size = 50;
> let color = '#ff3e00';
>
> let canvas;
>
> $: {
> const context = canvas.getContext('2d');
> context.clearRect(0, 0, canvas.width, canvas.height);
>
> // this will re-run whenever `color` or `size` change
> context.fillStyle = color;
> context.fillRect(0, 0, size, size);
> }
> </script>
>
> <canvas bind:this={canvas} width="100" height="100" />
> ```
>
> This only worked at the top level of a component.
# What are runes?
> [!NOTE] **rune** /ro͞on/ _noun_
>
> A letter or mark used as a mystical or magic symbol.
Runes are symbols that you use in `.svelte` and `.svelte.js`/`.svelte.ts` files to control the Svelte compiler. If you think of Svelte as a language, runes are part of the syntax — they are _keywords_.
Runes have a `$` prefix and look like functions:
```js
let message = $state('hello');
```
They differ from normal JavaScript functions in important ways, however:
- You don't need to import them — they are part of the language
- They're not values — you can't assign them to a variable or pass them as arguments to a function
- Just like JavaScript keywords, they are only valid in certain positions (the compiler will help you if you put them in the wrong place)
> [!LEGACY]
> Runes didn't exist prior to Svelte 5.
# $state
The `$state` rune allows you to create _reactive state_, which means that your UI _reacts_ when it changes.
```svelte
<script>
let count = $state(0);
</script>
<button onclick={() => count++}>
clicks: {count}
</button>
```
Unlike other frameworks you may have encountered, there is no API for interacting with state — `count` is just a number, rather than an object or a function, and you can update it like you would update any other variable.
### Deep state
If `$state` is used with an array or a simple object, the result is a deeply reactive _state proxy_. [Proxies](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) allow Svelte to run code when you read or write properties, including via methods like `array.push(...)`, triggering granular updates.
> [!NOTE] Classes like `Set` and `Map` will not be proxied, but Svelte provides reactive implementations for various built-ins like these that can be imported from [`svelte/reactivity`](./svelte-reactivity).
State is proxified recursively until Svelte finds something other than an array or simple object. In a case like this...
```js
let todos = $state([
{
done: false,
text: 'add more todos'
}
]);
```
...modifying an individual todo's property will trigger updates to anything in your UI that depends on that specific property:
```js
let todos = [{ done: false, text: 'add more todos' }];
// ---cut---
todos[0].done = !todos[0].done;
```
If you push a new object to the array, it will also be proxified:
```js
let todos = [{ done: false, text: 'add more todos' }];
// ---cut---
todos.push({
done: false,
text: 'eat lunch'
});
```
> [!NOTE] When you update properties of proxies, the original object is _not_ mutated.
Note that if you destructure a reactive value, the references are not reactive — as in normal JavaScript, they are evaluated at the point of destructuring:
```js
let todos = [{ done: false, text: 'add more todos' }];
// ---cut---
let { done, text } = todos[0];
// this will not affect the value of `done`
todos[0].done = !todos[0].done;
```
### Classes
You can also use `$state` in class fields (whether public or private):
```js
// @errors: 7006 2554
class Todo {
done = $state(false);
text = $state();
constructor(text) {
this.text = text;
}
reset() {
this.text = '';
this.done = false;
}
}
```
> [!NOTE] The compiler transforms `done` and `text` into `get`/`set` methods on the class prototype referencing private fields. This means the properties are not enumerable.
When calling methods in JavaScript, the value of [`this`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) matters. This won't work, because `this` inside the `reset` method will be the `<button>` rather than the `Todo`:
```svelte
<button onclick={todo.reset}>
reset
</button>
```
You can either use an inline function...
```svelte
<button onclick=+++{() => todo.reset()}>+++
reset
</button>
```
...or use an arrow function in the class definition:
```js
// @errors: 7006 2554
class Todo {
done = $state(false);
text = $state();
constructor(text) {
this.text = text;
}
+++reset = () => {+++
this.text = '';
this.done = false;
}
}
```
## `$state.raw`
In cases where you don't want objects and arrays to be deeply reactive you can use `$state.raw`.
State declared with `$state.raw` cannot be mutated; it can only be _reassigned_. In other words, rather than assigning to a property of an object, or using an array method like `push`, replace the object or array altogether if you'd like to update it:
```js
let person = $state.raw({
name: 'Heraclitus',
age: 49
});
// this will have no effect
person.age += 1;
// this will work, because we're creating a new person
person = {
name: 'Heraclitus',
age: 50
};
```
This can improve performance with large arrays and objects that you weren't planning to mutate anyway, since it avoids the cost of making them reactive. Note that raw state can _contain_ reactive state (for example, a raw array of reactive objects).
## `$state.snapshot`
To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snapshot`:
```svelte
<script>
let counter = $state({ count: 0 });
function onclick() {
// Will log `{ count: ... }` rather than `Proxy { ... }`
console.log($state.snapshot(counter));
}
</script>
```
This is handy when you want to pass some state to an external library or API that doesn't expect a proxy, such as `structuredClone`.
## Passing state into functions
JavaScript is a _pass-by-value_ language — when you call a function, the arguments are the _values_ rather than the _variables_. In other words:
```js
/// file: index.js
// @filename: index.js
// ---cut---
/**
* @param {number} a
* @param {number} b
*/
function add(a, b) {
return a + b;
}
let a = 1;
let b = 2;
let total = add(a, b);
console.log(total); // 3
a = 3;
b = 4;
console.log(total); // still 3!
```
If `add` wanted to have access to the _current_ values of `a` and `b`, and to return the current `total` value, you would need to use functions instead:
```js
/// file: index.js
// @filename: index.js
// ---cut---
/**
* @param {() => number} getA
* @param {() => number} getB
*/
function add(+++getA, getB+++) {
return +++() => getA() + getB()+++;
}
let a = 1;
let b = 2;
let total = add+++(() => a, () => b)+++;
console.log(+++total()+++); // 3
a = 3;
b = 4;
console.log(+++total()+++); // 7
```
State in Svelte is no different — when you reference something declared with the `$state` rune...
```js
let a = +++$state(1)+++;
let b = +++$state(2)+++;
```
...you're accessing its _current value_.
Note that 'functions' is broad — it encompasses properties of proxies and [`get`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/get)/[`set`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/set) properties...
```js
/// file: index.js
// @filename: index.js
// ---cut---
/**
* @param {{ a: number, b: number }} input
*/
function add(input) {
return {
get value() {
return input.a + input.b;
}
};
}
let input = $state({ a: 1, b: 2 });
let total = add(input);
console.log(total.value); // 3
input.a = 3;
input.b = 4;
console.log(total.value); // 7
```
...though if you find yourself writing code like that, consider using [classes](#Classes) instead.
# $derived
Derived state is declared with the `$derived` rune:
```svelte
<script>
let count = $state(0);
let doubled = $derived(count * 2);
</script>
<button onclick={() => count++}>
{doubled}
</button>
<p>{count} doubled is {doubled}</p>
```
The expression inside `$derived(...)` should be free of side-effects. Svelte will disallow state changes (e.g. `count++`) inside derived expressions.
As with `$state`, you can mark class fields as `$derived`.
> [!NOTE] Code in Svelte components is only executed once at creation. Without the `$derived` rune, `doubled` would maintain its original value even when `count` changes.
## `$derived.by`
Sometimes you need to create complex derivations that don't fit inside a short expression. In these cases, you can use `$derived.by` which accepts a function as its argument.
```svelte
<script>
let numbers = $state([1, 2, 3]);
let total = $derived.by(() => {
let total = 0;
for (const n of numbers) {
total += n;
}
return total;
});
</script>
<button onclick={() => numbers.push(numbers.length + 1)}>
{numbers.join(' + ')} = {total}
</button>
```
In essence, `$derived(expression)` is equivalent to `$derived.by(() => expression)`.
## Understanding dependencies
Anything read synchronously inside the `$derived` expression (or `$derived.by` function body) is considered a _dependency_ of the derived state. When the state changes, the derived will be marked as _dirty_ and recalculated when it is next read.
To exempt a piece of state from being treated as a dependency, use [`untrack`](svelte#untrack).
## Update propagation
Svelte uses something called _push-pull reactivity_ — when state is updated, everything that depends on the state (whether directly or indirectly) is immediately notified of the change (the 'push'), but derived values are not re-evaluated until they are actually read (the 'pull').
If the new value of a derived is referentially identical to its previous value, downstream updates will be skipped. In other words, Svelte will only update the text inside the button when `large` changes, not when `count` changes, even though `large` depends on `count`:
```svelte
<script>
let count = $state(0);
let large = $derived(count > 10);
</script>
<button onclick={() => count++}>
{large}
</button>
```
# $effect
Effects are what make your application _do things_. When Svelte runs an effect function, it tracks which pieces of state (and derived state) are accessed (unless accessed inside [`untrack`](svelte#untrack)), and re-runs the function when that state later changes.
Most of the effects in a Svelte app are created by Svelte itself — they're the bits that update the text in `<h1>hello {name}!</h1>` when `name` changes, for example.
But you can also create your own effects with the `$effect` rune, which is useful when you need to synchronize an external system (whether that's a library, or a `<canvas>` element, or something across a network) with state inside your Svelte app.
> [!NOTE] Avoid overusing `$effect`! When you do too much work in effects, code often becomes difficult to understand and maintain. See [when not to use `$effect`](#When-not-to-use-$effect) to learn about alternative approaches.
Your effects run after the component has been mounted to the DOM, and in a [microtask](https://developer.mozilla.org/en-US/docs/Web/API/HTML_DOM_API/Microtask_guide) after state changes ([demo](/playground/untitled#H4sIAAAAAAAAE31S246bMBD9lZF3pSRSAqTVvrCAVPUP2sdSKY4ZwJJjkD0hSVH-vbINuWxXfQH5zMyZc2ZmZLVUaFn6a2R06ZGlHmBrpvnBvb71fWQHVOSwPbf4GS46TajJspRlVhjZU1HqkhQSWPkHIYdXS5xw-Zas3ueI6FRn7qHFS11_xSRZhIxbFtcDtw7SJb1iXaOg5XIFeQGjzyPRaevYNOGZIJ8qogbpe8CWiy_VzEpTXiQUcvPDkSVrSNZz1UlW1N5eLcqmpdXUvaQ4BmqlhZNUCgxuzFHDqUWNAxrYeUM76AzsnOsdiJbrBp_71lKpn3RRbii-4P3f-IMsRxS-wcDV_bL4PmSdBa2wl7pKnbp8DMgVvJm8ZNskKRkEM_OzyOKQFkgqOYBQ3Nq89Ns0nbIl81vMFN-jKoLMTOr-SOBOJS-Z8f5Y6D1wdcR8dFqvEBdetK-PHwj-z-cH8oHPY54wRJ8Ys7iSQ3Bg3VA9azQbmC9k35kKzYa6PoVtfwbbKVnBixBiGn7Pq0rqJoUtHiCZwAM3jdTPWCVtr_glhVrhecIa3vuksJ_b7TqFs4DPyriSjd5IwoNNQaAmNI-ESfR2p8zimzvN1swdCkvJHPH6-_oX8o1SgcIDAAA=)):
```svelte
<script>
let size = $state(50);
let color = $state('#ff3e00');
let canvas;
$effect(() => {
const context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
// this will re-run whenever `color` or `size` change
context.fillStyle = color;
context.fillRect(0, 0, size, size);
});
</script>
<canvas bind:this={canvas} width="100" height="100" />
```
Re-runs are batched (i.e. changing `color` and `size` in the same moment won't cause two separate runs), and happen after any DOM updates have been applied.
You can place `$effect` anywhere, not just at the top level of a component, as long as it is called during component initialization (or while a parent effect is active). It is then tied to the lifecycle of the component (or parent effect) and will therefore destroy itself when the component unmounts (or the parent effect is destroyed).
You can return a function from `$effect`, which will run immediately before the effect re-runs, and before it is destroyed ([demo](/playground/untitled#H4sIAAAAAAAAE42RQY-bMBCF_8rI2kPopiXpMQtIPfbeW6m0xjyKtWaM7CFphPjvFVB2k2oPe7LmzXzyezOjaqxDVKefo5JrD3VaBLVXrLu5-tb3X-IZTmat0hHv6cazgCWqk8qiCbaXouRSHISMH1gop4coWrA7JE9bp7PO2QjjuY5vA8fDYZ3hUh7QNDCy2yWUFzTOUilpSj9aG-linaMKFGACtKCmSwvGGYGeLQvCWbtnMq3m34grajxHoa1JOUXI93_V_Sfz7Oz7Mafj0ypN-zvHm8dSAmQITP_xaUq2IU1GO1dp80I2Uh_82dao92Rl9R8GvgF0QrbrUFstcFeq0PgAkha0LoICPoeB4w1SJUvsZcj4rvcMlvmvGlGCv6J-DeSgw2vabQnJlm55p7nM0rcTctYei3HZxZSl7XHVqkHEM3k2zpqXfFyj393zU05fpyI6f0HI0hUoPoamC9roKDeo2ivBH1EnCQOmX9NfYw2GHrgCAAA=)).
```svelte
<script>
let count = $state(0);
let milliseconds = $state(1000);
$effect(() => {
// This will be recreated whenever `milliseconds` changes
const interval = setInterval(() => {
count += 1;
}, milliseconds);
return () => {
// if a callback is provided, it will run
// a) immediately before the effect re-runs
// b) when the component is destroyed
clearInterval(interval);
};
});
</script>
<h1>{count}</h1>
<button onclick={() => (milliseconds *= 2)}>slower</button>
<button onclick={() => (milliseconds /= 2)}>faster</button>
```
### Understanding dependencies
`$effect` automatically picks up any reactive values (`$state`, `$derived`, `$props`) that are _synchronously_ read inside its function body (including indirectly, via function calls) and registers them as dependencies. When those dependencies change, the `$effect` schedules a rerun.
Values that are read _asynchronously_ — after an `await` or inside a `setTimeout`, for example — will not be tracked. Here, the canvas will be repainted when `color` changes, but not when `size` changes ([demo](/playground/untitled#H4sIAAAAAAAAE31T246bMBD9lZF3pWSlBEirfaEQqdo_2PatVIpjBrDkGGQPJGnEv1e2IZfVal-wfHzmzJyZ4cIqqdCy9M-F0blDlnqArZjmB3f72XWRHVCRw_bc4me4aDWhJstSlllhZEfbQhekkMDKfwg5PFvihMvX5OXH_CJa1Zrb0-Kpqr5jkiwC48rieuDWQbqgZ6wqFLRcvkC-hYvnkWi1dWqa8ESQTxFRjfQWsOXiWzmr0sSLhEJu3p1YsoJkNUcdZUnN9dagrBu6FVRQHAM10sJRKgUG16bXcGxQ44AGdt7SDkTDdY02iqLHnJVU6hedlWuIp94JW6Tf8oBt_8GdTxlF0b4n0C35ZLBzXb3mmYn3ae6cOW74zj0YVzDNYXRHFt9mprNgHfZSl6mzml8CMoLvTV6wTZIUDEJv5us2iwMtiJRyAKG4tXnhl8O0yhbML0Wm-B7VNlSSSd31BG7z8oIZZ6dgIffAVY_5xdU9Qrz1Bnx8fCfwtZ7v8Qc9j3nB8PqgmMWlHIID6-bkVaPZwDySfWtKNGtquxQ23Qlsq2QJT0KIqb8dL0up6xQ2eIBkAg_c1FI_YqW0neLnFCqFpwmreedJYT7XX8FVOBfwWRhXstZrSXiwKQjUhOZeMIleb5JZfHWn2Yq5pWEpmR7Hv-N_wEqT8hEEAAA=)):
```ts
// @filename: index.ts
declare let canvas: {
width: number;
height: number;
getContext(type: '2d', options?: CanvasRenderingContext2DSettings): CanvasRenderingContext2D;
};
declare let color: string;
declare let size: number;
// ---cut---
$effect(() => {
const context = canvas.getContext('2d');
context.clearRect(0, 0, canvas.width, canvas.height);
// this will re-run whenever `color` changes...
context.fillStyle = color;
setTimeout(() => {
// ...but not when `size` changes
context.fillRect(0, 0, size, size);
}, 0);
});
```
An effect only reruns when the object it reads changes, not when a property inside it changes. (If you want to observe changes _inside_ an object at dev time, you can use [`$inspect`]($inspect).)
```svelte
<script>
let state = $state({ value: 0 });
let derived = $derived({ value: state.value * 2 });
// this will run once, because `state` is never reassigned (only mutated)
$effect(() => {
state;
});
// this will run whenever `state.value` changes...
$effect(() => {
state.value;
});
// ...and so will this, because `derived` is a new object each time
$effect(() => {
derived;
});
</script>
<button onclick={() => (state.value += 1)}>
{state.value}
</button>
<p>{state.value} doubled is {derived.value}</p>
```
An effect only depends on the values that it read the last time it ran. This has interesting implications for effects that have conditional code.
For instance, if `a` is `true` in the code snippet below, the code inside the `if` block will run and `b` will be evaluated. As such, changes to either `a` or `b` [will cause the effect to re-run](/playground/untitled#H4sIAAAAAAAAE3VQzWrDMAx-FdUU4kBp71li6EPstOxge0ox8-QQK2PD-N1nLy2F0Z2Evj9_chKkP1B04pnYscc3cRCT8xhF95IEf8-Vq0DBr8rzPB_jJ3qumNERH-E2ECNxiRF9tIubWY00lgcYNAywj6wZJS8rtk83wjwgCrXHaULLUrYwKEgVGrnkx-Dx6MNFNstK5OjSbFGbwE0gdXuT_zGYrjmAuco515Hr1p_uXak3K3MgCGS9s-9D2grU-judlQYXIencnzad-tdR79qZrMyvw9wd5Z8Yv1h09dz8mn8AkM7Pfo0BAAA=).
Conversely, if `a` is `false`, `b` will not be evaluated, and the effect will _only_ re-run when `a` changes.
```ts
let a = false;
let b = false;
// ---cut---
$effect(() => {
console.log('running');
if (a) {
console.log('b:', b);
}
});
```
## `$effect.pre`
In rare cases, you may need to run code _before_ the DOM updates. For this we can use the `$effect.pre` rune:
```svelte
<script>
import { tick } from 'svelte';
let div = $state();
let messages = $state([]);
// ...
$effect.pre(() => {
if (!div) return; // not yet mounted
// reference `messages` array length so that this code re-runs whenever it changes
messages.length;
// autoscroll when new messages are added
if (div.offsetHeight + div.scrollTop > div.scrollHeight - 20) {
tick().then(() => {
div.scrollTo(0, div.scrollHeight);
});
}
});
</script>
<div bind:this={div}>
{#each messages as message}
<p>{message}</p>
{/each}
</div>
```
Apart from the timing, `$effect.pre` works exactly like `$effect`.
## `$effect.tracking`
The `$effect.tracking` rune is an advanced feature that tells you whether or not the code is running inside a tracking context, such as an effect or inside your template ([demo](/playground/untitled#H4sIAAAAAAAACn3PwYrCMBDG8VeZDYIt2PYeY8Dn2HrIhqkU08nQjItS-u6buAt7UDzmz8ePyaKGMWBS-nNRcmdU-hHUTpGbyuvI3KZvDFLal0v4qvtIgiSZUSb5eWSxPfWSc4oB2xDP1XYk8HHiSHkICeXKeruDDQ4Demlldv4y0rmq6z10HQwuJMxGVv4mVVXDwcJS0jP9u3knynwtoKz1vifT_Z9Jhm0WBCcOTlDD8kyspmML5qNpHg40jc3fFryJ0iWsp_UHgz3180oBAAA=)):
```svelte
<script>
console.log('in component setup:', $effect.tracking()); // false
$effect(() => {
console.log('in effect:', $effect.tracking()); // true
});
</script>
<p>in template: {$effect.tracking()}</p> <!-- true -->
```
It is used to implement abstractions like [`createSubscriber`](/docs/svelte/svelte-reactivity#createSubscriber), which will create listeners to update reactive values but _only_ if those values are being tracked (rather than, for example, read inside an event handler).
## `$effect.root`
The `$effect.root` rune is an advanced feature that creates a non-tracked scope that doesn't auto-cleanup. This is useful for nested effects that you want to manually control. This rune also allows for the creation of effects outside of the component initialisation phase.
```svelte
<script>
let count = $state(0);
const cleanup = $effect.root(() => {
$effect(() => {
console.log(count);
});
return () => {
console.log('effect root cleanup');
};
});
</script>
```
## When not to use `$effect`
In general, `$effect` is best considered something of an escape hatch — useful for things like analytics and direct DOM manipulation — rather than a tool you should use frequently. In particular, avoid using it to synchronise state. Instead of this...
```svelte
<script>
let count = $state(0);
let doubled = $state();
// don't do this!
$effect(() => {
doubled = count * 2;
});
</script>
```
...do this:
```svelte
<script>
let count = $state(0);
let doubled = $derived(count * 2);
</script>
```
> [!NOTE] For things that are more complicated than a simple expression like `count * 2`, you can also use `$derived.by`.
You might be tempted to do something convoluted with effects to link one value to another. The following example shows two inputs for "money spent" and "money left" that are connected to each other. If you update one, the other should update accordingly. Don't use effects for this ([demo](/playground/untitled#H4sIAAAAAAAACpVRy26DMBD8FcvKgUhtoIdeHBwp31F6MGSJkBbHwksEQvx77aWQqooq9bgzOzP7mGTdIHipPiZJowOpGJAv0po2VmfnDv4OSBErjYdneHWzBJaCjcx91TWOToUtCIEE3cig0OIty44r5l1oDtjOkyFIsv3GINQ_CNYyGegd1DVUlCR7oU9iilDUcP8S8roYs9n8p2wdYNVFm4csTx872BxNCcjr5I11fdgonEkXsjP2CoUUZWMv6m6wBz2x7yxaM-iJvWeRsvSbSVeUy5i0uf8vKA78NIeJLSZWv1I8jQjLdyK4XuTSeIdmVKJGGI4LdjVOiezwDu1yG74My8PLCQaSiroe5s_5C2PHrkVGAgAA)):
```svelte
<script>
let total = 100;
let spent = $state(0);
let left = $state(total);
$effect(() => {
left = total - spent;
});
$effect(() => {
spent = total - left;
});
</script>
<label>
<input type="range" bind:value={spent} max={total} />
{spent}/{total} spent
</label>
<label>
<input type="range" bind:value={left} max={total} />
{left}/{total} left
</label>
```
Instead, use callbacks where possible ([demo](/playground/untitled#H4sIAAAAAAAACo1SMW6EMBD8imWluFMSIEUaDiKlvy5lSOHjlhOSMRZeTiDkv8deMEEJRcqdmZ1ZjzzxqpZgePo5cRw18JQA_sSVaPz0rnVk7iDRYxdhYA8vW4Wg0NnwzJRdrfGtUAVKQIYtCsly9pIkp4AZ7cQOezAoEA7JcWUkVBuCdol0dNWrEutWsV5fHfnhPQ5wZJMnCwyejxCh6G6A0V3IHk4zu_jOxzzPBxBld83PTr7xXrb3rUNw8PbiYJ3FP22oTIoLSComq5XuXTeu8LzgnVA3KDgj13wiQ8taRaJ82rzXskYM-URRlsXktejjgNLoo9e4fyf70_8EnwncySX1GuunX6kGRwnzR_BgaPNaGy3FmLJKwrCUeBM6ZUn0Cs2mOlp3vwthQJ5i14P9st9vZqQlsQIAAA==)):
```svelte
<script>
let total = 100;
let spent = $state(0);
let left = $state(total);
function updateSpent(e) {
spent = +e.target.value;
left = total - spent;
}
function updateLeft(e) {
left = +e.target.value;
spent = total - left;
}
</script>
<label>
<input type="range" value={spent} oninput={updateSpent} max={total} />
{spent}/{total} spent
</label>
<label>
<input type="range" value={left} oninput={updateLeft} max={total} />
{left}/{total} left
</label>
```
If you need to use bindings, for whatever reason (for example when you want some kind of "writable `$derived`"), consider using getters and setters to synchronise state ([demo](/playground/untitled#H4sIAAAAAAAACpWRwW6DMBBEf8WyekikFOihFwcq9TvqHkyyQUjGsfCCQMj_XnvBNKpy6Qn2DTOD1wu_tRocF18Lx9kCFwT4iRvVxenT2syNoDGyWjl4xi93g2AwxPDSXfrW4oc0EjUgwzsqzSr2VhTnxJwNHwf24lAhHIpjVDZNwy1KS5wlNoGMSg9wOCYksQccerMlv65p51X0p_Xpdt_4YEy9yTkmV3z4MJT579-bUqsaNB2kbI0dwlnCgirJe2UakJzVrbkKaqkWivasU1O1ULxnOVk3JU-Uxti0p_-vKO4no_enbQ_yXhnZn0aHs4b1jiJMK7q2zmo1C3bTMG3LaZQVrMjeoSPgaUtkDxePMCEX2Ie6b_8D4WyJJEwCAAA=)):
```svelte
<script>
let total = 100;
let spent = $state(0);
let left = {
get value() {
return total - spent;
},
set value(v) {
spent = total - v;
}
};
</script>
<label>
<input type="range" bind:value={spent} max={total} />
{spent}/{total} spent
</label>
<label>
<input type="range" bind:value={left.value} max={total} />
{left.value}/{total} left
</label>
```
If you absolutely have to update `$state` within an effect and run into an infinite loop because you read and write to the same `$state`, use [untrack](svelte#untrack).
# $props
The inputs to a component are referred to as _props_, which is short for _properties_. You pass props to components just like you pass attributes to elements:
```svelte
<!--- file: App.svelte --->
<script>
import MyComponent from './MyComponent.svelte';
</script>
<MyComponent adjective="cool" />
```
On the other side, inside `MyComponent.svelte`, we can receive props with the `$props` rune...
```svelte
<!--- file: MyComponent.svelte --->
<script>
let props = $props();
</script>
<p>this component is {props.adjective}</p>
```
...though more commonly, you'll [_destructure_](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Destructuring_assignment) your props:
```svelte
<!--- file: MyComponent.svelte --->
<script>
let +++{ adjective }+++ = $props();
</script>
<p>this component is {+++adjective+++}</p>
```
## Fallback values
Destructuring allows us to declare fallback values, which are used if the parent component does not set a given prop:
```js
let { adjective = 'happy' } = $props();
```
> [!NOTE] Fallback values are not turned into reactive state proxies (see [Updating props](#Updating-props) for more info)
## Renaming props
We can also use the destructuring assignment to rename props, which is necessary if they're invalid identifiers, or a JavaScript keyword like `super`:
```js
let { super: trouper = 'lights are gonna find me' } = $props();
```
## Rest props
Finally, we can use a _rest property_ to get, well, the rest of the props:
```js
let { a, b, c, ...others } = $props();
```
## Updating props
References to a prop inside a component update when the prop itself updates — when `count` changes in `App.svelte`, it will also change inside `Child.svelte`. But the child component is able to temporarily override the prop value, which can be useful for unsaved ephemeral state ([demo](/playground/untitled#H4sIAAAAAAAAE6WQ0WrDMAxFf0WIQR0Wmu3VTQJln7HsIfVcZubIxlbGRvC_DzuBraN92qPula50tODZWB1RPi_IX16jLALWSOOUq6P3-_ihLWftNEZ9TVeOWBNHlNhGFYznfqCBzeRdYHh6M_YVzsFNsNs3pdpGd4eBcqPVDMrNxNDBXeSRtXioDgO1zU8ataeZ2RE4Utao924RFXQ9iHXwvoPHKpW1xY4g_Bg0cSVhKS0p560Za95612ZC02ONrD8ZJYdZp_rGQ37ff_mSP86Np2TWZaNNmdcH56P4P67K66_SXoK9pG-5dF5Z9QEAAA==)):
```svelte
<!--- file: App.svelte --->
<script>
import Child from './Child.svelte';
let count = $state(0);
</script>
<button onclick={() => (count += 1)}>
clicks (parent): {count}
</button>
<Child {count} />
```
```svelte
<!--- file: Child.svelte --->
<script>
let { count } = $props();
</script>
<button onclick={() => (count += 1)}>
clicks (child): {count}
</button>
```
While you can temporarily _reassign_ props, you should not _mutate_ props unless they are [bindable]($bindable).
If the prop is a regular object, the mutation will have no effect ([demo](/playground/untitled#H4sIAAAAAAAAE3WQwU7DMBBEf2W1QmorQgJXk0RC3PkBwiExG9WQrC17U4Es_ztKUkQp9OjxzM7bjcjtSKjwyfKNp1aLORA4b13ADHszUED1HFE-3eyaBcy-Mw_O5eFAg8xa1wb6T9eWhVgCKiyD9sZJ3XAjZnTWCzzuzfAKvbcjbPJieR2jm_uGy-InweXqtd0baaliBG0nFgW3kBIUNWYo9CGoxE-UsgvIpw2_oc9-LmAPJBCPDJCggqvlVtvdH9puErEMlvVg9HsVtzuoaojzkKKAfRuALVDfk5ZZW0fmy05wXcFdwyktlUs-KIinljTXrRVnm7-kL9dYLVbUAQAA)):
```svelte
<!--- file: App.svelte --->
<script>
import Child from './Child.svelte';
</script>
<Child object={{ count: 0 }} />
```
```svelte
<!--- file: Child.svelte --->
<script>
let { object } = $props();
</script>
<button onclick={() => {
// has no effect
object.count += 1
}}>
clicks: {object.count}
</button>
```
If the prop is a reactive state proxy, however, then mutations _will_ have an effect but you will see an [`ownership_invalid_mutation`](runtime-warnings#Client-warnings-ownership_invalid_mutation) warning, because the component is mutating state that does not 'belong' to it ([demo](/playground/untitled#H4sIAAAAAAAAE3WR0U7DMAxFf8VESBuiauG1WycheOEbKA9p67FA6kSNszJV-XeUZhMw2GN8r-1znUmQ7FGU4pn2UqsOes-SlSGRia3S6ET5Mgk-2OiJBZGdOh6szd0eNcdaIx3-V28NMRI7UYq1awdleVNTzaq3ZmB43CndwXYwPSzyYn4dWxermqJRI4Np3rFlqODasWRcTtAaT1zCHYSbVU3r4nsyrdPMKTUFKDYiE4yfLEoePIbsQpqfy3_nOVMuJIqg0wk1RFg7GOuWfwEbz2wIDLVatR_VtLyBagNTHFIUMCqtoZXeIfAOU1JoUJsR2IC3nWTMjt7GM4yKdyBhlAMpesvhydCC0y_i0ZagHByMh26WzUhXUUxKnpbcVnBfUwhznJnNlac7JkuIURL-2VVfwxflyrWcSQIAAA==)):
```svelte
<!--- file: App.svelte --->
<script>
import Child from './Child.svelte';
let object = $state({count: 0});
</script>
<Child {object} />
```
```svelte
<!--- file: Child.svelte --->
<script>
let { object } = $props();
</script>
<button onclick={() => {
// will cause the count below to update,
// but with a warning. Don't mutate
// objects you don't own!
object.count += 1
}}>
clicks: {object.count}
</button>
```
The fallback value of a prop not declared with `$bindable` is left untouched — it is not turned into a reactive state proxy — meaning mutations will not cause updates ([demo](/playground/untitled#H4sIAAAAAAAAE3WQwU7DMBBEf2VkIbUVoYFraCIh7vwA4eC4G9Wta1vxpgJZ_nfkBEQp9OjxzOzTRGHlkUQlXpy9G0gq1idCL43ppDrAD84HUYheGwqieo2CP3y2Z0EU3-En79fhRIaz1slA_-nKWSbLQVRiE9SgPTetbVkfvRsYzztttugHd8RiXU6vr-jisbWb8idhN7O3bEQhmN5ZVDyMlIorcOddv_Eufq4AGmJEuG5PilEjQrnRcoV7JCTUuJlGWq7-YHYjs7NwVhmtDnVcrlA3iLmzLLGTAdaB-j736h68Oxv-JM1I0AFjoG1OzPfX023c1nhobUoT39QeKsRzS8owM8DFTG_pE6dcVl70AQAA))
```svelte
<!--- file: Child.svelte --->
<script>
let { object = { count: 0 } } = $props();
</script>
<button onclick={() => {
// has no effect if the fallback value is used
object.count += 1
}}>
clicks: {object.count}
</button>
```
In summary: don't mutate props. Either use callback props to communicate changes, or — if parent and child should share the same object — use the [`$bindable`]($bindable) rune.
## Type safety
You can add type safety to your components by annotating your props, as you would with any other variable declaration. In TypeScript that might look like this...
```svelte
<script lang="ts">
let { adjective }: { adjective: string } = $props();
</script>
```
...while in JSDoc you can do this:
```svelte
<script>
/** @type {{ adjective: string }} */
let { adjective } = $props();
</script>
```
You can, of course, separate the type declaration from the annotation:
```svelte
<script lang="ts">
interface Props {
adjective: string;
}
let { adjective }: Props = $props();
</script>
```
> [!NOTE] Interfaces for native DOM elements are provided in the `svelte/elements` module (see [Typing wrapper components](typescript#Typing-wrapper-components))
Adding types is recommended, as it ensures that people using your component can easily discover which props they should provide.
## `$props.id()`
This rune, added in version 5.20.0, generates an ID that is unique to the current component instance. When hydrating a server-rendered component, the value will be consistent between server and client.
This is useful for linking elements via attributes like `for` and `aria-labelledby`.
```svelte
<script>
const uid = $props.id();
</script>
<form>
<label for="{uid}-firstname">First Name: </label>
<input id="{uid}-firstname" type="text" />
<label for="{uid}-lastname">Last Name: </label>
<input id="{uid}-lastname" type="text" />
</form>
```
# $bindable
Ordinarily, props go one way, from parent to child. This makes it easy to understand how data flows around your app.
In Svelte, component props can be _bound_, which means that data can also flow _up_ from child to parent. This isn't something you should do often, but it can simplify your code if used sparingly and carefully.
It also means that a state proxy can be _mutated_ in the child.
> [!NOTE] Mutation is also possible with normal props, but is strongly discouraged — Svelte will warn you if it detects that a component is mutating state it does not 'own'.
To mark a prop as bindable, we use the `$bindable` rune:
<!-- prettier-ignore -->
```svelte
/// file: FancyInput.svelte
<script>
let { value = $bindable(), ...props } = $props();
</script>
<input bind:value={value} {...props} />
<style>
input {
font-family: 'Comic Sans MS';
color: deeppink;
}
</style>
```
Now, a component that uses `<FancyInput>` can add the [`bind:`](bind) directive ([demo](/playground/untitled#H4sIAAAAAAAAE3WQwWrDMBBEf2URBSfg2nfFMZRCoYeecqx6UJx1IyqvhLUONcb_XqSkTUOSk1az7DBvJtEai0HI90nw6FHIJIhckO7i78n7IhzQctS2OuAtvXHESByEFFVoeuO5VqTYdN71DC-amvGV_MDQ9q6DrCjP0skkWymKJxYZOgxBfyKs4SGwZlxke7TWZcuVoqo8-1P1z3lraCcP2g64nk4GM5S1osrXf0JV-lrkgvGbheR-wDm_g30V8JL-1vpOCZFogpQsEsWcemtxscyhKArfOx9gjps0Lq4hzRVfemaYfu-PoIqqwKPFY_XpaIqj4tYRP7a6M3aUkD27zjSw0RTgbZN6Z8WNs66XsEP03tBXUueUJFlelvYx_wCuI3leNwIAAA==)):
<!-- prettier-ignore -->
```svelte
/// App.svelte
<script>
import FancyInput from './FancyInput.svelte';
let message = $state('hello');
</script>
<FancyInput bind:value={message} />
<p>{message}</p>
```
The parent component doesn't _have_ to use `bind:` — it can just pass a normal prop. Some parents don't want to listen to what their children have to say.
In this case, you can specify a fallback value for when no prop is passed at all:
```js
/// file: FancyInput.svelte
let { value = $bindable('fallback'), ...props } = $props();
```
# $inspect
> [!NOTE] `$inspect` only works during development. In a production build it becomes a noop.
The `$inspect` rune is roughly equivalent to `console.log`, with the exception that it will re-run whenever its argument changes. `$inspect` tracks reactive state deeply, meaning that updating something inside an object or array using fine-grained reactivity will cause it to re-fire ([demo](/playground/untitled#H4sIAAAAAAAACkWQ0YqDQAxFfyUMhSotdZ-tCvu431AXtGOqQ2NmmMm0LOK_r7Utfby5JzeXTOpiCIPKT5PidkSVq2_n1F7Jn3uIcEMSXHSw0evHpAjaGydVzbUQCmgbWaCETZBWMPlKj29nxBDaHj_edkAiu12JhdkYDg61JGvE_s2nR8gyuBuiJZuDJTyQ7eE-IEOzog1YD80Lb0APLfdYc5F9qnFxjiKWwbImo6_llKRQVs-2u91c_bD2OCJLkT3JZasw7KLA2XCX31qKWE6vIzNk1fKE0XbmYrBTufiI8-_8D2cUWBA_AQAA)):
```svelte
<script>
let count = $state(0);
let message = $state('hello');
$inspect(count, message); // will console.log when `count` or `message` change
</script>
<button onclick={() => count++}>Increment</button>
<input bind:value={message} />
```
## $inspect(...).with
`$inspect` returns a property `with`, which you can invoke with a callback, which will then be invoked instead of `console.log`. The first argument to the callback is either `"init"` or `"update"`; subsequent arguments are the values passed to `$inspect` ([demo](/playground/untitled#H4sIAAAAAAAACkVQ24qDMBD9lSEUqlTqPlsj7ON-w7pQG8c2VCchmVSK-O-bKMs-DefKYRYx6BG9qL4XQd2EohKf1opC8Nsm4F84MkbsTXAqMbVXTltuWmp5RAZlAjFIOHjuGLOP_BKVqB00eYuKs82Qn2fNjyxLtcWeyUE2sCRry3qATQIpJRyD7WPVMf9TW-7xFu53dBcoSzAOrsqQNyOe2XUKr0Xi5kcMvdDB2wSYO-I9vKazplV1-T-d6ltgNgSG1KjVUy7ZtmdbdjqtzRcphxMS1-XubOITJtPrQWMvKnYB15_1F7KKadA_AQAA)):
```svelte
<script>
let count = $state(0);
$inspect(count).with((type, count) => {
if (type === 'update') {
debugger; // or `console.trace`, or whatever you want
}
});
</script>
<button onclick={() => count++}>Increment</button>
```
A convenient way to find the origin of some change is to pass `console.trace` to `with`:
```js
// @errors: 2304
$inspect(stuff).with(console.trace);
```
## $inspect.trace(...)
This rune, added in 5.14, causes the surrounding function to be _traced_ in development. Any time the function re-runs as part of an [effect]($effect) or a [derived]($derived), information will be printed to the console about which pieces of reactive state caused the effect to fire.
```svelte
<script>
import { doSomeWork } from './elsewhere';
$effect(() => {
+++$inspect.trace();+++
doSomeWork();
});
</script>
```
`$inspect.trace` takes an optional first argument which will be used as the label.
# $host
When compiling a component as a custom element, the `$host` rune provides access to the host element, allowing you to (for example) dispatch custom events ([demo](/playground/untitled#H4sIAAAAAAAAE41Ry2rDMBD8FSECtqkTt1fHFpSSL-ix7sFRNkTEXglrnTYY_3uRlDgxTaEHIfYxs7szA9-rBizPPwZOZwM89wmecqxbF70as7InaMjltrWFR3mpkQDJ8pwXVnbKkKiwItUa3RGLVtk7gTHQXRDR2lXda4CY1D0SK9nCUk0QPyfrCovsRoNFe17aQOAwGncgO2gBqRzihJXiQrEs2csYOhQ-7HgKHaLIbpRhhBG-I2eD_8ciM4KnnOCbeE5dD2P6h0Dz0-Yi_arNhPLJXBtSGi2TvSXdbpqwdsXvjuYsC1veabvvUTog2ylrapKH2G2XsMFLS4uDthQnq2t1cwKkGOGLvYU5PvaQxLsxOkPmsm97Io1Mo2yUPF6VnOZFkw1RMoopKLKAE_9gmGxyDFMwMcwN-Bx_ABXQWmOtAgAA)):
<!-- prettier-ignore -->
```svelte
/// file: Stepper.svelte
<svelte:options customElement="my-stepper" />
<script>
function dispatch(type) {
+++$host()+++.dispatchEvent(new CustomEvent(type));
}
</script>
<button onclick={() => dispatch('decrement')}>decrement</button>
<button onclick={() => dispatch('increment')}>increment</button>
```
<!-- prettier-ignore -->
```svelte
/// file: App.svelte
<script>
import './Stepper.svelte';
let count = $state(0);
</script>
<my-stepper
ondecrement={() => count -= 1}
onincrement={() => count += 1}
></my-stepper>
<p>count: {count}</p>
```
# Basic markup
Markup inside a Svelte component can be thought of as HTML++.
## Tags
A lowercase tag, like `<div>`, denotes a regular HTML element. A capitalised tag or a tag that uses dot notation, such as `<Widget>` or `<my.stuff>`, indicates a _component_.
```svelte
<script>
import Widget from './Widget.svelte';
</script>
<div>
<Widget />
</div>
```
## Element attributes
By default, attributes work exactly like their HTML counterparts.
```svelte
<div class="foo">
<button disabled>can't touch this</button>
</div>
```
As in HTML, values may be unquoted.
<!-- prettier-ignore -->
```svelte
<input type=checkbox />
```
Attribute values can contain JavaScript expressions.
```svelte
<a href="page/{p}">page {p}</a>
```
Or they can _be_ JavaScript expressions.
```svelte
<button disabled={!clickable}>...</button>
```
Boolean attributes are included on the element if their value is [truthy](https://developer.mozilla.org/en-US/docs/Glossary/Truthy) and excluded if it's [falsy](https://developer.mozilla.org/en-US/docs/Glossary/Falsy).
All other attributes are included unless their value is [nullish](https://developer.mozilla.org/en-US/docs/Glossary/Nullish) (`null` or `undefined`).
```svelte
<input required={false} placeholder="This input field is not required" />
<div title={null}>This div has no title attribute</div>
```
> [!NOTE] Quoting a singular expression does not affect how the value is parsed, but in Svelte 6 it will cause the value to be coerced to a string:
>
> <!-- prettier-ignore -->
> ```svelte
> <button disabled="{number !== 42}">...</button>
> ```
When the attribute name and value match (`name={name}`), they can be replaced with `{name}`.
```svelte
<button {disabled}>...</button>
<!-- equivalent to
<button disabled={disabled}>...</button>
-->
```
## Component props
By convention, values passed to components are referred to as _properties_ or _props_ rather than _attributes_, which are a feature of the DOM.
As with elements, `name={name}` can be replaced with the `{name}` shorthand.
```svelte
<Widget foo={bar} answer={42} text="hello" />
```
_Spread attributes_ allow many attributes or properties to be passed to an element or component at once.
An element or component can have multiple spread attributes, interspersed with regular ones.
```svelte
<Widget {...things} />
```
## Events
Listening to DOM events is possible by adding attributes to the element that start with `on`. For example, to listen to the `click` event, add the `onclick` attribute to a button:
```svelte
<button onclick={() => console.log('clicked')}>click me</button>
```
Event attributes are case sensitive. `onclick` listens to the `click` event, `onClick` listens to the `Click` event, which is different. This ensures you can listen to custom events that have uppercase characters in them.
Because events are just attributes, the same rules as for attributes apply:
- you can use the shorthand form: `<button {onclick}>click me</button>`
- you can spread them: `<button {...thisSpreadContainsEventAttributes}>click me</button>`
Timing-wise, event attributes always fire after events from bindings (e.g. `oninput` always fires after an update to `bind:value`). Under the hood, some event handlers are attached directly with `addEventListener`, while others are _delegated_.
When using `ontouchstart` and `ontouchmove` event attributes, the handlers are [passive](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#using_passive_listeners) for better performance. This greatly improves responsiveness by allowing the browser to scroll the document immediately, rather than waiting to see if the event handler calls `event.preventDefault()`.
In the very rare cases that you need to prevent these event defaults, you should use [`on`](svelte-events#on) instead (for example inside an action).
### Event delegation
To reduce memory footprint and increase performance, Svelte uses a technique called event delegation. This means that for certain events — see the list below — a single event listener at the application root takes responsibility for running any handlers on the event's path.
There are a few gotchas to be aware of:
- when you manually dispatch an event with a delegated listener, make sure to set the `{ bubbles: true }` option or it won't reach the application root
- when using `addEventListener` directly, avoid calling `stopPropagation` or the event won't reach the application root and handlers won't be invoked. Similarly, handlers added manually inside the application root will run _before_ handlers added declaratively deeper in the DOM (with e.g. `onclick={...}`), in both capturing and bubbling phases. For these reasons it's better to use the `on` function imported from `svelte/events` rather than `addEventListener`, as it will ensure that order is preserved and `stopPropagation` is handled correctly.
The following event handlers are delegated:
- `beforeinput`
- `click`
- `change`
- `dblclick`
- `contextmenu`
- `focusin`
- `focusout`
- `input`
- `keydown`
- `keyup`
- `mousedown`
- `mousemove`
- `mouseout`
- `mouseover`
- `mouseup`
- `pointerdown`
- `pointermove`
- `pointerout`
- `pointerover`
- `pointerup`
- `touchend`
- `touchmove`
- `touchstart`
## Text expressions
A JavaScript expression can be included as text by surrounding it with curly braces.
```svelte
{expression}
```
Curly braces can be included in a Svelte template by using their [HTML entity](https://developer.mozilla.org/docs/Glossary/Entity) strings: `{`, `{`, or `{` for `{` and `}`, `}`, or `}` for `}`.
If you're using a regular expression (`RegExp`) [literal notation](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp#literal_notation_and_constructor), you'll need to wrap it in parentheses.
<!-- prettier-ignore -->
```svelte
<h1>Hello {name}!</h1>
<p>{a} + {b} = {a + b}.</p>
<div>{(/^[A-Za-z ]+$/).test(value) ? x : y}</div>
```
The expression will be stringified and escaped to prevent code injections. If you want to render HTML, use the `{@html}` tag instead.
```svelte
{@html potentiallyUnsafeHtmlString}
```
> [!NOTE] Make sure that you either escape the passed string or only populate it with values that are under your control in order to prevent [XSS attacks](https://owasp.org/www-community/attacks/xss/)
## Comments
You can use HTML comments inside components.
```svelte
<!-- this is a comment! --><h1>Hello world</h1>
```
Comments beginning with `svelte-ignore` disable warnings for the next block of markup. Usually, these are accessibility warnings; make sure that you're disabling them for a good reason.
```svelte
<!-- svelte-ignore a11y-autofocus -->
<input bind:value={name} autofocus />
```
You can add a special comment starting with `@component` that will show up when hovering over the component name in other files.
````svelte
<!--
@component
- You can use markdown here.
- You can also use code blocks here.
- Usage:
```html
<Main name="Arethra">
```
-->
<script>
let { name } = $props();
</script>
<main>
<h1>
Hello, {name}
</h1>
</main>
````
# {#if ...}
```svelte
<!--- copy: false --->
{#if expression}...{/if}
```
```svelte
<!--- copy: false --->
{#if expression}...{:else if expression}...{/if}
```
```svelte
<!--- copy: false --->
{#if expression}...{:else}...{/if}
```
Content that is conditionally rendered can be wrapped in an if block.
```svelte
{#if answer === 42}
<p>what was the question?</p>
{/if}
```
Additional conditions can be added with `{:else if expression}`, optionally ending in an `{:else}` clause.
```svelte
{#if porridge.temperature > 100}
<p>too hot!</p>
{:else if 80 > porridge.temperature}
<p>too cold!</p>
{:else}
<p>just right!</p>
{/if}
```
(Blocks don't have to wrap elements, they can also wrap text within elements.)
# {#each ...}
```svelte
<!--- copy: false --->
{#each expression as name}...{/each}
```
```svelte
<!--- copy: false --->
{#each expression as name, index}...{/each}
```
Iterating over values can be done with an each block. The values in question can be arrays, array-like objects (i.e. anything with a `length` property), or iterables like `Map` and `Set` — in other words, anything that can be used with `Array.from`.
```svelte
<h1>Shopping list</h1>
<ul>
{#each items as item}
<li>{item.name} x {item.qty}</li>
{/each}
</ul>
```
An each block can also specify an _index_, equivalent to the second argument in an `array.map(...)` callback:
```svelte
{#each items as item, i}
<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}
```
## Keyed each blocks
```svelte
<!--- copy: false --->
{#each expression as name (key)}...{/each}
```
```svelte
<!--- copy: false --->
{#each expression as name, index (key)}...{/each}
```
If a _key_ expression is provided — which must uniquely identify each list item — Svelte will use it to diff the list when data changes, rather than adding or removing items at the end. The key can be any object, but strings and numbers are recommended since they allow identity to persist when the objects themselves change.
```svelte
{#each items as item (item.id)}
<li>{item.name} x {item.qty}</li>
{/each}
<!-- or with additional index value -->
{#each items as item, i (item.id)}
<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}
```
You can freely use destructuring and rest patterns in each blocks.
```svelte
{#each items as { id, name, qty }, i (id)}
<li>{i + 1}: {name} x {qty}</li>
{/each}
{#each objects as { id, ...rest }}
<li><span>{id}</span><MyComponent {...rest} /></li>
{/each}
{#each items as [id, ...rest]}
<li><span>{id}</span><MyComponent values={rest} /></li>
{/each}
```
## Each blocks without an item
```svelte
<!--- copy: false --->
{#each expression}...{/each}
```
```svelte
<!--- copy: false --->
{#each expression, index}...{/each}
```
In case you just want to render something `n` times, you can omit the `as` part ([demo](/playground/untitled#H4sIAAAAAAAAE3WR0W7CMAxFf8XKNAk0WsSeUEaRpn3Guoc0MbQiJFHiMlDVf18SOrZJ48259_jaVgZmxBEZZ28thgCNFV6xBdt1GgPj7wOji0t2EqI-wa_OleGEmpLWiID_6dIaQkMxhm1UdwKpRQhVzWSaVORJNdvWpqbhAYVsYQCNZk8thzWMC_DCHMZk3wPSThNQ088I3mghD9UwSwHwlLE5PMIzVFUFq3G7WUZ2OyUvU3JOuZU332wCXTRmtPy1NgzXZtUFp8WFw9536uWqpbIgPEaDsJBW90cTOHh0KGi2XsBq5-cT6-3nPauxXqHnsHJnCFZ3CvJVkyuCQ0mFF9TZyCQ162WGvteLKfG197Y3iv_pz_fmS68Hxt8iPBPj5HscP8YvCNX7uhYCAAA=)):
```svelte
<div class="chess-board">
{#each { length: 8 }, rank}
{#each { length: 8 }, file}
<div class:black={(rank + file) % 2 === 1}></div>
{/each}
{/each}
</div>
```
## Else blocks
```svelte
<!--- copy: false --->
{#each expression as name}...{:else}...{/each}
```
An each block can also have an `{:else}` clause, which is rendered if the list is empty.
```svelte
{#each todos as todo}
<p>{todo.text}</p>
{:else}
<p>No tasks today!</p>
{/each}
```
# {#key ...}
```svelte
<!--- copy: false --->
{#key expression}...{/key}
```
Key blocks destroy and recreate their contents when the value of an expression changes. When used around components, this will cause them to be reinstantiated and reinitialised:
```svelte
{#key value}
<Component />
{/key}
```
It's also useful if you want a transition to play whenever a value changes:
```svelte
{#key value}
<div transition:fade>{value}</div>
{/key}
```
# {#await ...}
```svelte
<!--- copy: false --->
{#await expression}...{:then name}...{:catch name}...{/await}
```
```svelte
<!--- copy: false --->
{#await expression}...{:then name}...{/await}
```
```svelte
<!--- copy: false --->
{#await expression then name}...{/await}
```
```svelte
<!--- copy: false --->
{#await expression catch name}...{/await}
```
Await blocks allow you to branch on the three possible states of a [`Promise`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise) — pending, fulfilled or rejected.
```svelte
{#await promise}
<!-- promise is pending -->
<p>waiting for the promise to resolve...</p>
{:then value}
<!-- promise was fulfilled or not a Promise -->
<p>The value is {value}</p>
{:catch error}
<!-- promise was rejected -->
<p>Something went wrong: {error.message}</p>
{/await}
```
> [!NOTE] During server-side rendering, only the pending branch will be rendered.
>
> If the provided expression is not a `Promise`, only the `:then` branch will be rendered, including during server-side rendering.
The `catch` block can be omitted if you don't need to render anything when the promise rejects (or no error is possible).
```svelte
{#await promise}
<!-- promise is pending -->
<p>waiting for the promise to resolve...</p>
{:then value}
<!-- promise was fulfilled -->
<p>The value is {value}</p>
{/await}
```
If you don't care about the pending state, you can also omit the initial block.
```svelte
{#await promise then value}
<p>The value is {value}</p>
{/await}
```
Similarly, if you only want to show the error state, you can omit the `then` block.
```svelte
{#await promise catch error}
<p>The error is {error}</p>
{/await}
```
> [!NOTE] You can use `#await` with [`import(...)`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/import) to render components lazily:
>
> ```svelte
> {#await import('./Component.svelte') then { default: Component }}
> <Component />
> {/await}
> ```
# {#snippet ...}
```svelte
<!--- copy: false --->
{#snippet name()}...{/snippet}
```
```svelte
<!--- copy: false --->
{#snippet name(param1, param2, paramN)}...{/snippet}
```
Snippets, and [render tags](@render), are a way to create reusable chunks of markup inside your components. Instead of writing duplicative code like [this](/playground/untitled#H4sIAAAAAAAAE5VUYW-kIBD9K8Tmsm2yXXRzvQ-s3eR-R-0HqqOQKhAZb9sz_vdDkV1t000vRmHewMx7w2AflbIGG7GnPlK8gYhFv42JthG-m9Gwf6BGcLbVXZuPSGrzVho8ZirDGpDIhldgySN5GpEMez9kaNuckY1ANJZRamRuu2ZnhEZt6a84pvs43mzD4pMsUDDi8DMkQFYCGdkvsJwblFq5uCik9bmJ4JZwUkv1eoknWigX2eGNN6aGXa6bjV8ybP-X7sM36T58SVcrIIV2xVIaA41xeD5kKqWXuqpUJEefOqVuOkL9DfBchGrzWfu0vb-RpTd3o-zBR045Ga3HfuE5BmJpKauuhbPtENlUF2sqR9jqpsPSxWsMrlngyj3VJiyYjJXb1-lMa7IWC-iSk2M5Zzh-SJjShe-siq5kpZRPs55BbSGU5YPyte4vVV_VfFXxVb10dSLf17pS2lM5HnpPxw4Zpv6x-F57p0jI3OKlVnhv5V9wPQrNYQQ9D_f6aGHlC89fq1Z3qmDkJCTCweOGF4VUFSPJvD_DhreVdA0eu8ehJJ5x91dBaBkpWm3ureCFPt3uzRv56d4kdp-2euG38XZ6dsnd3ZmPG9yRBCrzRUvi-MccOdwz3qE-fOZ7AwAhlrtTUx3c76vRhSwlFBHDtoPhefgHX3dM0PkEAAA=)...
```svelte
{#each images as image}
{#if image.href}
<a href={image.href}>
<figure>
<img src={image.src} alt={image.caption} width={image.width} height={image.height} />
<figcaption>{image.caption}</figcaption>
</figure>
</a>
{:else}
<figure>
<img src={image.src} alt={image.caption} width={image.width} height={image.height} />
<figcaption>{image.caption}</figcaption>
</figure>
{/if}
{/each}
```
...you can write [this](/playground/untitled#H4sIAAAAAAAAE5VUYW-bMBD9KxbRlERKY4jWfSA02n5H6QcXDmwVbMs-lnaI_z6D7TTt1moTAnPvzvfenQ_GpBEd2CS_HxPJekjy5IfWyS7BFz0b9id0CM62ajDVjBS2MkLjqZQldoBE9KwFS-7I_YyUOPqlRGuqnKw5orY5pVpUduj3mitUln5LU3pI0_UuBp9FjTwnDr9AHETLMSeHK6xiGoWSLi9yYT034cwSRjohn17zcQPNFTs8s153sK9Uv_Yh0-5_5d7-o9zbD-UqCaRWrllSYZQxLw_HUhb0ta-y4NnJUxfUvc7QuLJSaO0a3oh2MLBZat8u-wsPnXzKQvTtVVF34xK5d69ThFmHEQ4SpzeVRediTG8rjD5vBSeN3E5JyHh6R1DQK9-iml5kjzQUN_lSgVU8DhYLx7wwjSvRkMDvTjiwF4zM1kXZ7DlF1eN3A7IG85e-zRrYEjjm0FkI4Cc7Ripm0pHOChexhcWXzreeZyRMU6Mk3ljxC9w4QH-cQZ_b3T5pjHxk1VNr1CDrnJy5QDh6XLO6FrLNSRb2l9gz0wo3S6m7HErSgLsPGMHkpDZK31jOanXeHPQz-eruLHUP0z6yTbpbrn223V70uMXNSpQSZjpL0y8hcxxpNqA6_ql3BQAxlxvfpQ_uT9GrWjQC6iRHM8D0MP0GQsIi92QEAAA=):
```svelte
{#snippet figure(image)}
<figure>
<img src={image.src} alt={image.caption} width={image.width} height={image.height} />
<figcaption>{image.caption}</figcaption>
</figure>
{/snippet}
{#each images as image}
{#if image.href}
<a href={image.href}>
{@render figure(image)}
</a>
{:else}
{@render figure(image)}
{/if}
{/each}
```
Like function declarations, snippets can have an arbitrary number of parameters, which can have default values, and you can destructure each parameter. You cannot use rest parameters, however.
## Snippet scope
Snippets can be declared anywhere inside your component. They can reference values declared outside themselves, for example in the `<script>` tag or in `{#each ...}` blocks ([demo](/playground/untitled#H4sIAAAAAAAAE12P0QrCMAxFfyWrwhSEvc8p-h1OcG5RC10bmkyQ0n-3HQPBx3vCPUmCemiDrOpLULYbUdXqTKR2Sj6UA7_RCKbMbvJ9Jg33XpMcW9uKQYEAIzJ3T4QD3LSUDE-PnYA4YET4uOkGMc3W5B3xZrtvbVP9HDas2GqiZHqhMW6Tr9jGbG_oOCMImcUCwrIpFk1FqRyqpRpn0cmjHdAvnrIzuscyq_4nd3dPPD01ukE_NA6qFj9hvMYvGjJADw8BAAA=))...
```svelte
<script>
let { message = `it's great to see you!` } = $props();
</script>
{#snippet hello(name)}
<p>hello {name}! {message}!</p>
{/snippet}
{@render hello('alice')}
{@render hello('bob')}
```
...and they are 'visible' to everything in the same lexical scope (i.e. siblings, and children of those siblings):
```svelte
<div>
{#snippet x()}
{#snippet y()}...{/snippet}
<!-- this is fine -->
{@render y()}
{/snippet}
<!-- this will error, as `y` is not in scope -->
{@render y()}
</div>
<!-- this will also error, as `x` is not in scope -->
{@render x()}
```
Snippets can reference themselves and each other ([demo](/playground/untitled#H4sIAAAAAAAAE2WPTQqDMBCFrxLiRqH1Zysi7TlqF1YnENBJSGJLCYGeo5tesUeosfYH3c2bee_jjaWMd6BpfrAU6x5oTvdS0g01V-mFPkNnYNRaDKrxGxto5FKCIaeu1kYwFkauwsoUWtZYPh_3W5FMY4U2mb3egL9kIwY0rbhgiO-sDTgjSEqSTvIDs-jiOP7i_MHuFGAL6p9BtiSbOTl0GtzCuihqE87cqtyam6WRGz_vRcsZh5bmRg3gju4Fptq_kzQBAAA=)):
```svelte
{#snippet blastoff()}
<span>🚀</span>
{/snippet}
{#snippet countdown(n)}
{#if n > 0}
<span>{n}...</span>
{@render countdown(n - 1)}
{:else}
{@render blastoff()}
{/if}
{/snippet}
{@render countdown(10)}
```
## Passing snippets to components
Within the template, snippets are values just like any other. As such, they can be passed to components as props ([demo](/playground/untitled#H4sIAAAAAAAAE3VS247aMBD9lZGpBGwDASRegonaPvQL2qdlH5zYEKvBNvbQLbL875VzAcKyj3PmzJnLGU8UOwqSkd8KJdaCk4TsZS0cyV49wYuJuQiQpGd-N2bu_ooaI1YwJ57hpVYoFDqSEepKKw3mO7VDeTTaIvxiRS1gb_URxvO0ibrS8WanIrHUyiHs7Vmigy28RmyHHmKvDMbMmFq4cQInvGSwTsBYWYoMVhCSB2rBFFPsyl0uruTlR3JZCWvlTXl1Yy_mawiR_rbZKZrellJ-5JQ0RiBUgnFhJ9OGR7HKmwVoilXeIye8DOJGfYCgRlZ3iE876TBsZPX7hPdteO75PC4QaIo8vwNPePmANQ2fMeEFHrLD7rR1jTNkW986E8C3KwfwVr8HSHOSEBT_kGRozyIkn_zQveXDL3rIfPJHtUDwzShJd_Qk3gQCbOGLsdq4yfTRJopRuin3I7nv6kL7ARRjmLdBDG3uv1mhuLA3V2mKtqNEf_oCn8p9aN-WYqH5peP4kWBl1UwJzAEPT9U7K--0fRrrWnPTXpCm1_EVdXjpNmlA8G1hPPyM1fKgMqjFHjctXGjLhZ05w0qpDhksGrybuNEHtJnCalZWsuaTlfq6nPaaBSv_HKw-K57BjzOiVj9ZKQYKzQjZodYFqydYTRN4gPhVzTDO2xnma3HsVWjaLjT8nbfwHy7Q5f2dBAAA)):
```svelte
<script>
import Table from './Table.svelte';
const fruits = [
{ name: 'apples', qty: 5, price: 2 },
{ name: 'bananas', qty: 10, price: 1 },
{ name: 'cherries', qty: 20, price: 0.5 }
];
</script>
{#snippet header()}
<th>fruit</th>
<th>qty</th>
<th>price</th>
<th>total</th>
{/snippet}
{#snippet row(d)}
<td>{d.name}</td>
<td>{d.qty}</td>
<td>{d.price}</td>
<td>{d.qty * d.price}</td>
{/snippet}
<Table data={fruits} {header} {row} />
```
Think about it like passing content instead of data to a component. The concept is similar to slots in web components.
As an authoring convenience, snippets declared directly _inside_ a component implicitly become props _on_ the component ([demo](/playground/untitled#H4sIAAAAAAAAE3VSTa_aMBD8Kyu_SkAbCA-JSzBR20N_QXt6vIMTO8SqsY29tI2s_PcqTiB8vaPHs7MzuxuIZgdBMvJLo0QlOElIJZXwJHsLBBvb_XUASc7Mb9Yu_B-hsMMK5sUzvDQahUZPMkJ96aTFfKd3KA_WOISfrFACKmcOMFmk8TWUTjY73RFLoz1C5U4SPWzhrcN2GKDrlcGEWauEnyRwxCaDdQLWyVJksII2uaMWTDPNLtzX5YX8-kgua-GcHJVXI3u5WEPb0d83O03TMZSmfRzOkG1Db7mNacOL19JagVALxoWbztq-H8U6j0SaYp2P2BGbOyQ2v8PQIFMXLKRDk177pq0zf6d8bMrzwBdd0pamyPMb-IjNEzS2f86Gz_Dwf-2F9nvNSUJQ_EOSoTuJNvngqK5v4Pas7n4-OCwlEEJcQTIMO-nSQwtb-GSdsX46e9gbRoP9yGQ11I0rEuycunu6PHx1QnPhxm3SFN15MOlYEFJZtf0dUywMbwZOeBGsrKNLYB54-1R9WNqVdki7usim6VmQphf7mnpshiQRhNAXdoOfMyX3OgMlKtz0cGEcF27uLSul3mewjPjgOOoDukxjPS9rqfh0pb-8zs6aBSt_7505aZ7B9xOi0T9YKW4UooVsr0zB1BTrWQJ3EL-oWcZ572GxFoezCk37QLe3897-B2i2U62uBAAA)):
```svelte
<!-- this is semantically the same as the above -->
<Table data={fruits}>
{#snippet header()}
<th>fruit</th>
<th>qty</th>
<th>price</th>
<th>total</th>
{/snippet}
{#snippet row(d)}
<td>{d.name}</td>
<td>{d.qty}</td>
<td>{d.price}</td>
<td>{d.qty * d.price}</td>
{/snippet}
</Table>
```
Any content inside the component tags that is _not_ a snippet declaration implicitly becomes part of the `children` snippet ([demo](/playground/untitled#H4sIAAAAAAAAE3WOQQrCMBBFrzIMggql3ddY1Du4si5sOmIwnYRkFKX07lKqglqX8_7_w2uRDw1hjlsWI5ZqTPBoLEXMdy3K3fdZDzB5Ndfep_FKVnpWHSKNce1YiCVijirqYLwUJQOYxrsgsLmIOIZjcA1M02w4n-PpomSVvTclqyEutDX6DA2pZ7_ABIVugrmEC3XJH92P55_G39GodCmWBFrQJ2PrQAwdLGHig_NxNv9xrQa1dhWIawrv1Wzeqawa8953D-8QOmaEAQAA)):
```svelte
<!--- file: App.svelte --->
<Button>click me</Button>
```
```svelte
<!--- file: Button.svelte --->
<script>
let { children } = $props();
</script>
<!-- result will be <button>click me</button> -->
<button>{@render children()}</button>
```
> [!NOTE] Note that you cannot have a prop called `children` if you also have content inside the component — for this reason, you should avoid having props with that name
You can declare snippet props as being optional. You can either use optional chaining to not render anything if the snippet isn't set...
```svelte
<script>
let { children } = $props();
</script>
{@render children?.()}
```
...or use an `#if` block to render fallback content:
```svelte
<script>
let { children } = $props();
</script>
{#if children}
{@render children()}
{:else}
fallback content
{/if}
```
## Typing snippets
Snippets implement the `Snippet` interface imported from `'svelte'`:
```svelte
<script lang="ts">
import type { Snippet } from 'svelte';
interface Props {
data: any[];
children: Snippet;
row: Snippet<[any]>;
}
let { data, children, row }: Props = $props();
</script>
```
With this change, red squigglies will appear if you try and use the component without providing a `data` prop and a `row` snippet. Notice that the type argument provided to `Snippet` is a tuple, since snippets can have multiple parameters.
We can tighten things up further by declaring a generic, so that `data` and `row` refer to the same type:
```svelte
<script lang="ts" generics="T">
import type { Snippet } from 'svelte';
let {
data,
children,
row
}: {
data: T[];
children: Snippet;
row: Snippet<[T]>;
} = $props();
</script>
```
## Exporting snippets
Snippets declared at the top level of a `.svelte` file can be exported from a `<script module>` for use in other components, provided they don't reference any declarations in a non-module `<script>` (whether directly or indirectly, via other snippets) ([demo](/playground/untitled#H4sIAAAAAAAAE3WPwY7CMAxEf8UyB1hRgdhjl13Bga8gHFJipEqtGyUGFUX5dxJUtEB3b9bYM_MckHVLWOKut50TMuC5tpbEY4GnuiGP5T6gXG0-ykLSB8vW2oW_UCNZq7Snv_Rjx0Kc4kpc-6OrrfwoVlK3uQ4CaGMgwsl1LUwXy0f54J9-KV4vf20cNo7YkMu22aqAz4-oOLUI9YKluDPF4h_at-hX5PFyzA1tZ84N3fGpf8YfUU6GvDumLqDKmEqCjjCHUEX4hqDTWCU5PJ6Or38c4g1cPu9tnAEAAA==)):
```svelte
<script module>
export { add };
</script>
{#snippet add(a, b)}
{a} + {b} = {a + b}
{/snippet}
```
> [!NOTE]
> This requires Svelte 5.5.0 or newer
## Programmatic snippets
Snippets can be created programmatically with the [`createRawSnippet`](svelte#createRawSnippet) API. This is intended for advanced use cases.
## Snippets and slots
In Svelte 4, content can be passed to components using [slots](legacy-slots). Snippets are more powerful and flexible, and as such slots are deprecated in Svelte 5.
# {@render ...}
To render a [snippet](snippet), use a `{@render ...}` tag.
```svelte
{#snippet sum(a, b)}
<p>{a} + {b} = {a + b}</p>
{/snippet}
{@render sum(1, 2)}
{@render sum(3, 4)}
{@render sum(5, 6)}
```
The expression can be an identifier like `sum`, or an arbitrary JavaScript expression:
```svelte
{@render (cool ? coolSnippet : lameSnippet)()}
```
## Optional snippets
If the snippet is potentially undefined — for example, because it's an incoming prop — then you can use optional chaining to only render it when it _is_ defined:
```svelte
{@render children?.()}
```
Alternatively, use an [`{#if ...}`](if) block with an `:else` clause to render fallback content:
```svelte
{#if children}
{@render children()}
{:else}
<p>fallback content</p>
{/if}
```
# {@html ...}
To inject raw HTML into your component, use the `{@html ...}` tag:
```svelte
<article>
{@html content}
</article>
```
> [!NOTE] Make sure that you either escape the passed string or only populate it with values that are under your control in order to prevent [XSS attacks](https://owasp.org/www-community/attacks/xss/). Never render unsanitized content.
The expression should be valid standalone HTML — this will not work, because `</div>` is not valid HTML:
```svelte
{@html '<div>'}content{@html '</div>'}
```
It also will not compile Svelte code.
## Styling
Content rendered this way is 'invisible' to Svelte and as such will not receive [scoped styles](scoped-styles) — in other words, this will not work, and the `a` and `img` styles will be regarded as unused:
<!-- prettier-ignore -->
```svelte
<article>
{@html content}
</article>
<style>
article {
a { color: hotpink }
img { width: 100% }
}
</style>
```
Instead, use the `:global` modifier to target everything inside the `<article>`:
<!-- prettier-ignore -->
```svelte
<style>
article +++:global+++ {
a { color: hotpink }
img { width: 100% }
}
</style>
```
# {@const ...}
The `{@const ...}` tag defines a local constant.
```svelte
{#each boxes as box}
{@const area = box.width * box.height}
{box.width} * {box.height} = {area}
{/each}
```
`{@const}` is only allowed as an immediate child of a block — `{#if ...}`, `{#each ...}`, `{#snippet ...}` and so on — a `<Component />` or a `<svelte:boundary>`.
# {@debug ...}
The `{@debug ...}` tag offers an alternative to `console.log(...)`. It logs the values of specific variables whenever they change, and pauses code execution if you have devtools open.
```svelte
<script>
let user = {
firstname: 'Ada',
lastname: 'Lovelace'
};
</script>
{@debug user}
<h1>Hello {user.firstname}!</h1>
```
`{@debug ...}` accepts a comma-separated list of variable names (not arbitrary expressions).
```svelte
<!-- Compiles -->
{@debug user}
{@debug user1, user2, user3}
<!-- WON'T compile -->
{@debug user.firstname}
{@debug myArray[0]}
{@debug !isReady}
{@debug typeof user === 'object'}
```
The `{@debug}` tag without any arguments will insert a `debugger` statement that gets triggered when _any_ state changes, as opposed to the specified variables.
# bind:
Data ordinarily flows down, from parent to child. The `bind:` directive allows data to flow the other way, from child to parent.
The general syntax is `bind:property={expression}`, where `expression` is an _lvalue_ (i.e. a variable or an object property). When the expression is an identifier with the same name as the property, we can omit the expression — in other words these are equivalent:
<!-- prettier-ignore -->
```svelte
<input bind:value={value} />
<input bind:value />
```
Svelte creates an event listener that updates the bound value. If an element already has a listener for the same event, that listener will be fired before the bound value is updated.
Most bindings are _two-way_, meaning that changes to the value will affect the element and vice versa. A few bindings are _readonly_, meaning that changing their value will have no effect on the element.
## Function bindings
You can also use `bind:property={get, set}`, where `get` and `set` are functions, allowing you to perform validation and transformation:
```svelte
<input bind:value={
() => value,
(v) => value = v.toLowerCase()}
/>
```
In the case of readonly bindings like [dimension bindings](#Dimensions), the `get` value should be `null`:
```svelte
<div
bind:clientWidth={null, redraw}
bind:clientHeight={null, redraw}
>...</div>
```
> [!NOTE]
> Function bindings are available in Svelte 5.9.0 and newer.
## `<input bind:value>`
A `bind:value` directive on an `<input>` element binds the input's `value` property:
<!-- prettier-ignore -->
```svelte
<script>
let message = $state('hello');
</script>
<input bind:value={message} />
<p>{message}</p>
```
In the case of a numeric input (`type="number"` or `type="range"`), the value will be coerced to a number ([demo](/playground/untitled#H4sIAAAAAAAAE6WPwYoCMQxAfyWEPeyiOOqx2w74Hds9pBql0IllmhGXYf5dKqwiyILsLXnwwsuI-5i4oPkaUX8yo7kCnKNQV7dNzoty4qSVBSr8jG-Poixa0KAt2z5mbb14TaxA4OCtKCm_rz4-f2m403WltrlrYhMFTtcLNkoeFGqZ8yhDF7j3CCHKzpwoDexGmqCL4jwuPUJHZ-dxVcfmyYGe5MAv-La5pbxYFf5Z9Zf_UJXb-sEMquFgJJhBmGyTW5yj8lnRaD_w9D1dAKSSj7zqAQAA)):
```svelte
<script>
let a = $state(1);
let b = $state(2);
</script>
<label>
<input type="number" bind:value={a} min="0" max="10" />
<input type="range" bind:value={a} min="0" max="10" />
</label>
<label>
<input type="number" bind:value={b} min="0" max="10" />
<input type="range" bind:value={b} min="0" max="10" />
</label>
<p>{a} + {b} = {a + b}</p>
```
If the input is empty or invalid (in the case of `type="number"`), the value is `undefined`.
Since 5.6.0, if an `<input>` has a `defaultValue` and is part of a form, it will revert to that value instead of the empty string when the form is reset. Note that for the initial render the value of the binding takes precedence unless it is `null` or `undefined`.
```svelte
<script>
let value = $state('');
</script>
<form>
<input bind:value defaultValue="not the empty string">
<input type="reset" value="Reset">
</form>
```
> [!NOTE]
> Use reset buttons sparingly, and ensure that users won't accidentally click them while trying to submit the form.
## `<input bind:checked>`
Checkbox and radio inputs can be bound with `bind:checked`:
```svelte
<label>
<input type="checkbox" bind:checked={accepted} />
Accept terms and conditions
</label>
```
Since 5.6.0, if an `<input>` has a `defaultChecked` attribute and is part of a form, it will revert to that value instead of `false` when the form is reset. Note that for the initial render the value of the binding takes precedence unless it is `null` or `undefined`.
```svelte
<script>
let checked = $state(true);
</script>
<form>
<input type="checkbox" bind:checked defaultChecked={true}>
<input type="reset" value="Reset">
</form>
```
## `<input bind:group>`
Inputs that work together can use `bind:group`.
```svelte
<script>
let tortilla = $state('Plain');
/** @type {Array<string>} */
let fillings = $state([]);
</script>
<!-- grouped radio inputs are mutually exclusive -->
<input type="radio" bind:group={tortilla} value="Plain" />
<input type="radio" bind:group={tortilla} value="Whole wheat" />
<input type="radio" bind:group={tortilla} value="Spinach" />
<!-- grouped checkbox inputs populate an array -->
<input type="checkbox" bind:group={fillings} value="Rice" />
<input type="checkbox" bind:group={fillings} value="Beans" />
<input type="checkbox" bind:group={fillings} value="Cheese" />
<input type="checkbox" bind:group={fillings} value="Guac (extra)" />
```
> [!NOTE] `bind:group` only works if the inputs are in the same Svelte component.
## `<input bind:files>`
On `<input>` elements with `type="file"`, you can use `bind:files` to get the [`FileList` of selected files](https://developer.mozilla.org/en-US/docs/Web/API/FileList). When you want to update the files programmatically, you always need to use a `FileList` object. Currently `FileList` objects cannot be constructed directly, so you need to create a new [`DataTransfer`](https://developer.mozilla.org/en-US/docs/Web/API/DataTransfer) object and get `files` from there.
```svelte
<script>
let files = $state();
function clear() {
files = new DataTransfer().files; // null or undefined does not work
}
</script>
<label for="avatar">Upload a picture:</label>
<input accept="image/png, image/jpeg" bind:files id="avatar" name="avatar" type="file" />
<button onclick={clear}>clear</button>
```
`FileList` objects also cannot be modified, so if you want to e.g. delete a single file from the list, you need to create a new `DataTransfer` object and add the files you want to keep.
> [!NOTE] `DataTransfer` may not be available in server-side JS runtimes. Leaving the state that is bound to `files` uninitialized prevents potential errors if components are server-side rendered.
## `<select bind:value>`
A `<select>` value binding corresponds to the `value` property on the selected `<option>`, which can be any value (not just strings, as is normally the case in the DOM).
```svelte
<select bind:value={selected}>
<option value={a}>a</option>
<option value={b}>b</option>
<option value={c}>c</option>
</select>
```
A `<select multiple>` element behaves similarly to a checkbox group. The bound variable is an array with an entry corresponding to the `value` property of each selected `<option>`.
```svelte
<select multiple bind:value={fillings}>
<option value="Rice">Rice</option>
<option value="Beans">Beans</option>
<option value="Cheese">Cheese</option>
<option value="Guac (extra)">Guac (extra)</option>
</select>
```
When the value of an `<option>` matches its text content, the attribute can be omitted.
```svelte
<select multiple bind:value={fillings}>
<option>Rice</option>
<option>Beans</option>
<option>Cheese</option>
<option>Guac (extra)</option>
</select>
```
You can give the `<select>` a default value by adding a `selected` attribute to the`<option>` (or options, in the case of `<select multiple>`) that should be initially selected. If the `<select>` is part of a form, it will revert to that selection when the form is reset. Note that for the initial render the value of the binding takes precedence if it's not `undefined`.
```svelte
<select bind:value={selected}>
<option value={a}>a</option>
<option value={b} selected>b</option>
<option value={c}>c</option>
</select>
```
## `<audio>`
`<audio>` elements have their own set of bindings — five two-way ones...
- [`currentTime`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/currentTime)
- [`playbackRate`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/playbackRate)
- [`paused`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/paused)
- [`volume`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/volume)
- [`muted`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/muted)
...and seven readonly ones:
- [`duration`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/duration)
- [`buffered`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/buffered)
- [`paused`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/paused)
- [`seekable`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/seekable)
- [`seeking`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/seeking_event)
- [`ended`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/ended)
- [`readyState`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/readyState)
```svelte
<audio src={clip} bind:duration bind:currentTime bind:paused></audio>
```
## `<video>`
`<video>` elements have all the same bindings as [`<audio>`](#audio) elements, plus readonly [`videoWidth`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLVideoElement/videoWidth) and [`videoHeight`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLVideoElement/videoHeight) bindings.
## `<img>`
`<img>` elements have two readonly bindings:
- [`naturalWidth`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/naturalWidth)
- [`naturalHeight`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLImageElement/naturalHeight)
## `<details bind:open>`
`<details>` elements support binding to the `open` property.
```svelte
<details bind:open={isOpen}>
<summary>How do you comfort a JavaScript bug?</summary>
<p>You console it.</p>
</details>
```
## Contenteditable bindings
Elements with the `contenteditable` attribute support the following bindings:
- [`innerHTML`](https://developer.mozilla.org/en-US/docs/Web/API/Element/innerHTML)
- [`innerText`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/innerText)
- [`textContent`](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent)
> [!NOTE] There are [subtle differences between `innerText` and `textContent`](https://developer.mozilla.org/en-US/docs/Web/API/Node/textContent#differences_from_innertext).
<!-- for some reason puts the comment and html on same line -->
<!-- prettier-ignore -->
```svelte
<div contenteditable="true" bind:innerHTML={html} />
```
## Dimensions
All visible elements have the following readonly bindings, measured with a `ResizeObserver`:
- [`clientWidth`](https://developer.mozilla.org/en-US/docs/Web/API/Element/clientWidth)
- [`clientHeight`](https://developer.mozilla.org/en-US/docs/Web/API/Element/clientHeight)
- [`offsetWidth`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/offsetWidth)
- [`offsetHeight`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLElement/offsetHeight)
```svelte
<div bind:offsetWidth={width} bind:offsetHeight={height}>
<Chart {width} {height} />
</div>
```
> [!NOTE] `display: inline` elements do not have a width or height (except for elements with 'intrinsic' dimensions, like `<img>` and `<canvas>`), and cannot be observed with a `ResizeObserver`. You will need to change the `display` style of these elements to something else, such as `inline-block`.
## bind:this
```svelte
<!--- copy: false --->
bind:this={dom_node}
```
To get a reference to a DOM node, use `bind:this`. The value will be `undefined` until the component is mounted — in other words, you should read it inside an effect or an event handler, but not during component initialisation:
```svelte
<script>
/** @type {HTMLCanvasElement} */
let canvas;
$effect(() => {
const ctx = canvas.getContext('2d');
drawStuff(ctx);
});
</script>
<canvas bind:this={canvas} />
```
Components also support `bind:this`, allowing you to interact with component instances programmatically.
```svelte
<!--- file: App.svelte --->
<ShoppingCart bind:this={cart} />
<button onclick={() => cart.empty()}> Empty shopping cart </button>
```
```svelte
<!--- file: ShoppingCart.svelte --->
<script>
// All instance exports are available on the instance object
export function empty() {
// ...
}
</script>
```
## bind:_property_ for components
```svelte
bind:property={variable}
```
You can bind to component props using the same syntax as for elements.
```svelte
<Keypad bind:value={pin} />
```
While Svelte props are reactive without binding, that reactivity only flows downward into the component by default. Using `bind:property` allows changes to the property from within the component to flow back up out of the component.
To mark a property as bindable, use the [`$bindable`]($bindable) rune:
```svelte
<script>
let { readonlyProperty, bindableProperty = $bindable() } = $props();
</script>
```
Declaring a property as bindable means it _can_ be used using `bind:`, not that it _must_ be used using `bind:`.
Bindable properties can have a fallback value:
```svelte
<script>
let { bindableProperty = $bindable('fallback value') } = $props();
</script>
```
This fallback value _only_ applies when the property is _not_ bound. When the property is bound and a fallback value is present, the parent is expected to provide a value other than `undefined`, else a runtime error is thrown. This prevents hard-to-reason-about situations where it's unclear which value should apply.
# use:
Actions are functions that are called when an element is mounted. They are added with the `use:` directive, and will typically use an `$effect` so that they can reset any state when the element is unmounted:
```svelte
<!--- file: App.svelte --->
<script>
/** @type {import('svelte/action').Action} */
function myaction(node) {
// the node has been mounted in the DOM
$effect(() => {
// setup goes here
return () => {
// teardown goes here
};
});
}
</script>
<div use:myaction>...</div>
```
An action can be called with an argument:
```svelte
<!--- file: App.svelte --->
<script>
/** @type {import('svelte/action').Action} */
function myaction(node, +++data+++) {
// ...
}
</script>
<div use:myaction={+++data+++}>...</div>
```
The action is only called once (but not during server-side rendering) — it will _not_ run again if the argument changes.
> [!LEGACY]
> Prior to the `$effect` rune, actions could return an object with `update` and `destroy` methods, where `update` would be called with the latest value of the argument if it changed. Using effects is preferred.
## Typing
The `Action` interface receives three optional type arguments — a node type (which can be `Element`, if the action applies to everything), a parameter, and any custom event handlers created by the action:
```svelte
<!--- file: App.svelte --->
<script>
/**
* @type {import('svelte/action').Action<
* HTMLDivElement,
* undefined,
* {
* onswiperight: (e: CustomEvent) => void;
* onswipeleft: (e: CustomEvent) => void;
* // ...
* }
* >}
*/
function gestures(node) {
$effect(() => {
// ...
node.dispatchEvent(new CustomEvent('swipeleft'));
// ...
node.dispatchEvent(new CustomEvent('swiperight'));
});
}
</script>
<div
use:gestures
onswipeleft={next}
onswiperight={prev}
>...</div>
```
# transition:
A _transition_ is triggered by an element entering or leaving the DOM as a result of a state change.
When a block (such as an `{#if ...}` block) is transitioning out, all elements inside it, including those that do not have their own transitions, are kept in the DOM until every transition in the block has been completed.
The `transition:` directive indicates a _bidirectional_ transition, which means it can be smoothly reversed while the transition is in progress.
```svelte
<script>
+++import { fade } from 'svelte/transition';+++
let visible = $state(false);
</script>
<button onclick={() => visible = !visible}>toggle</button>
{#if visible}
<div +++transition:fade+++>fades in and out</div>
{/if}
```
## Built-in transitions
A selection of built-in transitions can be imported from the [`svelte/transition`](svelte-transition) module.
## Local vs global
Transitions are local by default. Local transitions only play when the block they belong to is created or destroyed, _not_ when parent blocks are created or destroyed.
```svelte
{#if x}
{#if y}
<p transition:fade>fades in and out only when y changes</p>
<p transition:fade|global>fades in and out when x or y change</p>
{/if}
{/if}
```
## Transition parameters
Transitions can have parameters.
(The double `{{curlies}}` aren't a special syntax; this is an object literal inside an expression tag.)
```svelte
{#if visible}
<div transition:fade={{ duration: 2000 }}>fades in and out over two seconds</div>
{/if}
```
## Custom transition functions
```js
/// copy: false
// @noErrors
transition = (node: HTMLElement, params: any, options: { direction: 'in' | 'out' | 'both' }) => {
delay?: number,
duration?: number,
easing?: (t: number) => number,
css?: (t: number, u: number) => string,
tick?: (t: number, u: number) => void
}
```
Transitions can use custom functions. If the returned object has a `css` function, Svelte will generate keyframes for a [web animation](https://developer.mozilla.org/en-US/docs/Web/API/Web_Animations_API).
The `t` argument passed to `css` is a value between `0` and `1` after the `easing` function has been applied. _In_ transitions run from `0` to `1`, _out_ transitions run from `1` to `0` — in other words, `1` is the element's natural state, as though no transition had been applied. The `u` argument is equal to `1 - t`.
The function is called repeatedly _before_ the transition begins, with different `t` and `u` arguments.
```svelte
<!--- file: App.svelte --->
<script>
import { elasticOut } from 'svelte/easing';
/** @type {boolean} */
export let visible;
/**
* @param {HTMLElement} node
* @param {{ delay?: number, duration?: number, easing?: (t: number) => number }} params
*/
function whoosh(node, params) {
const existingTransform = getComputedStyle(node).transform.replace('none', '');
return {
delay: params.delay || 0,
duration: params.duration || 400,
easing: params.easing || elasticOut,
css: (t, u) => `transform: ${existingTransform} scale(${t})`
};
}
</script>
{#if visible}
<div in:whoosh>whooshes in</div>
{/if}
```
A custom transition function can also return a `tick` function, which is called _during_ the transition with the same `t` and `u` arguments.
> [!NOTE] If it's possible to use `css` instead of `tick`, do so — web animations can run off the main thread, preventing jank on slower devices.
```svelte
<!--- file: App.svelte --->
<script>
export let visible = false;
/**
* @param {HTMLElement} node
* @param {{ speed?: number }} params
*/
function typewriter(node, { speed = 1 }) {
const valid = node.childNodes.length === 1 && node.childNodes[0].nodeType === Node.TEXT_NODE;
if (!valid) {
throw new Error(`This transition only works on elements with a single text node child`);
}
const text = node.textContent;
const duration = text.length / (speed * 0.01);
return {
duration,
tick: (t) => {
const i = ~~(text.length * t);
node.textContent = text.slice(0, i);
}
};
}
</script>
{#if visible}
<p in:typewriter={{ speed: 1 }}>The quick brown fox jumps over the lazy dog</p>
{/if}
```
If a transition returns a function instead of a transition object, the function will be called in the next microtask. This allows multiple transitions to coordinate, making [crossfade effects](/tutorial/deferred-transitions) possible.
Transition functions also receive a third argument, `options`, which contains information about the transition.
Available values in the `options` object are:
- `direction` - one of `in`, `out`, or `both` depending on the type of transition
## Transition events
An element with transitions will dispatch the following events in addition to any standard DOM events:
- `introstart`
- `introend`
- `outrostart`
- `outroend`
```svelte
{#if visible}
<p
transition:fly={{ y: 200, duration: 2000 }}
onintrostart={() => (status = 'intro started')}
onoutrostart={() => (status = 'outro started')}
onintroend={() => (status = 'intro ended')}
onoutroend={() => (status = 'outro ended')}
>
Flies in and out
</p>
{/if}
```
# in: and out:
The `in:` and `out:` directives are identical to [`transition:`](transition), except that the resulting transitions are not bidirectional — an `in` transition will continue to 'play' alongside the `out` transition, rather than reversing, if the block is outroed while the transition is in progress. If an out transition is aborted, transitions will restart from scratch.
```svelte
<script>
import { fade, fly } from 'svelte/transition';
let visible = $state(false);
</script>
<label>
<input type="checkbox" bind:checked={visible}>
visible
</label>
{#if visible}
<div in:fly={{ y: 200 }} out:fade>flies in, fades out</div>
{/if}
```
# animate:
An animation is triggered when the contents of a [keyed each block](each#Keyed-each-blocks) are re-ordered. Animations do not run when an element is added or removed, only when the index of an existing data item within the each block changes. Animate directives must be on an element that is an _immediate_ child of a keyed each block.
Animations can be used with Svelte's [built-in animation functions](svelte-animate) or [custom animation functions](#Custom-animation-functions).
```svelte
<!-- When `list` is reordered the animation will run -->
{#each list as item, index (item)}
<li animate:flip>{item}</li>
{/each}
```
## Animation Parameters
As with actions and transitions, animations can have parameters.
(The double `{{curlies}}` aren't a special syntax; this is an object literal inside an expression tag.)
```svelte
{#each list as item, index (item)}
<li animate:flip={{ delay: 500 }}>{item}</li>
{/each}
```
## Custom animation functions
```js
/// copy: false
// @noErrors
animation = (node: HTMLElement, { from: DOMRect, to: DOMRect } , params: any) => {
delay?: number,
duration?: number,
easing?: (t: number) => number,
css?: (t: number, u: number) => string,
tick?: (t: number, u: number) => void
}
```
Animations can use custom functions that provide the `node`, an `animation` object and any `parameters` as arguments. The `animation` parameter is an object containing `from` and `to` properties each containing a [DOMRect](https://developer.mozilla.org/en-US/docs/Web/API/DOMRect#Properties) describing the geometry of the element in its `start` and `end` positions. The `from` property is the DOMRect of the element in its starting position, and the `to` property is the DOMRect of the element in its final position after the list has been reordered and the DOM updated.
If the returned object has a `css` method, Svelte will create a [web animation](https://developer.mozilla.org/en-US/docs/Web/API/Web_Animations_API) that plays on the element.
The `t` argument passed to `css` is a value that goes from `0` and `1` after the `easing` function has been applied. The `u` argument is equal to `1 - t`.
The function is called repeatedly _before_ the animation begins, with different `t` and `u` arguments.
<!-- TODO: Types -->
```svelte
<!--- file: App.svelte --->
<script>
import { cubicOut } from 'svelte/easing';
/**
* @param {HTMLElement} node
* @param {{ from: DOMRect; to: DOMRect }} states
* @param {any} params
*/
function whizz(node, { from, to }, params) {
const dx = from.left - to.left;
const dy = from.top - to.top;
const d = Math.sqrt(dx * dx + dy * dy);
return {
delay: 0,
duration: Math.sqrt(d) * 120,
easing: cubicOut,
css: (t, u) => `transform: translate(${u * dx}px, ${u * dy}px) rotate(${t * 360}deg);`
};
}
</script>
{#each list as item, index (item)}
<div animate:whizz>{item}</div>
{/each}
```
A custom animation function can also return a `tick` function, which is called _during_ the animation with the same `t` and `u` arguments.
> [!NOTE] If it's possible to use `css` instead of `tick`, do so — web animations can run off the main thread, preventing jank on slower devices.
```svelte
<!--- file: App.svelte --->
<script>
import { cubicOut } from 'svelte/easing';
/**
* @param {HTMLElement} node
* @param {{ from: DOMRect; to: DOMRect }} states
* @param {any} params
*/
function whizz(node, { from, to }, params) {
const dx = from.left - to.left;
const dy = from.top - to.top;
const d = Math.sqrt(dx * dx + dy * dy);
return {
delay: 0,
duration: Math.sqrt(d) * 120,
easing: cubicOut,
tick: (t, u) => Object.assign(node.style, { color: t > 0.5 ? 'Pink' : 'Blue' })
};
}
</script>
{#each list as item, index (item)}
<div animate:whizz>{item}</div>
{/each}
```
# style:
The `style:` directive provides a shorthand for setting multiple styles on an element.
```svelte
<!-- These are equivalent -->
<div style:color="red">...</div>
<div style="color: red;">...</div>
```
The value can contain arbitrary expressions:
```svelte
<div style:color={myColor}>...</div>
```
The shorthand form is allowed:
```svelte
<div style:color>...</div>
```
Multiple styles can be set on a single element:
```svelte
<div style:color style:width="12rem" style:background-color={darkMode ? 'black' : 'white'}>...</div>
```
To mark a style as important, use the `|important` modifier:
```svelte
<div style:color|important="red">...</div>
```
When `style:` directives are combined with `style` attributes, the directives will take precedence:
```svelte
<div style="color: blue;" style:color="red">This will be red</div>
```
# class
There are two ways to set classes on elements: the `class` attribute, and the `class:` directive.
## Attributes
Primitive values are treated like any other attribute:
```svelte
<div class={large ? 'large' : 'small'}>...</div>
```
> [!NOTE]
> For historical reasons, falsy values (like `false` and `NaN`) are stringified (`class="false"`), though `class={undefined}` (or `null`) cause the attribute to be omitted altogether. In a future version of Svelte, all falsy values will cause `class` to be omitted.
### Objects and arrays
Since Svelte 5.16, `class` can be an object or array, and is converted to a string using [clsx](https://github.com/lukeed/clsx).
If the value is an object, the truthy keys are added:
```svelte
<script>
let { cool } = $props();
</script>
<!-- results in `class="cool"` if `cool` is truthy,
`class="lame"` otherwise -->
<div class={{ cool, lame: !cool }}>...</div>
```
If the value is an array, the truthy values are combined:
```svelte
<!-- if `faded` and `large` are both truthy, results in
`class="saturate-0 opacity-50 scale-200"` -->
<div class={[faded && 'saturate-0 opacity-50', large && 'scale-200']}>...</div>
```
Note that whether we're using the array or object form, we can set multiple classes simultaneously with a single condition, which is particularly useful if you're using things like Tailwind.
Arrays can contain arrays and objects, and clsx will flatten them. This is useful for combining local classes with props, for example:
```svelte
<!--- file: Button.svelte --->
<script>
let props = $props();
</script>
<button {...props} class={['cool-button', props.class]}>
{@render props.children?.()}
</button>
```
The user of this component has the same flexibility to use a mixture of objects, arrays and strings:
```svelte
<!--- file: App.svelte --->
<script>
import Button from './Button.svelte';
let useTailwind = $state(false);
</script>
<Button
onclick={() => useTailwind = true}
class={{ 'bg-blue-700 sm:w-1/2': useTailwind }}
>
Accept the inevitability of Tailwind
</Button>
```
Svelte also exposes the `ClassValue` type, which is the type of value that the `class` attribute on elements accept. This is useful if you want to use a type-safe class name in component props:
```svelte
<script lang="ts">
import type { ClassValue } from 'svelte/elements';
const props: { class: ClassValue } = $props();
</script>
<div class={['original', props.class]}>...</div>
```
## The `class:` directive
Prior to Svelte 5.16, the `class:` directive was the most convenient way to set classes on elements conditionally.
```svelte
<!-- These are equivalent -->
<div class={{ cool, lame: !cool }}>...</div>
<div class:cool={cool} class:lame={!cool}>...</div>
```
As with other directives, we can use a shorthand when the name of the class coincides with the value:
```svelte
<div class:cool class:lame={!cool}>...</div>
```
> [!NOTE] Unless you're using an older version of Svelte, consider avoiding `class:`, since the attribute is more powerful and composable.
# Control flow
- if
- each
- await (or move that into some kind of data loading section?)
- NOT: key (move into transition section, because that's the common use case)
Svelte augments HTML with control flow blocks to be able to express conditionally rendered content or lists.
The syntax between these blocks is the same:
- `{#` denotes the start of a block
- `{:` denotes a different branch part of the block. Depending on the block, there can be multiple of these
- `{/` denotes the end of a block
## {#if ...}
## {#each ...}
```svelte
<!--- copy: false --->
{#each expression as name}...{/each}
```
```svelte
<!--- copy: false --->
{#each expression as name, index}...{/each}
```
```svelte
<!--- copy: false --->
{#each expression as name (key)}...{/each}
```
```svelte
<!--- copy: false --->
{#each expression as name, index (key)}...{/each}
```
```svelte
<!--- copy: false --->
{#each expression as name}...{:else}...{/each}
```
Iterating over lists of values can be done with an each block.
```svelte
<h1>Shopping list</h1>
<ul>
{#each items as item}
<li>{item.name} x {item.qty}</li>
{/each}
</ul>
```
You can use each blocks to iterate over any array or array-like value — that is, any object with a `length` property.
An each block can also specify an _index_, equivalent to the second argument in an `array.map(...)` callback:
```svelte
{#each items as item, i}
<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}
```
If a _key_ expression is provided — which must uniquely identify each list item — Svelte will use it to diff the list when data changes, rather than adding or removing items at the end. The key can be any object, but strings and numbers are recommended since they allow identity to persist when the objects themselves change.
```svelte
{#each items as item (item.id)}
<li>{item.name} x {item.qty}</li>
{/each}
<!-- or with additional index value -->
{#each items as item, i (item.id)}
<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}
```
You can freely use destructuring and rest patterns in each blocks.
```svelte
{#each items as { id, name, qty }, i (id)}
<li>{i + 1}: {name} x {qty}</li>
{/each}
{#each objects as { id, ...rest }}
<li><span>{id}</span><MyComponent {...rest} /></li>
{/each}
{#each items as [id, ...rest]}
<li><span>{id}</span><MyComponent values={rest} /></li>
{/each}
```
An each block can also have an `{:else}` clause, which is rendered if the list is empty.
```svelte
{#each todos as todo}
<p>{todo.text}</p>
{:else}
<p>No tasks today!</p>
{/each}
```
It is possible to iterate over iterables like `Map` or `Set`. Iterables need to be finite and static (they shouldn't change while being iterated over). Under the hood, they are transformed to an array using `Array.from` before being passed off to rendering. If you're writing performance-sensitive code, try to avoid iterables and use regular arrays as they are more performant.
## Other block types
Svelte also provides [`#snippet`](snippets), [`#key`](transitions-and-animations) and [`#await`](data-fetching) blocks. You can find out more about them in their respective sections.
# Data fetching
Fetching data is a fundamental part of apps interacting with the outside world. Svelte is unopinionated with how you fetch your data. The simplest way would be using the built-in `fetch` method:
```svelte
<script>
let response = $state();
fetch('/api/data').then(async (r) => (response = r.json()));
</script>
```
While this works, it makes working with promises somewhat unergonomic. Svelte alleviates this problem using the `#await` block.
## {#await ...}
## SvelteKit loaders
Fetching inside your components is great for simple use cases, but it's prone to data loading waterfalls and makes code harder to work with because of the promise handling. SvelteKit solves this problem by providing a opinionated data loading story that is coupled to its router. Learn more about it [in the docs](../kit).
# Scoped styles
Svelte components can include a `<style>` element containing CSS that belongs to the component. This CSS is _scoped_ by default, meaning that styles will not apply to any elements on the page outside the component in question.
This works by adding a class to affected elements, which is based on a hash of the component styles (e.g. `svelte-123xyz`).
```svelte
<style>
p {
/* this will only affect <p> elements in this component */
color: burlywood;
}
</style>
```
## Specificity
Each scoped selector receives a [specificity](https://developer.mozilla.org/en-US/docs/Web/CSS/Specificity) increase of 0-1-0, as a result of the scoping class (e.g. `.svelte-123xyz`) being added to the selector. This means that (for example) a `p` selector defined in a component will take precedence over a `p` selector defined in a global stylesheet, even if the global stylesheet is loaded later.
In some cases, the scoping class must be added to a selector multiple times, but after the first occurrence it is added with `:where(.svelte-xyz123)` in order to not increase specificity further.
## Scoped keyframes
If a component defines `@keyframes`, the name is scoped to the component using the same hashing approach. Any `animation` rules in the component will be similarly adjusted:
```svelte
<style>
.bouncy {
animation: bounce 10s;
}
/* these keyframes are only accessible inside this component */
@keyframes bounce {
/* ... */
}
</style>
```
# Global styles
## :global(...)
To apply styles to a single selector globally, use the `:global(...)` modifier:
```svelte
<style>
:global(body) {
/* applies to <body> */
margin: 0;
}
div :global(strong) {
/* applies to all <strong> elements, in any component,
that are inside <div> elements belonging
to this component */
color: goldenrod;
}
p:global(.big.red) {
/* applies to all <p> elements belonging to this component
with `class="big red"`, even if it is applied
programmatically (for example by a library) */
}
</style>
```
If you want to make @keyframes that are accessible globally, you need to prepend your keyframe names with `-global-`.
The `-global-` part will be removed when compiled, and the keyframe will then be referenced using just `my-animation-name` elsewhere in your code.
```svelte
<style>
@keyframes -global-my-animation-name {
/* code goes here */
}
</style>
```
## :global
To apply styles to a group of selectors globally, create a `:global {...}` block:
```svelte
<style>
:global {
/* applies to every <div> in your application */
div { ... }
/* applies to every <p> in your application */
p { ... }
}
.a :global {
/* applies to every `.b .c .d` element, in any component,
that is inside an `.a` element in this component */
.b .c .d {...}
}
</style>
```
> [!NOTE] The second example above could also be written as an equivalent `.a :global .b .c .d` selector, where everything after the `:global` is unscoped, though the nested form is preferred.
# Custom properties
You can pass CSS custom properties — both static and dynamic — to components:
```svelte
<Slider
bind:value
min={0}
max={100}
--track-color="black"
--thumb-color="rgb({r} {g} {b})"
/>
```
The above code essentially desugars to this:
```svelte
<svelte-css-wrapper style="display: contents; --track-color: black; --thumb-color: rgb({r} {g} {b})">
<Slider
bind:value
min={0}
max={100}
/>
</svelte-css-wrapper>
```
For an SVG element, it would use `<g>` instead:
```svelte
<g style="--track-color: black; --thumb-color: rgb({r} {g} {b})">
<Slider
bind:value
min={0}
max={100}
/>
</g>
```
Inside the component, we can read these custom properties (and provide fallback values) using [`var(...)`](https://developer.mozilla.org/en-US/docs/Web/CSS/Using_CSS_custom_properties):
```svelte
<style>
.track {
background: var(--track-color, #aaa);
}
.thumb {
background: var(--thumb-color, blue);
}
</style>
```
You don't _have_ to specify the values directly on the component; as long as the custom properties are defined on a parent element, the component can use them. It's common to define custom properties on the `:root` element in a global stylesheet so that they apply to your entire application.
> [!NOTE] While the extra element will not affect layout, it _will_ affect any CSS selectors that (for example) use the `>` combinator to target an element directly inside the component's container.
# Nested <style> elements
There can only be one top-level `<style>` tag per component.
However, it is possible to have a `<style>` tag nested inside other elements or logic blocks.
In that case, the `<style>` tag will be inserted as-is into the DOM; no scoping or processing will be done on the `<style>` tag.
```svelte
<div>
<style>
/* this style tag will be inserted as-is */
div {
/* this will apply to all `<div>` elements in the DOM */
color: red;
}
</style>
</div>
```
# <svelte:boundary>
```svelte
<svelte:boundary onerror={handler}>...</svelte:boundary>
```
> [!NOTE]
> This feature was added in 5.3.0
Boundaries allow you to guard against errors in part of your app from breaking the app as a whole, and to recover from those errors.
If an error occurs while rendering or updating the children of a `<svelte:boundary>`, or running any [`$effect`]($effect) functions contained therein, the contents will be removed.
Errors occurring outside the rendering process (for example, in event handlers or after a `setTimeout` or async work) are _not_ caught by error boundaries.
## Properties
For the boundary to do anything, one or both of `failed` and `onerror` must be provided.
### `failed`
If a `failed` snippet is provided, it will be rendered with the error that was thrown, and a `reset` function that recreates the contents ([demo](/playground/hello-world#H4sIAAAAAAAAE3VRy26DMBD8lS2tFCIh6JkAUlWp39Cq9EBg06CAbdlLArL87zWGKk8ORnhmd3ZnrD1WtOjFXqKO2BDGW96xqpBD5gXerm5QefG39mgQY9EIWHxueRMinLosti0UPsJLzggZKTeilLWgLGc51a3gkuCjKQ7DO7cXZotgJ3kLqzC6hmex1SZnSXTWYHcrj8LJjWTk0PHoZ8VqIdCOKayPykcpuQxAokJaG1dGybYj4gw4K5u6PKTasSbjXKgnIDlA8VvUdo-pzonraBY2bsH7HAl78mKSHZpgIcuHjq9jXSpZSLixRlveKYQUXhQVhL6GPobXAAb7BbNeyvNUs4qfRg3OnELLj5hqH9eQZqCnoBwR9lYcQxuVXeBzc8kMF8yXY4yNJ5oGiUzP_aaf_waTRGJib5_Ad3P_vbCuaYxzeNpbU0eUMPAOKh7Yw1YErgtoXyuYlPLzc10_xo_5A91zkQL_AgAA)):
```svelte
<svelte:boundary>
<FlakyComponent />
{#snippet failed(error, reset)}
<button onclick={reset}>oops! try again</button>
{/snippet}
</svelte:boundary>
```
> [!NOTE]
> As with [snippets passed to components](snippet#Passing-snippets-to-components), the `failed` snippet can be passed explicitly as a property...
>
> ```svelte
> <svelte:boundary {failed}>...</svelte:boundary>
> ```
>
> ...or implicitly by declaring it directly inside the boundary, as in the example above.
### `onerror`
If an `onerror` function is provided, it will be called with the same two `error` and `reset` arguments. This is useful for tracking the error with an error reporting service...
```svelte
<svelte:boundary onerror={(e) => report(e)}>
...
</svelte:boundary>
```
...or using `error` and `reset` outside the boundary itself:
```svelte
<script>
let error = $state(null);
let reset = $state(() => {});
function onerror(e, r) {
error = e;
reset = r;
}
</script>
<svelte:boundary {onerror}>
<FlakyComponent />
</svelte:boundary>
{#if error}
<button onclick={() => {
error = null;
reset();
}}>
oops! try again
</button>
{/if}
```
If an error occurs inside the `onerror` function (or if you rethrow the error), it will be handled by a parent boundary if such exists.
# <svelte:window>
```svelte
<svelte:window onevent={handler} />
```
```svelte
<svelte:window bind:prop={value} />
```
The `<svelte:window>` element allows you to add event listeners to the `window` object without worrying about removing them when the component is destroyed, or checking for the existence of `window` when server-side rendering.
This element may only appear at the top level of your component — it cannot be inside a block or element.
```svelte
<script>
function handleKeydown(event) {
alert(`pressed the ${event.key} key`);
}
</script>
<svelte:window onkeydown={handleKeydown} />
```
You can also bind to the following properties:
- `innerWidth`
- `innerHeight`
- `outerWidth`
- `outerHeight`
- `scrollX`
- `scrollY`
- `online` — an alias for `window.navigator.onLine`
- `devicePixelRatio`
All except `scrollX` and `scrollY` are readonly.
```svelte
<svelte:window bind:scrollY={y} />
```
> [!NOTE] Note that the page will not be scrolled to the initial value to avoid accessibility issues. Only subsequent changes to the bound variable of `scrollX` and `scrollY` will cause scrolling. If you have a legitimate reason to scroll when the component is rendered, call `scrollTo()` in an `$effect`.
# <svelte:document>
```svelte
<svelte:document onevent={handler} />
```
```svelte
<svelte:document bind:prop={value} />
```
Similarly to `<svelte:window>`, this element allows you to add listeners to events on `document`, such as `visibilitychange`, which don't fire on `window`. It also lets you use [actions](use) on `document`.
As with `<svelte:window>`, this element may only appear the top level of your component and must never be inside a block or element.
```svelte
<svelte:document onvisibilitychange={handleVisibilityChange} use:someAction />
```
You can also bind to the following properties:
- `activeElement`
- `fullscreenElement`
- `pointerLockElement`
- `visibilityState`
All are readonly.
# <svelte:body>
```svelte
<svelte:body onevent={handler} />
```
Similarly to `<svelte:window>`, this element allows you to add listeners to events on `document.body`, such as `mouseenter` and `mouseleave`, which don't fire on `window`. It also lets you use [actions](use) on the `<body>` element.
As with `<svelte:window>` and `<svelte:document>`, this element may only appear the top level of your component and must never be inside a block or element.
```svelte
<svelte:body onmouseenter={handleMouseenter} onmouseleave={handleMouseleave} use:someAction />
```
# <svelte:head>
```svelte
<svelte:head>...</svelte:head>
```
This element makes it possible to insert elements into `document.head`. During server-side rendering, `head` content is exposed separately to the main `body` content.
As with `<svelte:window>`, `<svelte:document>` and `<svelte:body>`, this element may only appear at the top level of your component and must never be inside a block or element.
```svelte
<svelte:head>
<title>Hello world!</title>
<meta name="description" content="This is where the description goes for SEO" />
</svelte:head>
```
# <svelte:element>
```svelte
<svelte:element this={expression} />
```
The `<svelte:element>` element lets you render an element that is unknown at author time, for example because it comes from a CMS. Any properties and event listeners present will be applied to the element.
The only supported binding is `bind:this`, since Svelte's built-in bindings do not work with generic elements.
If `this` has a nullish value, the element and its children will not be rendered.
If `this` is the name of a [void element](https://developer.mozilla.org/en-US/docs/Glossary/Void_element) (e.g., `br`) and `<svelte:element>` has child elements, a runtime error will be thrown in development mode:
```svelte
<script>
let tag = $state('hr');
</script>
<svelte:element this={tag}>
This text cannot appear inside an hr element
</svelte:element>
```
Svelte tries its best to infer the correct namespace from the element's surroundings, but it's not always possible. You can make it explicit with an `xmlns` attribute:
```svelte
<svelte:element this={tag} xmlns="http://www.w3.org/2000/svg" />
```
`this` needs to be a valid DOM element tag, things like `#text` or `svelte:head` will not work.
# <svelte:options>
```svelte
<svelte:options option={value} />
```
The `<svelte:options>` element provides a place to specify per-component compiler options, which are detailed in the [compiler section](svelte-compiler#compile). The possible options are:
- `runes={true}` — forces a component into _runes mode_ (see the [Legacy APIs](legacy-overview) section)
- `runes={false}` — forces a component into _legacy mode_
- `namespace="..."` — the namespace where this component will be used, can be "html" (the default), "svg" or "mathml"
- `customElement={...}` — the [options](custom-elements#Component-options) to use when compiling this component as a custom element. If a string is passed, it is used as the `tag` option
- `css="injected"` — the component will inject its styles inline: During server side rendering, it's injected as a `<style>` tag in the `head`, during client side rendering, it's loaded via JavaScript
> [!LEGACY] Deprecated options
> Svelte 4 also included the following options. They are deprecated in Svelte 5 and non-functional in runes mode.
>
> - `immutable={true}` — you never use mutable data, so the compiler can do simple referential equality checks to determine if values have changed
> - `immutable={false}` — the default. Svelte will be more conservative about whether or not mutable objects have changed
> - `accessors={true}` — adds getters and setters for the component's props
> - `accessors={false}` — the default
```svelte
<svelte:options customElement="my-custom-element" />
```
# Stores
<!-- - how to use
- how to write
- TODO should the details for the store methods belong to the reference section? -->
A _store_ is an object that allows reactive access to a value via a simple _store contract_. The [`svelte/store` module](../svelte-store) contains minimal store implementations which fulfil this contract.
Any time you have a reference to a store, you can access its value inside a component by prefixing it with the `$` character. This causes Svelte to declare the prefixed variable, subscribe to the store at component initialisation and unsubscribe when appropriate.
Assignments to `$`-prefixed variables require that the variable be a writable store, and will result in a call to the store's `.set` method.
Note that the store must be declared at the top level of the component — not inside an `if` block or a function, for example.
Local variables (that do not represent store values) must _not_ have a `$` prefix.
```svelte
<script>
import { writable } from 'svelte/store';
const count = writable(0);
console.log($count); // logs 0
count.set(1);
console.log($count); // logs 1
$count = 2;
console.log($count); // logs 2
</script>
```
## When to use stores
Prior to Svelte 5, stores were the go-to solution for creating cross-component reactive states or extracting logic. With runes, these use cases have greatly diminished.
- when extracting logic, it's better to take advantage of runes' universal reactivity: You can use runes outside the top level of components and even place them into JavaScript or TypeScript files (using a `.svelte.js` or `.svelte.ts` file ending)
- when creating shared state, you can create a `$state` object containing the values you need and then manipulate said state
```ts
/// file: state.svelte.js
export const userState = $state({
name: 'name',
/* ... */
});
```
```svelte
<!--- file: App.svelte --->
<script>
import { userState } from './state.svelte.js';
</script>
<p>User name: {userState.name}</p>
<button onclick={() => {
userState.name = 'new name';
}}>
change name
</button>
```
Stores are still a good solution when you have complex asynchronous data streams or it's important to have more manual control over updating values or listening to changes. If you're familiar with RxJs and want to reuse that knowledge, the `$` also comes in handy for you.
## svelte/store
The `svelte/store` module contains a minimal store implementation which fulfil the store contract. It provides methods for creating stores that you can update from the outside, stores you can only update from the inside, and for combining and deriving stores.
### `writable`
Function that creates a store which has values that can be set from 'outside' components. It gets created as an object with additional `set` and `update` methods.
`set` is a method that takes one argument which is the value to be set. The store value gets set to the value of the argument if the store value is not already equal to it.
`update` is a method that takes one argument which is a callback. The callback takes the existing store value as its argument and returns the new value to be set to the store.
```js
/// file: store.js
import { writable } from 'svelte/store';
const count = writable(0);
count.subscribe((value) => {
console.log(value);
}); // logs '0'
count.set(1); // logs '1'
count.update((n) => n + 1); // logs '2'
```
If a function is passed as the second argument, it will be called when the number of subscribers goes from zero to one (but not from one to two, etc). That function will be passed a `set` function which changes the value of the store, and an `update` function which works like the `update` method on the store, taking a callback to calculate the store's new value from its old value. It must return a `stop` function that is called when the subscriber count goes from one to zero.
```js
/// file: store.js
import { writable } from 'svelte/store';
const count = writable(0, () => {
console.log('got a subscriber');
return () => console.log('no more subscribers');
});
count.set(1); // does nothing
const unsubscribe = count.subscribe((value) => {
console.log(value);
}); // logs 'got a subscriber', then '1'
unsubscribe(); // logs 'no more subscribers'
```
Note that the value of a `writable` is lost when it is destroyed, for example when the page is refreshed. However, you can write your own logic to sync the value to for example the `localStorage`.
### `readable`
Creates a store whose value cannot be set from 'outside', the first argument is the store's initial value, and the second argument to `readable` is the same as the second argument to `writable`.
```ts
import { readable } from 'svelte/store';
const time = readable(new Date(), (set) => {
set(new Date());
const interval = setInterval(() => {
set(new Date());
}, 1000);
return () => clearInterval(interval);
});
const ticktock = readable('tick', (set, update) => {
const interval = setInterval(() => {
update((sound) => (sound === 'tick' ? 'tock' : 'tick'));
}, 1000);
return () => clearInterval(interval);
});
```
### `derived`
Derives a store from one or more other stores. The callback runs initially when the first subscriber subscribes and then whenever the store dependencies change.
In the simplest version, `derived` takes a single store, and the callback returns a derived value.
```ts
// @filename: ambient.d.ts
import { type Writable } from 'svelte/store';
declare global {
const a: Writable<number>;
}
export {};
// @filename: index.ts
// ---cut---
import { derived } from 'svelte/store';
const doubled = derived(a, ($a) => $a * 2);
```
The callback can set a value asynchronously by accepting a second argument, `set`, and an optional third argument, `update`, calling either or both of them when appropriate.
In this case, you can also pass a third argument to `derived` — the initial value of the derived store before `set` or `update` is first called. If no initial value is specified, the store's initial value will be `undefined`.
```ts
// @filename: ambient.d.ts
import { type Writable } from 'svelte/store';
declare global {
const a: Writable<number>;
}
export {};
// @filename: index.ts
// @errors: 18046 2769 7006
// ---cut---
import { derived } from 'svelte/store';
const delayed = derived(
a,
($a, set) => {
setTimeout(() => set($a), 1000);
},
2000
);
const delayedIncrement = derived(a, ($a, set, update) => {
set($a);
setTimeout(() => update((x) => x + 1), 1000);
// every time $a produces a value, this produces two
// values, $a immediately and then $a + 1 a second later
});
```
If you return a function from the callback, it will be called when a) the callback runs again, or b) the last subscriber unsubscribes.
```ts
// @filename: ambient.d.ts
import { type Writable } from 'svelte/store';
declare global {
const frequency: Writable<number>;
}
export {};
// @filename: index.ts
// ---cut---
import { derived } from 'svelte/store';
const tick = derived(
frequency,
($frequency, set) => {
const interval = setInterval(() => {
set(Date.now());
}, 1000 / $frequency);
return () => {
clearInterval(interval);
};
},
2000
);
```
In both cases, an array of arguments can be passed as the first argument instead of a single store.
```ts
// @filename: ambient.d.ts
import { type Writable } from 'svelte/store';
declare global {
const a: Writable<number>;
const b: Writable<number>;
}
export {};
// @filename: index.ts
// ---cut---
import { derived } from 'svelte/store';
const summed = derived([a, b], ([$a, $b]) => $a + $b);
const delayed = derived([a, b], ([$a, $b], set) => {
setTimeout(() => set($a + $b), 1000);
});
```
### `readonly`
This simple helper function makes a store readonly. You can still subscribe to the changes from the original one using this new readable store.
```js
import { readonly, writable } from 'svelte/store';
const writableStore = writable(1);
const readableStore = readonly(writableStore);
readableStore.subscribe(console.log);
writableStore.set(2); // console: 2
// @errors: 2339
readableStore.set(2); // ERROR
```
### `get`
Generally, you should read the value of a store by subscribing to it and using the value as it changes over time. Occasionally, you may need to retrieve the value of a store to which you're not subscribed. `get` allows you to do so.
> [!NOTE] This works by creating a subscription, reading the value, then unsubscribing. It's therefore not recommended in hot code paths.
```ts
// @filename: ambient.d.ts
import { type Writable } from 'svelte/store';
declare global {
const store: Writable<string>;
}
export {};
// @filename: index.ts
// ---cut---
import { get } from 'svelte/store';
const value = get(store);
```
## Store contract
```ts
// @noErrors
store = { subscribe: (subscription: (value: any) => void) => (() => void), set?: (value: any) => void }
```
You can create your own stores without relying on [`svelte/store`](../svelte-store), by implementing the _store contract_:
1. A store must contain a `.subscribe` method, which must accept as its argument a subscription function. This subscription function must be immediately and synchronously called with the store's current value upon calling `.subscribe`. All of a store's active subscription functions must later be synchronously called whenever the store's value changes.
2. The `.subscribe` method must return an unsubscribe function. Calling an unsubscribe function must stop its subscription, and its corresponding subscription function must not be called again by the store.
3. A store may _optionally_ contain a `.set` method, which must accept as its argument a new value for the store, and which synchronously calls all of the store's active subscription functions. Such a store is called a _writable store_.
For interoperability with RxJS Observables, the `.subscribe` method is also allowed to return an object with an `.unsubscribe` method, rather than return the unsubscription function directly. Note however that unless `.subscribe` synchronously calls the subscription (which is not required by the Observable spec), Svelte will see the value of the store as `undefined` until it does.
# Context
<!-- - get/set/hasContext
- how to use, best practises (like encapsulating them) -->
Most state is component-level state that lives as long as its component lives. There's also section-wide or app-wide state however, which also needs to be handled somehow.
The easiest way to do that is to create global state and just import that.
```ts
/// file: state.svelte.js
export const myGlobalState = $state({
user: {
/* ... */
}
/* ... */
});
```
```svelte
<!--- file: App.svelte --->
<script>
import { myGlobalState } from './state.svelte.js';
// ...
</script>
```
This has a few drawbacks though:
- it only safely works when your global state is only used client-side - for example, when you're building a single page application that does not render any of your components on the server. If your state ends up being managed and updated on the server, it could end up being shared between sessions and/or users, causing bugs
- it may give the false impression that certain state is global when in reality it should only used in a certain part of your app
To solve these drawbacks, Svelte provides a few `context` primitives which alleviate these problems.
## Setting and getting context
To associate an arbitrary object with the current component, use `setContext`.
```svelte
<script>
import { setContext } from 'svelte';
setContext('key', value);
</script>
```
The context is then available to children of the component (including slotted content) with `getContext`.
```svelte
<script>
import { getContext } from 'svelte';
const value = getContext('key');
</script>
```
`setContext` and `getContext` solve the above problems:
- the state is not global, it's scoped to the component. That way it's safe to render your components on the server and not leak state
- it's clear that the state is not global but rather scoped to a specific component tree and therefore can't be used in other parts of your app
> [!NOTE] `setContext`/`getContext` must be called during component initialisation.
Context is not inherently reactive. If you need reactive values in context then you can pass a `$state` object into context, whose properties _will_ be reactive.
```svelte
<!--- file: Parent.svelte --->
<script>
import { setContext } from 'svelte';
let value = $state({ count: 0 });
setContext('counter', value);
</script>
<button onclick={() => value.count++}>increment</button>
```
```svelte
<!--- file: Child.svelte --->
<script>
import { getContext } from 'svelte';
const value = getContext('counter');
</script>
<p>Count is {value.count}</p>
```
To check whether a given `key` has been set in the context of a parent component, use `hasContext`.
```svelte
<script>
import { hasContext } from 'svelte';
if (hasContext('key')) {
// do something
}
</script>
```
You can also retrieve the whole context map that belongs to the closest parent component using `getAllContexts`. This is useful, for example, if you programmatically create a component and want to pass the existing context to it.
```svelte
<script>
import { getAllContexts } from 'svelte';
const contexts = getAllContexts();
</script>
```
## Encapsulating context interactions
The above methods are very unopinionated about how to use them. When your app grows in scale, it's worthwhile to encapsulate setting and getting the context into functions and properly type them.
```ts
// @errors: 2304
import { getContext, setContext } from 'svelte';
let userKey = Symbol('user');
export function setUserContext(user: User) {
setContext(userKey, user);
}
export function getUserContext(): User {
return getContext(userKey) as User;
}
```
# Lifecycle hooks
<!-- - onMount/onDestroy
- mention that `$effect` might be better for your use case
- beforeUpdate/afterUpdate with deprecation notice?
- or skip this entirely and only have it in the reference docs? -->
In Svelte 5, the component lifecycle consists of only two parts: Its creation and its destruction. Everything in-between — when certain state is updated — is not related to the component as a whole; only the parts that need to react to the state change are notified. This is because under the hood the smallest unit of change is actually not a component, it's the (render) effects that the component sets up upon component initialization. Consequently, there's no such thing as a "before update"/"after update" hook.
## `onMount`
The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM. It must be called during the component's initialisation (but doesn't need to live _inside_ the component; it can be called from an external module).
`onMount` does not run inside a component that is rendered on the server.
```svelte
<script>
import { onMount } from 'svelte';
onMount(() => {
console.log('the component has mounted');
});
</script>
```
If a function is returned from `onMount`, it will be called when the component is unmounted.
```svelte
<script>
import { onMount } from 'svelte';
onMount(() => {
const interval = setInterval(() => {
console.log('beep');
}, 1000);
return () => clearInterval(interval);
});
</script>
```
> [!NOTE] This behaviour will only work when the function passed to `onMount` _synchronously_ returns a value. `async` functions always return a `Promise`, and as such cannot _synchronously_ return a function.
## `onDestroy`
Schedules a callback to run immediately before the component is unmounted.
Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the
only one that runs inside a server-side component.
<div class="ts-block">
```dts
function onDestroy(fn: () => any): void;
```
</div>
Schedules a callback to run immediately before the component is unmounted.
Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the only one that runs inside a server-side component.
```svelte
<script>
import { onDestroy } from 'svelte';
onDestroy(() => {
console.log('the component is being destroyed');
});
</script>
```
## `tick`
While there's no "after update" hook, you can use `tick` to ensure that the UI is updated before continuing. `tick` returns a promise that resolves once any pending state changes have been applied, or in the next microtask if there are none.
```svelte
<script>
import { tick } from 'svelte';
$effect.pre(() => {
console.log('the component is about to update');
tick().then(() => {
console.log('the component just updated');
});
});
</script>
```
## Deprecated: `beforeUpdate` / `afterUpdate`
Svelte 4 contained hooks that ran before and after the component as a whole was updated. For backwards compatibility, these hooks were shimmed in Svelte 5 but not available inside components that use runes.
```svelte
<script>
import { beforeUpdate, afterUpdate } from 'svelte';
beforeUpdate(() => {
console.log('the component is about to update');
});
afterUpdate(() => {
console.log('the component just updated');
});
</script>
```
Instead of `beforeUpdate` use `$effect.pre` and instead of `afterUpdate` use `$effect` instead - these runes offer more granular control and only react to the changes you're actually interested in.
### Chat window example
To implement a chat window that autoscrolls to the bottom when new messages appear (but only if you were _already_ scrolled to the bottom), we need to measure the DOM before we update it.
In Svelte 4, we do this with `beforeUpdate`, but this is a flawed approach — it fires before _every_ update, whether it's relevant or not. In the example below, we need to introduce checks like `updatingMessages` to make sure we don't mess with the scroll position when someone toggles dark mode.
With runes, we can use `$effect.pre`, which behaves the same as `$effect` but runs before the DOM is updated. As long as we explicitly reference `messages` inside the effect body, it will run whenever `messages` changes, but _not_ when `theme` changes.
`beforeUpdate`, and its equally troublesome counterpart `afterUpdate`, are therefore deprecated in Svelte 5.
- [Before](/playground/untitled#H4sIAAAAAAAAE31WXa_bNgz9K6yL1QmWOLlrC-w6H8MeBgwY9tY9NfdBtmlbiywZkpyPBfnvo2zLcZK28AWuRPGI5OGhkEuQc4EmiL9eAskqDOLg97oOZoE9125jDigs0t6oRqfOsjap5rXd7uTO8qpW2sIFEsyVxn_qjFmcAcstar-xPN3DFXKtKgi768IVgQku0ELj3Lgs_kZjWIEGNpAzYXDlHWyJFZI1zJjeh4O5uvl_DY8oUkVeVoFuJKYls-_CGYS25Aboj0EtWNqel0wWoBoLTGZgmdgDS9zW4Uz4NsrswPHoyutN4xInkylstnBxdmIhh8m7xzqmoNE2Wq46n1RJQzEbq4g-JQSl7e-HDx-GdaTy3KD9E3lRWvj5Zu9QX1QN20dj7zyHz8s-1S6lW7Cpz3RnXTcm04hIlfdFuO8p2mQ5-3a06cqjrn559bF_2NHOnRZ5I1PLlXQNyQT-hedMHeUEDyjtdMxsa4n2eIbNhlTwhyRthaOKOmYtniwF6pwt0wXa6MBEg0OibZec27gz_dk3UrZ6hB2LLYoiv521Yd8Gt-foTrfhiCDP0lC9VUUhcDLU49Xe_9943cNvEArHfAjxeBTovvXiNpFynfEDpIIZs9kFbg52QbeNHWZzebz32s7xHco3nJAJl1nshmhz8dYOQJDyZetnbb2gTWe-vEeWlrfpZMavr56ldb29eNt6UXvgwgFbp_WC0tl2RK25rGk6lYz3nUI2lzvBXGHhPZPGWmKUXFNBKqdaW259wl_aHbiqoVIZdpE60Nax6IOujT0LbFFxIVTCxCRR2XloUcYNvSbnGHKBp763jHoj59xiZWJI0Wm0P_m3MSS985xkasn-cFq20xTDy3J5KFcjgUTD69BHdcHIjz431z28IqlxGcPSfdFnrGDZn6gD6lyo45zyHAD-btczf-98nhQxHEvKfeUtOVkSejD3q-9X7JbzjGtsdUxlKdFU8qGsT78uaw848syWMXz85Waq2Gnem4mAn3prweq4q6Y3JEpnqMmnPoFRgmd3ySW0LLRqSKlwYHriCvJvUs2yjMaaoA-XzTXLeGMe45zmhv_XAno3Mj0xF7USuqNvnE9H343QHlq-eAgxpbTPNR9yzUkgLjwSR0NK4wKoxy-jDg-9vy8sUSToakzW-9fX13Em9Q8T6Z26uZhBN36XUYo5q7ggLXBZoub2Ofv7g6GCZfTxe034NCjiudXj7Omla0eTfo7QBPOcYxbE7qG-vl3_B1G-_i_JCAAA)
- [After](/playground/untitled#H4sIAAAAAAAAE31WXa-jNhD9K7PsdknUQJLurtRLPqo-VKrU1327uQ8GBnBjbGSb5KZR_nvHgMlXtyIS9njO-MyZGZRzUHCBJkhez4FkNQZJ8HvTBLPAnhq3MQcUFmlvVKszZ1mbTPPGbndyZ3ndKG3hDJZne7hAoVUNYY8JV-RBPgIt2AprhA18MpZZnIQ50_twuvLHNRrDSjRXj9fwiCJTBLIKdCsxq5j9EM4gtBU3QD8GjWBZd14xWYJqLTCZg2ViDyx1W4cz4dv0hsiB49FRHkyfsCgws3GjcTKZwmYLZ2feWc9o1W8zJQ2Fb62i5JUQRNRHgs-fx3WsisKg_RN5WVn4-WrvUd9VA9tH4-AcwbfFQIpkLWByvWzqSe2sk3kyjUlOec_XPU-3TRaz_75tuvKoi19e3OvipSpamVmupJM2F_gXnnJ1lBM8oLQjHceys8R7PMFms4HwD2lRhzeEe-EsvluSrHe2TJdo4wMTLY48XKwPzm0KGm2r5ajFtRYU4TWOY7-ddWHfxhDP0QkQhnf5PWRnVVkKnIx8fZsOb5dR16nwG4TCCRdCMphWQ7z1_DoOcp3zA2SCGbPZBa5jd0G_TRxmc36Me-mG6A7l60XIlMs8ce2-OXtrDyBItdz6qVjPadObzx-RZdV1nJjx64tXad1sz962njceOHfAzmk9JzrbXqg1lw3NkZL7vgE257t-uMDcO6attSSokpmgFqVMO2U93e_dDlzOUKsc-3t6zNZp6K9cG3sS2KGSUqiUiUmq8tNYoJwbmvpTAoXA96GyjCojI26xNglk6DpwOPm7NdRYp4ia0JL94bTqRiGB5WJxqFY37RGPoz3c6i4jP3rcUA7wmhqNywQW7om_YQ2L4UQdUBdCHSPiOQJ8bFcxHzeK0jKBY0XcV95SkCWlD9t-9eOM3TLKucauiyktJdpaPqT19ddF4wFHntsqgS-_XE01e48GMwnw02AtWZP02QyGVOkcNfk072CU4PkduZSWpVYt9SkcmJ64hPwHpWF5ziVls3wIFmmW89Y83vMeGf5PBxjcyPSkXNy10J18t3x6-a6CDtBq6SGklNKeazFyLahB3PVIGo2UbhOgGi9vKjzW_j6xVFFD17difXx5ebll0vwvkcGpn4sZ9MN3vqFYsJoL6gUuK9TcPrO_PxgzWMRfflSEr2NHPJf6lj1957rRpH8CNMG84JgHidUtXt4u_wK21LXERAgAAA==)
<!-- prettier-ignore -->
```svelte
<script>
import { ---beforeUpdate, afterUpdate,--- tick } from 'svelte';
---let updatingMessages = false;---
let theme = +++$state('dark')+++;
let messages = +++$state([])+++;
let viewport;
---beforeUpdate(() => {---
+++$effect.pre(() => {+++
---if (!updatingMessages) return;---
+++messages;+++
const autoscroll = viewport && viewport.offsetHeight + viewport.scrollTop > viewport.scrollHeight - 50;
if (autoscroll) {
tick().then(() => {
viewport.scrollTo(0, viewport.scrollHeight);
});
}
---updatingMessages = false;---
});
function handleKeydown(event) {
if (event.key === 'Enter') {
const text = event.target.value;
if (!text) return;
---updatingMessages = true;---
messages = [...messages, text];
event.target.value = '';
}
}
function toggle() {
toggleValue = !toggleValue;
}
</script>
<div class:dark={theme === 'dark'}>
<div bind:this={viewport}>
{#each messages as message}
<p>{message}</p>
{/each}
</div>
<input +++onkeydown+++={handleKeydown} />
<button +++onclick+++={toggle}> Toggle dark mode </button>
</div>
```
# Imperative component API
<!-- better title needed?
- mount
- unmount
- render
- hydrate
- how they interact with each other -->
Every Svelte application starts by imperatively creating a root component. On the client this component is mounted to a specific element. On the server, you want to get back a string of HTML instead which you can render. The following functions help you achieve those tasks.
## `mount`
Instantiates a component and mounts it to the given target:
```js
// @errors: 2322
import { mount } from 'svelte';
import App from './App.svelte';
const app = mount(App, {
target: document.querySelector('#app'),
props: { some: 'property' }
});
```
You can mount multiple components per page, and you can also mount from within your application, for example when creating a tooltip component and attaching it to the hovered element.
Note that unlike calling `new App(...)` in Svelte 4, things like effects (including `onMount` callbacks, and action functions) will not run during `mount`. If you need to force pending effects to run (in the context of a test, for example) you can do so with `flushSync()`.
## `unmount`
Unmounts a component that was previously created with [`mount`](#mount) or [`hydrate`](#hydrate).
If `options.outro` is `true`, [transitions](transition) will play before the component is removed from the DOM:
```js
import { mount, unmount } from 'svelte';
import App from './App.svelte';
const app = mount(App, { target: document.body });
// later
unmount(app, { outro: true });
```
Returns a `Promise` that resolves after transitions have completed if `options.outro` is true, or immediately otherwise.
## `render`
Only available on the server and when compiling with the `server` option. Takes a component and returns an object with `body` and `head` properties on it, which you can use to populate the HTML when server-rendering your app:
```js
// @errors: 2724 2305 2307
import { render } from 'svelte/server';
import App from './App.svelte';
const result = render(App, {
props: { some: 'property' }
});
result.body; // HTML for somewhere in this <body> tag
result.head; // HTML for somewhere in this <head> tag
```
## `hydrate`
Like `mount`, but will reuse up any HTML rendered by Svelte's SSR output (from the [`render`](#render) function) inside the target and make it interactive:
```js
// @errors: 2322
import { hydrate } from 'svelte';
import App from './App.svelte';
const app = hydrate(App, {
target: document.querySelector('#app'),
props: { some: 'property' }
});
```
As with `mount`, effects will not run during `hydrate` — use `flushSync()` immediately afterwards if you need them to.
# Testing
Testing helps you write and maintain your code and guard against regressions. Testing frameworks help you with that, allowing you to describe assertions or expectations about how your code should behave. Svelte is unopinionated about which testing framework you use — you can write unit tests, integration tests, and end-to-end tests using solutions like [Vitest](https://vitest.dev/), [Jasmine](https://jasmine.github.io/), [Cypress](https://www.cypress.io/) and [Playwright](https://playwright.dev/).
## Unit and integration testing using Vitest
Unit tests allow you to test small isolated parts of your code. Integration tests allow you to test parts of your application to see if they work together. If you're using Vite (including via SvelteKit), we recommend using [Vitest](https://vitest.dev/).
To get started, install Vitest:
```bash
npm install -D vitest
```
Then adjust your `vite.config.js`:
<!-- prettier-ignore -->
```js
/// file: vite.config.js
import { defineConfig } from +++'vitest/config'+++;
export default defineConfig({
// ...
// Tell Vitest to use the `browser` entry points in `package.json` files, even though it's running in Node
resolve: process.env.VITEST
? {
conditions: ['browser']
}
: undefined
});
```
> [!NOTE] If loading the browser version of all your packages is undesirable, because (for example) you also test backend libraries, [you may need to resort to an alias configuration](https://github.com/testing-library/svelte-testing-library/issues/222#issuecomment-1909993331)
You can now write unit tests for code inside your `.js/.ts` files:
```js
/// file: multiplier.svelte.test.js
import { flushSync } from 'svelte';
import { expect, test } from 'vitest';
import { multiplier } from './multiplier.svelte.js';
test('Multiplier', () => {
let double = multiplier(0, 2);
expect(double.value).toEqual(0);
double.set(5);
expect(double.value).toEqual(10);
});
```
```js
/// file: multiplier.svelte.js
/**
* @param {number} initial
* @param {number} k
*/
export function multiplier(initial, k) {
let count = $state(initial);
return {
get value() {
return count * k;
},
/** @param {number} c */
set: (c) => {
count = c;
}
};
}
```
### Using runes inside your test files
Since Vitest processes your test files the same way as your source files, you can use runes inside your tests as long as the filename includes `.svelte`:
```js
/// file: multiplier.svelte.test.js
import { flushSync } from 'svelte';
import { expect, test } from 'vitest';
import { multiplier } from './multiplier.svelte.js';
test('Multiplier', () => {
let count = $state(0);
let double = multiplier(() => count, 2);
expect(double.value).toEqual(0);
count = 5;
expect(double.value).toEqual(10);
});
```
```js
/// file: multiplier.svelte.js
/**
* @param {() => number} getCount
* @param {number} k
*/
export function multiplier(getCount, k) {
return {
get value() {
return getCount() * k;
}
};
}
```
If the code being tested uses effects, you need to wrap the test inside `$effect.root`:
```js
/// file: logger.svelte.test.js
import { flushSync } from 'svelte';
import { expect, test } from 'vitest';
import { logger } from './logger.svelte.js';
test('Effect', () => {
const cleanup = $effect.root(() => {
let count = $state(0);
// logger uses an $effect to log updates of its input
let log = logger(() => count);
// effects normally run after a microtask,
// use flushSync to execute all pending effects synchronously
flushSync();
expect(log.value).toEqual([0]);
count = 1;
flushSync();
expect(log.value).toEqual([0, 1]);
});
cleanup();
});
```
```js
/// file: logger.svelte.js
/**
* @param {() => any} getValue
*/
export function logger(getValue) {
/** @type {any[]} */
let log = $state([]);
$effect(() => {
log.push(getValue());
});
return {
get value() {
return log;
}
};
}
```
### Component testing
It is possible to test your components in isolation using Vitest.
> [!NOTE] Before writing component tests, think about whether you actually need to test the component, or if it's more about the logic _inside_ the component. If so, consider extracting out that logic to test it in isolation, without the overhead of a component
To get started, install jsdom (a library that shims DOM APIs):
```bash
npm install -D jsdom
```
Then adjust your `vite.config.js`:
```js
/// file: vite.config.js
import { defineConfig } from 'vitest/config';
export default defineConfig({
plugins: [
/* ... */
],
test: {
// If you are testing components client-side, you need to setup a DOM environment.
// If not all your files should have this environment, you can use a
// `// @vitest-environment jsdom` comment at the top of the test files instead.
environment: 'jsdom'
},
// Tell Vitest to use the `browser` entry points in `package.json` files, even though it's running in Node
resolve: process.env.VITEST
? {
conditions: ['browser']
}
: undefined
});
```
After that, you can create a test file in which you import the component to test, interact with it programmatically and write expectations about the results:
```js
/// file: component.test.js
import { flushSync, mount, unmount } from 'svelte';
import { expect, test } from 'vitest';
import Component from './Component.svelte';
test('Component', () => {
// Instantiate the component using Svelte's `mount` API
const component = mount(Component, {
target: document.body, // `document` exists because of jsdom
props: { initial: 0 }
});
expect(document.body.innerHTML).toBe('<button>0</button>');
// Click the button, then flush the changes so you can synchronously write expectations
document.body.querySelector('button').click();
flushSync();
expect(document.body.innerHTML).toBe('<button>1</button>');
// Remove the component from the DOM
unmount(component);
});
```
While the process is very straightforward, it is also low level and somewhat brittle, as the precise structure of your component may change frequently. Tools like [@testing-library/svelte](https://testing-library.com/docs/svelte-testing-library/intro/) can help streamline your tests. The above test could be rewritten like this:
```js
/// file: component.test.js
import { render, screen } from '@testing-library/svelte';
import userEvent from '@testing-library/user-event';
import { expect, test } from 'vitest';
import Component from './Component.svelte';
test('Component', async () => {
const user = userEvent.setup();
render(Component);
const button = screen.getByRole('button');
expect(button).toHaveTextContent(0);
await user.click(button);
expect(button).toHaveTextContent(1);
});
```
When writing component tests that involve two-way bindings, context or snippet props, it's best to create a wrapper component for your specific test and interact with that. `@testing-library/svelte` contains some [examples](https://testing-library.com/docs/svelte-testing-library/example).
## E2E tests using Playwright
E2E (short for 'end to end') tests allow you to test your full application through the eyes of the user. This section uses [Playwright](https://playwright.dev/) as an example, but you can also use other solutions like [Cypress](https://www.cypress.io/) or [NightwatchJS](https://nightwatchjs.org/).
To get started with Playwright, either install it via [the VS Code extension](https://playwright.dev/docs/getting-started-vscode), or install it from the command line using `npm init playwright`. It is also part of the setup CLI when you run `npx sv create`.
After you've done that, you should have a `tests` folder and a Playwright config. You may need to adjust that config to tell Playwright what to do before running the tests - mainly starting your application at a certain port:
```js
/// file: playwright.config.js
const config = {
webServer: {
command: 'npm run build && npm run preview',
port: 4173
},
testDir: 'tests',
testMatch: /(.+\.)?(test|spec)\.[jt]s/
};
export default config;
```
You can now start writing tests. These are totally unaware of Svelte as a framework, so you mainly interact with the DOM and write assertions.
```js
// @errors: 2307 7031
/// file: tests/hello-world.spec.js
import { expect, test } from '@playwright/test';
test('home page has expected h1', async ({ page }) => {
await page.goto('/');
await expect(page.locator('h1')).toBeVisible();
});
```
# TypeScript
<!-- - [basically what we have today](https://svelte.dev/docs/typescript)
- built-in support, but only for type-only features
- generics
- using `Component` and the other helper types
- using `svelte-check` -->
You can use TypeScript within Svelte components. IDE extensions like the [Svelte VS Code extension](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode) will help you catch errors right in your editor, and [`svelte-check`](https://www.npmjs.com/package/svelte-check) does the same on the command line, which you can integrate into your CI.
## `<script lang="ts">`
To use TypeScript inside your Svelte components, add `lang="ts"` to your `script` tags:
```svelte
<script lang="ts">
let name: string = 'world';
function greet(name: string) {
alert(`Hello, ${name}!`);
}
</script>
<button onclick={(e: Event) => greet(e.target.innerText)}>
{name as string}
</button>
```
Doing so allows you to use TypeScript's _type-only_ features. That is, all features that just disappear when transpiling to JavaScript, such as type annotations or interface declarations. Features that require the TypeScript compiler to output actual code are not supported. This includes:
- using enums
- using `private`, `protected` or `public` modifiers in constructor functions together with initializers
- using features that are not yet part of the ECMAScript standard (i.e. not level 4 in the TC39 process) and therefore not implemented yet within Acorn, the parser we use for parsing JavaScript
If you want to use one of these features, you need to setup up a `script` preprocessor.
## Preprocessor setup
To use non-type-only TypeScript features within Svelte components, you need to add a preprocessor that will turn TypeScript into JavaScript.
```ts
/// file: svelte.config.js
// @noErrors
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
const config = {
// Note the additional `{ script: true }`
preprocess: vitePreprocess({ script: true })
};
export default config;
```
### Using SvelteKit or Vite
The easiest way to get started is scaffolding a new SvelteKit project by typing `npx sv create`, following the prompts and choosing the TypeScript option.
```ts
/// file: svelte.config.js
// @noErrors
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
const config = {
preprocess: vitePreprocess()
};
export default config;
```
If you don't need or want all the features SvelteKit has to offer, you can scaffold a Svelte-flavoured Vite project instead by typing `npm create vite@latest` and selecting the `svelte-ts` option.
In both cases, a `svelte.config.js` with `vitePreprocess` will be added. Vite/SvelteKit will read from this config file.
### Other build tools
If you're using tools like Rollup or Webpack instead, install their respective Svelte plugins. For Rollup that's [rollup-plugin-svelte](https://github.com/sveltejs/rollup-plugin-svelte) and for Webpack that's [svelte-loader](https://github.com/sveltejs/svelte-loader). For both, you need to install `typescript` and `svelte-preprocess` and add the preprocessor to the plugin config (see the respective READMEs for more info). If you're starting a new project, you can also use the [rollup](https://github.com/sveltejs/template) or [webpack](https://github.com/sveltejs/template-webpack) template to scaffold the setup from a script.
> [!NOTE] If you're starting a new project, we recommend using SvelteKit or Vite instead
## tsconfig.json settings
When using TypeScript, make sure your `tsconfig.json` is setup correctly.
- Use a [`target`](https://www.typescriptlang.org/tsconfig/#target) of at least `ES2022`, or a `target` of at least `ES2015` alongside [`useDefineForClassFields`](https://www.typescriptlang.org/tsconfig/#useDefineForClassFields). This ensures that rune declarations on class fields are not messed with, which would break the Svelte compiler
- Set [`verbatimModuleSyntax`](https://www.typescriptlang.org/tsconfig/#verbatimModuleSyntax) to `true` so that imports are left as-is
- Set [`isolatedModules`](https://www.typescriptlang.org/tsconfig/#isolatedModules) to `true` so that each file is looked at in isolation. TypeScript has a few features which require cross-file analysis and compilation, which the Svelte compiler and tooling like Vite don't do.
## Typing `$props`
Type `$props` just like a regular object with certain properties.
```svelte
<script lang="ts">
import type { Snippet } from 'svelte';
interface Props {
requiredProperty: number;
optionalProperty?: boolean;
snippetWithStringArgument: Snippet<[string]>;
eventHandler: (arg: string) => void;
[key: string]: unknown;
}
let {
requiredProperty,
optionalProperty,
snippetWithStringArgument,
eventHandler,
...everythingElse
}: Props = $props();
</script>
<button onclick={() => eventHandler('clicked button')}>
{@render snippetWithStringArgument('hello')}
</button>
```
## Generic `$props`
Components can declare a generic relationship between their properties. One example is a generic list component that receives a list of items and a callback property that receives an item from the list. To declare that the `items` property and the `select` callback operate on the same types, add the `generics` attribute to the `script` tag:
```svelte
<script lang="ts" generics="Item extends { text: string }">
interface Props {
items: Item[];
select(item: Item): void;
}
let { items, select }: Props = $props();
</script>
{#each items as item}
<button onclick={() => select(item)}>
{item.text}
</button>
{/each}
```
The content of `generics` is what you would put between the `<...>` tags of a generic function. In other words, you can use multiple generics, `extends` and fallback types.
## Typing wrapper components
In case you're writing a component that wraps a native element, you may want to expose all the attributes of the underlying element to the user. In that case, use (or extend from) one of the interfaces provided by `svelte/elements`. Here's an example for a `Button` component:
```svelte
<script lang="ts">
import type { HTMLButtonAttributes } from 'svelte/elements';
let { children, ...rest }: HTMLButtonAttributes = $props();
</script>
<button {...rest}>
{@render children?.()}
</button>
```
Not all elements have a dedicated type definition. For those without one, use `SvelteHTMLElements`:
```svelte
<script lang="ts">
import type { SvelteHTMLElements } from 'svelte/elements';
let { children, ...rest }: SvelteHTMLElements['div'] = $props();
</script>
<div {...rest}>
{@render children?.()}
</div>
```
## Typing `$state`
You can type `$state` like any other variable.
```ts
let count: number = $state(0);
```
If you don't give `$state` an initial value, part of its types will be `undefined`.
```ts
// @noErrors
// Error: Type 'number | undefined' is not assignable to type 'number'
let count: number = $state();
```
If you know that the variable _will_ be defined before you first use it, use an `as` casting. This is especially useful in the context of classes:
```ts
class Counter {
count = $state() as number;
constructor(initial: number) {
this.count = initial;
}
}
```
## The `Component` type
Svelte components are of type `Component`. You can use it and its related types to express a variety of constraints.
Using it together with dynamic components to restrict what kinds of component can be passed to it:
```svelte
<script lang="ts">
import type { Component } from 'svelte';
interface Props {
// only components that have at most the "prop"
// property required can be passed
DynamicComponent: Component<{ prop: string }>;
}
let { DynamicComponent }: Props = $props();
</script>
<DynamicComponent prop="foo" />
```
> [!LEGACY] In Svelte 4, components were of type `SvelteComponent`
To extract the properties from a component, use `ComponentProps`.
```ts
import type { Component, ComponentProps } from 'svelte';
import MyComponent from './MyComponent.svelte';
function withProps<TComponent extends Component<any>>(
component: TComponent,
props: ComponentProps<TComponent>
) {}
// Errors if the second argument is not the correct props expected
// by the component in the first argument.
withProps(MyComponent, { foo: 'bar' });
```
To declare that a variable expects the constructor or instance type of a component:
```svelte
<script lang="ts">
import MyComponent from './MyComponent.svelte';
let componentConstructor: typeof MyComponent = MyComponent;
let componentInstance: MyComponent;
</script>
<MyComponent bind:this={componentInstance} />
```
## Enhancing built-in DOM types
Svelte provides a best effort of all the HTML DOM types that exist. Sometimes you may want to use experimental attributes or custom events coming from an action. In these cases, TypeScript will throw a type error, saying that it does not know these types. If it's a non-experimental standard attribute/event, this may very well be a missing typing from our [HTML typings](https://github.com/sveltejs/svelte/blob/main/packages/svelte/elements.d.ts). In that case, you are welcome to open an issue and/or a PR fixing it.
In case this is a custom or experimental attribute/event, you can enhance the typings like this:
```ts
/// file: additional-svelte-typings.d.ts
declare namespace svelteHTML {
// enhance elements
interface IntrinsicElements {
'my-custom-element': { someattribute: string; 'on:event': (e: CustomEvent<any>) => void };
}
// enhance attributes
interface HTMLAttributes<T> {
// If you want to use the beforeinstallprompt event
onbeforeinstallprompt?: (event: any) => any;
// If you want to use myCustomAttribute={..} (note: all lowercase)
mycustomattribute?: any; // You can replace any with something more specific if you like
}
}
```
Then make sure that `d.ts` file is referenced in your `tsconfig.json`. If it reads something like `"include": ["src/**/*"]` and your `d.ts` file is inside `src`, it should work. You may need to reload for the changes to take effect.
You can also declare the typings by augmenting the `svelte/elements` module like this:
```ts
/// file: additional-svelte-typings.d.ts
import { HTMLButtonAttributes } from 'svelte/elements';
declare module 'svelte/elements' {
export interface SvelteHTMLElements {
'custom-button': HTMLButtonAttributes;
}
// allows for more granular control over what element to add the typings to
export interface HTMLButtonAttributes {
veryexperimentalattribute?: string;
}
}
export {}; // ensure this is not an ambient module, else types will be overridden instead of augmented
```
# Custom elements
<!-- - [basically what we have today](https://svelte.dev/docs/custom-elements-api) -->
Svelte components can also be compiled to custom elements (aka web components) using the `customElement: true` compiler option. You should specify a tag name for the component using the `<svelte:options>` [element](svelte-options).
```svelte
<svelte:options customElement="my-element" />
<script>
let { name = 'world' } = $props();
</script>
<h1>Hello {name}!</h1>
<slot />
```
You can leave out the tag name for any of your inner components which you don't want to expose and use them like regular Svelte components. Consumers of the component can still name it afterwards if needed, using the static `element` property which contains the custom element constructor and which is available when the `customElement` compiler option is `true`.
```js
// @noErrors
import MyElement from './MyElement.svelte';
customElements.define('my-element', MyElement.element);
```
Once a custom element has been defined, it can be used as a regular DOM element:
```js
document.body.innerHTML = `
<my-element>
<p>This is some slotted content</p>
</my-element>
`;
```
Any [props](basic-markup#Component-props) are exposed as properties of the DOM element (as well as being readable/writable as attributes, where possible).
```js
// @noErrors
const el = document.querySelector('my-element');
// get the current value of the 'name' prop
console.log(el.name);
// set a new value, updating the shadow DOM
el.name = 'everybody';
```
Note that you need to list out all properties explicitly, i.e. doing `let props = $props()` without declaring `props` in the [component options](#Component-options) means that Svelte can't know which props to expose as properties on the DOM element.
## Component lifecycle
Custom elements are created from Svelte components using a wrapper approach. This means the inner Svelte component has no knowledge that it is a custom element. The custom element wrapper takes care of handling its lifecycle appropriately.
When a custom element is created, the Svelte component it wraps is _not_ created right away. It is only created in the next tick after the `connectedCallback` is invoked. Properties assigned to the custom element before it is inserted into the DOM are temporarily saved and then set on component creation, so their values are not lost. The same does not work for invoking exported functions on the custom element though, they are only available after the element has mounted. If you need to invoke functions before component creation, you can work around it by using the [`extend` option](#Component-options).
When a custom element written with Svelte is created or updated, the shadow DOM will reflect the value in the next tick, not immediately. This way updates can be batched, and DOM moves which temporarily (but synchronously) detach the element from the DOM don't lead to unmounting the inner component.
The inner Svelte component is destroyed in the next tick after the `disconnectedCallback` is invoked.
## Component options
When constructing a custom element, you can tailor several aspects by defining `customElement` as an object within `<svelte:options>` since Svelte 4. This object may contain the following properties:
- `tag: string`: an optional `tag` property for the custom element's name. If set, a custom element with this tag name will be defined with the document's `customElements` registry upon importing this component.
- `shadow`: an optional property that can be set to `"none"` to forgo shadow root creation. Note that styles are then no longer encapsulated, and you can't use slots
- `props`: an optional property to modify certain details and behaviors of your component's properties. It offers the following settings:
- `attribute: string`: To update a custom element's prop, you have two alternatives: either set the property on the custom element's reference as illustrated above or use an HTML attribute. For the latter, the default attribute name is the lowercase property name. Modify this by assigning `attribute: "<desired name>"`.
- `reflect: boolean`: By default, updated prop values do not reflect back to the DOM. To enable this behavior, set `reflect: true`.
- `type: 'String' | 'Boolean' | 'Number' | 'Array' | 'Object'`: While converting an attribute value to a prop value and reflecting it back, the prop value is assumed to be a `String` by default. This may not always be accurate. For instance, for a number type, define it using `type: "Number"`
You don't need to list all properties, those not listed will use the default settings.
- `extend`: an optional property which expects a function as its argument. It is passed the custom element class generated by Svelte and expects you to return a custom element class. This comes in handy if you have very specific requirements to the life cycle of the custom element or want to enhance the class to for example use [ElementInternals](https://developer.mozilla.org/en-US/docs/Web/API/ElementInternals#examples) for better HTML form integration.
```svelte
<svelte:options
customElement={{
tag: 'custom-element',
shadow: 'none',
props: {
name: { reflect: true, type: 'Number', attribute: 'element-index' }
},
extend: (customElementConstructor) => {
// Extend the class so we can let it participate in HTML forms
return class extends customElementConstructor {
static formAssociated = true;
constructor() {
super();
this.attachedInternals = this.attachInternals();
}
// Add the function here, not below in the component so that
// it's always available, not just when the inner Svelte component
// is mounted
randomIndex() {
this.elementIndex = Math.random();
}
};
}
}}
/>
<script>
let { elementIndex, attachedInternals } = $props();
// ...
function check() {
attachedInternals.checkValidity();
}
</script>
...
```
## Caveats and limitations
Custom elements can be a useful way to package components for consumption in a non-Svelte app, as they will work with vanilla HTML and JavaScript as well as [most frameworks](https://custom-elements-everywhere.com/). There are, however, some important differences to be aware of:
- Styles are _encapsulated_, rather than merely _scoped_ (unless you set `shadow: "none"`). This means that any non-component styles (such as you might have in a `global.css` file) will not apply to the custom element, including styles with the `:global(...)` modifier
- Instead of being extracted out as a separate .css file, styles are inlined into the component as a JavaScript string
- Custom elements are not generally suitable for server-side rendering, as the shadow DOM is invisible until JavaScript loads
- In Svelte, slotted content renders _lazily_. In the DOM, it renders _eagerly_. In other words, it will always be created even if the component's `<slot>` element is inside an `{#if ...}` block. Similarly, including a `<slot>` in an `{#each ...}` block will not cause the slotted content to be rendered multiple times
- The deprecated `let:` directive has no effect, because custom elements do not have a way to pass data to the parent component that fills the slot
- Polyfills are required to support older browsers
- You can use Svelte's context feature between regular Svelte components within a custom element, but you can't use them across custom elements. In other words, you can't use `setContext` on a parent custom element and read that with `getContext` in a child custom element.
- Don't declare properties or attributes starting with `on`, as their usage will be interpreted as an event listener. In other words, Svelte treats `<custom-element oneworld={true}></custom-element>` as `customElement.addEventListener('eworld', true)` (and not as `customElement.oneworld = true`)
# Svelte 4 migration guide
This migration guide provides an overview of how to migrate from Svelte version 3 to 4. See the linked PRs for more details about each change. Use the migration script to migrate some of these automatically: `npx svelte-migrate@latest svelte-4`
If you're a library author, consider whether to only support Svelte 4 or if it's possible to support Svelte 3 too. Since most of the breaking changes don't affect many people, this may be easily possible. Also remember to update the version range in your `peerDependencies`.
## Minimum version requirements
- Upgrade to Node 16 or higher. Earlier versions are no longer supported. ([#8566](https://github.com/sveltejs/svelte/issues/8566))
- If you are using SvelteKit, upgrade to 1.20.4 or newer ([sveltejs/kit#10172](https://github.com/sveltejs/kit/pull/10172))
- If you are using Vite without SvelteKit, upgrade to `vite-plugin-svelte` 2.4.1 or newer ([#8516](https://github.com/sveltejs/svelte/issues/8516))
- If you are using webpack, upgrade to webpack 5 or higher and `svelte-loader` 3.1.8 or higher. Earlier versions are no longer supported. ([#8515](https://github.com/sveltejs/svelte/issues/8515), [198dbcf](https://github.com/sveltejs/svelte/commit/198dbcf))
- If you are using Rollup, upgrade to `rollup-plugin-svelte` 7.1.5 or higher ([198dbcf](https://github.com/sveltejs/svelte/commit/198dbcf))
- If you are using TypeScript, upgrade to TypeScript 5 or higher. Lower versions might still work, but no guarantees are made about that. ([#8488](https://github.com/sveltejs/svelte/issues/8488))
## Browser conditions for bundlers
Bundlers must now specify the `browser` condition when building a frontend bundle for the browser. SvelteKit and Vite will handle this automatically for you. If you're using any others, you may observe lifecycle callbacks such as `onMount` not get called and you'll need to update the module resolution configuration.
- For Rollup this is done within the `@rollup/plugin-node-resolve` plugin by setting `browser: true` in its options. See the [`rollup-plugin-svelte`](https://github.com/sveltejs/rollup-plugin-svelte/#usage) documentation for more details
- For webpack this is done by adding `"browser"` to the `conditionNames` array. You may also have to update your `alias` config, if you have set it. See the [`svelte-loader`](https://github.com/sveltejs/svelte-loader#usage) documentation for more details
([#8516](https://github.com/sveltejs/svelte/issues/8516))
## Removal of CJS related output
Svelte no longer supports the CommonJS (CJS) format for compiler output and has also removed the `svelte/register` hook and the CJS runtime version. If you need to stay on the CJS output format, consider using a bundler to convert Svelte's ESM output to CJS in a post-build step. ([#8613](https://github.com/sveltejs/svelte/issues/8613))
## Stricter types for Svelte functions
There are now stricter types for `createEventDispatcher`, `Action`, `ActionReturn`, and `onMount`:
- `createEventDispatcher` now supports specifying that a payload is optional, required, or non-existent, and the call sites are checked accordingly ([#7224](https://github.com/sveltejs/svelte/issues/7224))
```ts
// @errors: 2554 2345
import { createEventDispatcher } from 'svelte';
const dispatch = createEventDispatcher<{
optional: number | null;
required: string;
noArgument: null;
}>();
// Svelte version 3:
dispatch('optional');
dispatch('required'); // I can still omit the detail argument
dispatch('noArgument', 'surprise'); // I can still add a detail argument
// Svelte version 4 using TypeScript strict mode:
dispatch('optional');
dispatch('required'); // error, missing argument
dispatch('noArgument', 'surprise'); // error, cannot pass an argument
```
- `Action` and `ActionReturn` have a default parameter type of `undefined` now, which means you need to type the generic if you want to specify that this action receives a parameter. The migration script will migrate this automatically ([#7442](https://github.com/sveltejs/svelte/pull/7442))
```ts
// @noErrors
---const action: Action = (node, params) => { ... } // this is now an error if you use params in any way---
+++const action: Action<HTMLElement, string> = (node, params) => { ... } // params is of type string+++
```
- `onMount` now shows a type error if you return a function asynchronously from it, because this is likely a bug in your code where you expect the callback to be called on destroy, which it will only do for synchronously returned functions ([#8136](https://github.com/sveltejs/svelte/issues/8136))
```js
// @noErrors
// Example where this change reveals an actual bug
onMount(
--- // someCleanup() not called because function handed to onMount is async
async () => {
const something = await foo();---
+++ // someCleanup() is called because function handed to onMount is sync
() => {
foo().then(something => {...});
// ...
return () => someCleanup();
}
);
```
## Custom Elements with Svelte
The creation of custom elements with Svelte has been overhauled and significantly improved. The `tag` option is deprecated in favor of the new `customElement` option:
```svelte
---<svelte:options tag="my-component" />---
+++<svelte:options customElement="my-component" />+++
```
This change was made to allow [more configurability](custom-elements#Component-options) for advanced use cases. The migration script will adjust your code automatically. The update timing of properties has changed slightly as well. ([#8457](https://github.com/sveltejs/svelte/issues/8457))
## SvelteComponentTyped is deprecated
`SvelteComponentTyped` is deprecated, as `SvelteComponent` now has all its typing capabilities. Replace all instances of `SvelteComponentTyped` with `SvelteComponent`.
```js
---import { SvelteComponentTyped } from 'svelte';---
+++import { SvelteComponent } from 'svelte';+++
---export class Foo extends SvelteComponentTyped<{ aProp: string }> {}---
+++export class Foo extends SvelteComponent<{ aProp: string }> {}+++
```
If you have used `SvelteComponent` as the component instance type previously, you may see a somewhat opaque type error now, which is solved by changing `: typeof SvelteComponent` to `: typeof SvelteComponent<any>`.
```svelte
<script>
import ComponentA from './ComponentA.svelte';
import ComponentB from './ComponentB.svelte';
import { SvelteComponent } from 'svelte';
let component: typeof SvelteComponent+++<any>+++;
function choseRandomly() {
component = Math.random() > 0.5 ? ComponentA : ComponentB;
}
</script>
<button on:click={choseRandomly}>random</button>
<svelte:element this={component} />
```
The migration script will do both automatically for you. ([#8512](https://github.com/sveltejs/svelte/issues/8512))
## Transitions are local by default
Transitions are now local by default to prevent confusion around page navigations. "local" means that a transition will not play if it's within a nested control flow block (`each/if/await/key`) and not the direct parent block but a block above it is created/destroyed. In the following example, the `slide` intro animation will only play when `success` goes from `false` to `true`, but it will _not_ play when `show` goes from `false` to `true`:
```svelte
{#if show}
...
{#if success}
<p in:slide>Success</p>
{/each}
{/if}
```
To make transitions global, add the `|global` modifier - then they will play when _any_ control flow block above is created/destroyed. The migration script will do this automatically for you. ([#6686](https://github.com/sveltejs/svelte/issues/6686))
## Default slot bindings
Default slot bindings are no longer exposed to named slots and vice versa:
```svelte
<script>
import Nested from './Nested.svelte';
</script>
<Nested let:count>
<p>
count in default slot - is available: {count}
</p>
<p slot="bar">
count in bar slot - is not available: {count}
</p>
</Nested>
```
This makes slot bindings more consistent as the behavior is undefined when for example the default slot is from a list and the named slot is not. ([#6049](https://github.com/sveltejs/svelte/issues/6049))
## Preprocessors
The order in which preprocessors are applied has changed. Now, preprocessors are executed in order, and within one group, the order is markup, script, style.
```js
// @errors: 2304
import { preprocess } from 'svelte/compiler';
const { code } = await preprocess(
source,
[
{
markup: () => {
console.log('markup-1');
},
script: () => {
console.log('script-1');
},
style: () => {
console.log('style-1');
}
},
{
markup: () => {
console.log('markup-2');
},
script: () => {
console.log('script-2');
},
style: () => {
console.log('style-2');
}
}
],
{
filename: 'App.svelte'
}
);
// Svelte 3 logs:
// markup-1
// markup-2
// script-1
// script-2
// style-1
// style-2
// Svelte 4 logs:
// markup-1
// script-1
// style-1
// markup-2
// script-2
// style-2
```
This could affect you for example if you are using `MDsveX` - in which case you should make sure it comes before any script or style preprocessor.
```js
// @noErrors
preprocess: [
--- vitePreprocess(),
mdsvex(mdsvexConfig)---
+++ mdsvex(mdsvexConfig),
vitePreprocess()+++
]
```
Each preprocessor must also have a name. ([#8618](https://github.com/sveltejs/svelte/issues/8618))
## New eslint package
`eslint-plugin-svelte3` is deprecated. It may still work with Svelte 4 but we make no guarantees about that. We recommend switching to our new package [eslint-plugin-svelte](https://github.com/sveltejs/eslint-plugin-svelte). See [this Github post](https://github.com/sveltejs/kit/issues/10242#issuecomment-1610798405) for an instruction how to migrate. Alternatively, you can create a new project using `npm create svelte@latest`, select the eslint (and possibly TypeScript) option and then copy over the related files into your existing project.
## Other breaking changes
- the `inert` attribute is now applied to outroing elements to make them invisible to assistive technology and prevent interaction. ([#8628](https://github.com/sveltejs/svelte/pull/8628))
- the runtime now uses `classList.toggle(name, boolean)` which may not work in very old browsers. Consider using a [polyfill](https://github.com/eligrey/classList.js) if you need to support these browsers. ([#8629](https://github.com/sveltejs/svelte/issues/8629))
- the runtime now uses the `CustomEvent` constructor which may not work in very old browsers. Consider using a [polyfill](https://github.com/theftprevention/event-constructor-polyfill/tree/master) if you need to support these browsers. ([#8775](https://github.com/sveltejs/svelte/pull/8775))
- people implementing their own stores from scratch using the `StartStopNotifier` interface (which is passed to the create function of `writable` etc) from `svelte/store` now need to pass an update function in addition to the set function. This has no effect on people using stores or creating stores using the existing Svelte stores. ([#6750](https://github.com/sveltejs/svelte/issues/6750))
- `derived` will now throw an error on falsy values instead of stores passed to it. ([#7947](https://github.com/sveltejs/svelte/issues/7947))
- type definitions for `svelte/internal` were removed to further discourage usage of those internal methods which are not public API. Most of these will likely change for Svelte 5
- Removal of DOM nodes is now batched which slightly changes its order, which might affect the order of events fired if you're using a `MutationObserver` on these elements ([#8763](https://github.com/sveltejs/svelte/pull/8763))
- if you enhanced the global typings through the `svelte.JSX` namespace before, you need to migrate this to use the `svelteHTML` namespace. Similarly if you used the `svelte.JSX` namespace to use type definitions from it, you need to migrate those to use the types from `svelte/elements` instead. You can find more information about what to do [here](https://github.com/sveltejs/language-tools/blob/master/docs/preprocessors/typescript.md#im-getting-deprecation-warnings-for-sveltejsx--i-want-to-migrate-to-the-new-typings)
# Svelte 5 migration guide
Version 5 comes with an overhauled syntax and reactivity system. While it may look different at first, you'll soon notice many similarities. This guide goes over the changes in detail and shows you how to upgrade. Along with it, we also provide information on _why_ we did these changes.
You don't have to migrate to the new syntax right away - Svelte 5 still supports the old Svelte 4 syntax, and you can mix and match components using the new syntax with components using the old and vice versa. We expect many people to be able to upgrade with only a few lines of code changed initially. There's also a [migration script](#Migration-script) that helps you with many of these steps automatically.
## Reactivity syntax changes
At the heart of Svelte 5 is the new runes API. Runes are basically compiler instructions that inform Svelte about reactivity. Syntactically, runes are functions starting with a dollar-sign.
### let -> $state
In Svelte 4, a `let` declaration at the top level of a component was implicitly reactive. In Svelte 5, things are more explicit: a variable is reactive when created using the `$state` rune. Let's migrate the counter to runes mode by wrapping the counter in `$state`:
```svelte
<script>
let count = +++$state(+++0+++)+++;
</script>
```
Nothing else changes. `count` is still the number itself, and you read and write directly to it, without a wrapper like `.value` or `getCount()`.
> [!DETAILS] Why we did this
> `let` being implicitly reactive at the top level worked great, but it meant that reactivity was constrained - a `let` declaration anywhere else was not reactive. This forced you to resort to using stores when refactoring code out of the top level of components for reuse. This meant you had to learn an entirely separate reactivity model, and the result often wasn't as nice to work with. Because reactivity is more explicit in Svelte 5, you can keep using the same API outside the top level of components. Head to [the tutorial](/tutorial) to learn more.
### $: -> $derived/$effect
In Svelte 4, a `$:` statement at the top level of a component could be used to declare a derivation, i.e. state that is entirely defined through a computation of other state. In Svelte 5, this is achieved using the `$derived` rune:
```svelte
<script>
let count = +++$state(+++0+++)+++;
---$:--- +++const+++ double = +++$derived(+++count * 2+++)+++;
</script>
```
As with `$state`, nothing else changes. `double` is still the number itself, and you read it directly, without a wrapper like `.value` or `getDouble()`.
A `$:` statement could also be used to create side effects. In Svelte 5, this is achieved using the `$effect` rune:
```svelte
<script>
let count = +++$state(+++0+++)+++;
---$:---+++$effect(() =>+++ {
if (count > 5) {
alert('Count is too high!');
}
}+++);+++
</script>
```
> [!DETAILS] Why we did this
> `$:` was a great shorthand and easy to get started with: you could slap a `$:` in front of most code and it would somehow work. This intuitiveness was also its drawback the more complicated your code became, because it wasn't as easy to reason about. Was the intent of the code to create a derivation, or a side effect? With `$derived` and `$effect`, you have a bit more up-front decision making to do (spoiler alert: 90% of the time you want `$derived`), but future-you and other developers on your team will have an easier time.
>
> There were also gotchas that were hard to spot:
>
> - `$:` only updated directly before rendering, which meant you could read stale values in-between rerenders
> - `$:` only ran once per tick, which meant that statements may run less often than you think
> - `$:` dependencies were determined through static analysis of the dependencies. This worked in most cases, but could break in subtle ways during a refactoring where dependencies would be for example moved into a function and no longer be visible as a result
> - `$:` statements were also ordered by using static analysis of the dependencies. In some cases there could be ties and the ordering would be wrong as a result, needing manual interventions. Ordering could also break while refactoring code and some dependencies no longer being visible as a result.
>
> Lastly, it wasn't TypeScript-friendly (our editor tooling had to jump through some hoops to make it valid for TypeScript), which was a blocker for making Svelte's reactivity model truly universal.
>
> `$derived` and `$effect` fix all of these by
>
> - always returning the latest value
> - running as often as needed to be stable
> - determining the dependencies at runtime, and therefore being immune to refactorings
> - executing dependencies as needed and therefore being immune to ordering problems
> - being TypeScript-friendly
### export let -> $props
In Svelte 4, properties of a component were declared using `export let`. Each property was one declaration. In Svelte 5, all properties are declared through the `$props` rune, through destructuring:
```svelte
<script>
---export let optional = 'unset';
export let required;---
+++let { optional = 'unset', required } = $props();+++
</script>
```
There are multiple cases where declaring properties becomes less straightforward than having a few `export let` declarations:
- you want to rename the property, for example because the name is a reserved identifier (e.g. `class`)
- you don't know which other properties to expect in advance
- you want to forward every property to another component
All these cases need special syntax in Svelte 4:
- renaming: `export { klass as class}`
- other properties: `$$restProps`
- all properties `$$props`
In Svelte 5, the `$props` rune makes this straightforward without any additional Svelte-specific syntax:
- renaming: use property renaming `let { class: klass } = $props();`
- other properties: use spreading `let { foo, bar, ...rest } = $props();`
- all properties: don't destructure `let props = $props();`
```svelte
<script>
---let klass = '';
export { klass as class};---
+++let { class: klass, ...rest } = $props();+++
</script>
<button class={klass} {...---$$restProps---+++rest+++}>click me</button>
```
> [!DETAILS] Why we did this
> `export let` was one of the more controversial API decisions, and there was a lot of debate about whether you should think about a property being `export`ed or `import`ed. `$props` doesn't have this trait. It's also in line with the other runes, and the general thinking reduces to "everything special to reactivity in Svelte is a rune".
>
> There were also a lot of limitations around `export let`, which required additional API, as shown above. `$props` unite this in one syntactical concept that leans heavily on regular JavaScript destructuring syntax.
## Event changes
Event handlers have been given a facelift in Svelte 5. Whereas in Svelte 4 we use the `on:` directive to attach an event listener to an element, in Svelte 5 they are properties like any other (in other words - remove the colon):
```svelte
<script>
let count = $state(0);
</script>
<button on---:---click={() => count++}>
clicks: {count}
</button>
```
Since they're just properties, you can use the normal shorthand syntax...
```svelte
<script>
let count = $state(0);
function onclick() {
count++;
}
</script>
<button {onclick}>
clicks: {count}
</button>
```
...though when using a named event handler function it's usually better to use a more descriptive name.
### Component events
In Svelte 4, components could emit events by creating a dispatcher with `createEventDispatcher`.
This function is deprecated in Svelte 5. Instead, components should accept _callback props_ - which means you then pass functions as properties to these components:
```svelte
<!--- file: App.svelte --->
<script>
import Pump from './Pump.svelte';
let size = $state(15);
let burst = $state(false);
function reset() {
size = 15;
burst = false;
}
</script>
<Pump
---on:---inflate={(power) => {
size += power---.detail---;
if (size > 75) burst = true;
}}
---on:---deflate={(power) => {
if (size > 0) size -= power---.detail---;
}}
/>
{#if burst}
<button onclick={reset}>new balloon</button>
<span class="boom">💥</span>
{:else}
<span class="balloon" style="scale: {0.01 * size}">
🎈
</span>
{/if}
```
```svelte
<!--- file: Pump.svelte --->
<script>
---import { createEventDispatcher } from 'svelte';
const dispatch = createEventDispatcher();
---
+++let { inflate, deflate } = $props();+++
let power = $state(5);
</script>
<button onclick={() => ---dispatch('inflate', power)---+++inflate(power)+++}>
inflate
</button>
<button onclick={() => ---dispatch('deflate', power)---+++deflate(power)+++}>
deflate
</button>
<button onclick={() => power--}>-</button>
Pump power: {power}
<button onclick={() => power++}>+</button>
```
### Bubbling events
Instead of doing `<button on:click>` to 'forward' the event from the element to the component, the component should accept an `onclick` callback prop:
```svelte
<script>
+++let { onclick } = $props();+++
</script>
<button ---on:click--- +++{onclick}+++>
click me
</button>
```
Note that this also means you can 'spread' event handlers onto the element along with other props instead of tediously forwarding each event separately:
```svelte
<script>
let props = $props();
</script>
<button ---{...$$props} on:click on:keydown on:all_the_other_stuff--- +++{...props}+++>
click me
</button>
```
### Event modifiers
In Svelte 4, you can add event modifiers to handlers:
```svelte
<button on:click|once|preventDefault={handler}>...</button>
```
Modifiers are specific to `on:` and as such do not work with modern event handlers. Adding things like `event.preventDefault()` inside the handler itself is preferable, since all the logic lives in one place rather than being split between handler and modifiers.
Since event handlers are just functions, you can create your own wrappers as necessary:
```svelte
<script>
function once(fn) {
return function (event) {
if (fn) fn.call(this, event);
fn = null;
};
}
function preventDefault(fn) {
return function (event) {
event.preventDefault();
fn.call(this, event);
};
}
</script>
<button onclick={once(preventDefault(handler))}>...</button>
```
There are three modifiers — `capture`, `passive` and `nonpassive` — that can't be expressed as wrapper functions, since they need to be applied when the event handler is bound rather than when it runs.
For `capture`, we add the modifier to the event name:
```svelte
<button onclickcapture={...}>...</button>
```
Changing the [`passive`](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#using_passive_listeners) option of an event handler, meanwhile, is not something to be done lightly. If you have a use case for it — and you probably don't! — then you will need to use an action to apply the event handler yourself.
### Multiple event handlers
In Svelte 4, this is possible:
```svelte
<button on:click={one} on:click={two}>...</button>
```
Duplicate attributes/properties on elements — which now includes event handlers — are not allowed. Instead, do this:
```svelte
<button
onclick={(e) => {
one(e);
two(e);
}}
>
...
</button>
```
When spreading props, local event handlers must go _after_ the spread, or they risk being overwritten:
```svelte
<button
{...props}
onclick={(e) => {
doStuff(e);
props.onclick?.(e);
}}
>
...
</button>
```
> [!DETAILS] Why we did this
> `createEventDispatcher` was always a bit boilerplate-y:
>
> - import the function
> - call the function to get a dispatch function
> - call said dispatch function with a string and possibly a payload
> - retrieve said payload on the other end through a `.detail` property, because the event itself was always a `CustomEvent`
>
> It was always possible to use component callback props, but because you had to listen to DOM events using `on:`, it made sense to use `createEventDispatcher` for component events due to syntactical consistency. Now that we have event attributes (`onclick`), it's the other way around: Callback props are now the more sensible thing to do.
>
> The removal of event modifiers is arguably one of the changes that seems like a step back for those who've liked the shorthand syntax of event modifiers. Given that they are not used that frequently, we traded a smaller surface area for more explicitness. Modifiers also were inconsistent, because most of them were only useable on DOM elements.
>
> Multiple listeners for the same event are also no longer possible, but it was something of an anti-pattern anyway, since it impedes readability: if there are many attributes, it becomes harder to spot that there are two handlers unless they are right next to each other. It also implies that the two handlers are independent, when in fact something like `event.stopImmediatePropagation()` inside `one` would prevent `two` from being called.
>
> By deprecating `createEventDispatcher` and the `on:` directive in favour of callback props and normal element properties, we:
>
> - reduce Svelte's learning curve
> - remove boilerplate, particularly around `createEventDispatcher`
> - remove the overhead of creating `CustomEvent` objects for events that may not even have listeners
> - add the ability to spread event handlers
> - add the ability to know which event handlers were provided to a component
> - add the ability to express whether a given event handler is required or optional
> - increase type safety (previously, it was effectively impossible for Svelte to guarantee that a component didn't emit a particular event)
## Snippets instead of slots
In Svelte 4, content can be passed to components using slots. Svelte 5 replaces them with snippets which are more powerful and flexible, and as such slots are deprecated in Svelte 5.
They continue to work, however, and you can pass snippets to a component that uses slots:
```svelte
<!--- file: Child.svelte --->
<slot />
<hr />
<slot name="foo" message="hello" />
```
```svelte
<!--- file: Parent.svelte --->
<script>
import Child from './Child.svelte';
</script>
<Child>
default child content
{#snippet foo({ message })}
message from child: {message}
{/snippet}
</Child>
```
(The reverse is not true — you cannot pass slotted content to a component that uses [`{@render ...}`](/docs/svelte/@render) tags.)
When using custom elements, you should still use `<slot />` like before. In a future version, when Svelte removes its internal version of slots, it will leave those slots as-is, i.e. output a regular DOM tag instead of transforming it.
### Default content
In Svelte 4, the easiest way to pass a piece of UI to the child was using a `<slot />`. In Svelte 5, this is done using the `children` prop instead, which is then shown with `{@render children()}`:
```svelte
<script>
+++let { children } = $props();+++
</script>
---<slot />---
+++{@render children?.()}+++
```
### Multiple content placeholders
If you wanted multiple UI placeholders, you had to use named slots. In Svelte 5, use props instead, name them however you like and `{@render ...}` them:
```svelte
<script>
+++let { header, main, footer } = $props();+++
</script>
<header>
---<slot name="header" />---
+++{@render header()}+++
</header>
<main>
---<slot name="main" />---
+++{@render main()}+++
</main>
<footer>
---<slot name="footer" />---
+++{@render footer()}+++
</footer>
```
### Passing data back up
In Svelte 4, you would pass data to a `<slot />` and then retrieve it with `let:` in the parent component. In Svelte 5, snippets take on that responsibility:
```svelte
<!--- file: App.svelte --->
<script>
import List from './List.svelte';
</script>
<List items={['one', 'two', 'three']} ---let:item--->
+++{#snippet item(text)}+++
<span>{text}</span>
+++{/snippet}+++
---<span slot="empty">No items yet</span>---
+++{#snippet empty()}
<span>No items yet</span>
{/snippet}+++
</List>
```
```svelte
<!--- file: List.svelte --->
<script>
let { items, +++item, empty+++ } = $props();
</script>
{#if items.length}
<ul>
{#each items as entry}
<li>
---<slot item={entry} />---
+++{@render item(entry)}+++
</li>
{/each}
</ul>
{:else}
---<slot name="empty" />---
+++{@render empty?.()}+++
{/if}
```
> [!DETAILS] Why we did this
> Slots were easy to get started with, but the more advanced the use case became, the more involved and confusing the syntax became:
>
> - the `let:` syntax was confusing to many people as it _creates_ a variable whereas all other `:` directives _receive_ a variable
> - the scope of a variable declared with `let:` wasn't clear. In the example above, it may look like you can use the `item` slot prop in the `empty` slot, but that's not true
> - named slots had to be applied to an element using the `slot` attribute. Sometimes you didn't want to create an element, so we had to add the `<svelte:fragment>` API
> - named slots could also be applied to a component, which changed the semantics of where `let:` directives are available (even today us maintainers often don't know which way around it works)
>
> Snippets solve all of these problems by being much more readable and clear. At the same time they're more powerful as they allow you to define sections of UI that you can render _anywhere_, not just passing them as props to a component.
## Migration script
By now you should have a pretty good understanding of the before/after and how the old syntax relates to the new syntax. It probably also became clear that a lot of these migrations are rather technical and repetitive - something you don't want to do by hand.
We thought the same, which is why we provide a migration script to do most of the migration automatically. You can upgrade your project by using `npx sv migrate svelte-5`. This will do the following things:
- bump core dependencies in your `package.json`
- migrate to runes (`let` -> `$state` etc)
- migrate to event attributes for DOM elements (`on:click` -> `onclick`)
- migrate slot creations to render tags (`<slot />` -> `{@render children()}`)
- migrate slot usages to snippets (`<div slot="x">...</div>` -> `{#snippet x()}<div>...</div>{/snippet}`)
- migrate obvious component creations (`new Component(...)` -> `mount(Component, ...)`)
You can also migrate a single component in VS Code through the `Migrate Component to Svelte 5 Syntax` command, or in our Playground through the `Migrate` button.
Not everything can be migrated automatically, and some migrations need manual cleanup afterwards. The following sections describe these in more detail.
### run
You may see that the migration script converts some of your `$:` statements to a `run` function which is imported from `svelte/legacy`. This happens if the migration script couldn't reliably migrate the statement to a `$derived` and concluded this is a side effect instead. In some cases this may be wrong and it's best to change this to use a `$derived` instead. In other cases it may be right, but since `$:` statements also ran on the server but `$effect` does not, it isn't safe to transform it as such. Instead, `run` is used as a stopgap solution. `run` mimics most of the characteristics of `$:`, in that it runs on the server once, and runs as `$effect.pre` on the client (`$effect.pre` runs _before_ changes are applied to the DOM; most likely you want to use `$effect` instead).
```svelte
<script>
---import { run } from 'svelte/legacy';---
---run(() => {---
+++$effect(() => {+++
// some side effect code
})
</script>
```
### Event modifiers
Event modifiers are not applicable to event attributes (e.g. you can't do `onclick|preventDefault={...}`). Therefore, when migrating event directives to event attributes, we need a function-replacement for these modifiers. These are imported from `svelte/legacy`, and should be migrated away from in favor of e.g. just using `event.preventDefault()`.
```svelte
<script>
---import { preventDefault } from 'svelte/legacy';---
</script>
<button
onclick={---preventDefault---((event) => {
+++event.preventDefault();+++
// ...
})}
>
click me
</button>
```
### Things that are not automigrated
The migration script does not convert `createEventDispatcher`. You need to adjust those parts manually. It doesn't do it because it's too risky because it could result in breakage for users of the component, which the migration script cannot find out.
The migration script does not convert `beforeUpdate/afterUpdate`. It doesn't do it because it's impossible to determine the actual intent of the code. As a rule of thumb you can often go with a combination of `$effect.pre` (runs at the same time as `beforeUpdate` did) and `tick` (imported from `svelte`, allows you to wait until changes are applied to the DOM and then do some work).
## Components are no longer classes
In Svelte 3 and 4, components are classes. In Svelte 5 they are functions and should be instantiated differently. If you need to manually instantiate components, you should use `mount` or `hydrate` (imported from `svelte`) instead. If you see this error using SvelteKit, try updating to the latest version of SvelteKit first, which adds support for Svelte 5. If you're using Svelte without SvelteKit, you'll likely have a `main.js` file (or similar) which you need to adjust:
```js
+++import { mount } from 'svelte';+++
import App from './App.svelte'
---const app = new App({ target: document.getElementById("app") });---
+++const app = mount(App, { target: document.getElementById("app") });+++
export default app;
```
`mount` and `hydrate` have the exact same API. The difference is that `hydrate` will pick up the Svelte's server-rendered HTML inside its target and hydrate it. Both return an object with the exports of the component and potentially property accessors (if compiled with `accessors: true`). They do not come with the `$on`, `$set` and `$destroy` methods you may know from the class component API. These are its replacements:
For `$on`, instead of listening to events, pass them via the `events` property on the options argument.
```js
+++import { mount } from 'svelte';+++
import App from './App.svelte'
---const app = new App({ target: document.getElementById("app") });
app.$on('event', callback);---
+++const app = mount(App, { target: document.getElementById("app"), events: { event: callback } });+++
```
> [!NOTE] Note that using `events` is discouraged — instead, [use callbacks](#Event-changes)
For `$set`, use `$state` instead to create a reactive property object and manipulate it. If you're doing this inside a `.js` or `.ts` file, adjust the ending to include `.svelte`, i.e. `.svelte.js` or `.svelte.ts`.
```js
+++import { mount } from 'svelte';+++
import App from './App.svelte'
---const app = new App({ target: document.getElementById("app"), props: { foo: 'bar' } });
app.$set({ foo: 'baz' });---
+++const props = $state({ foo: 'bar' });
const app = mount(App, { target: document.getElementById("app"), props });
props.foo = 'baz';+++
```
For `$destroy`, use `unmount` instead.
```js
+++import { mount, unmount } from 'svelte';+++
import App from './App.svelte'
---const app = new App({ target: document.getElementById("app"), props: { foo: 'bar' } });
app.$destroy();---
+++const app = mount(App, { target: document.getElementById("app") });
unmount(app);+++
```
As a stop-gap-solution, you can also use `createClassComponent` or `asClassComponent` (imported from `svelte/legacy`) instead to keep the same API known from Svelte 4 after instantiating.
```js
+++import { createClassComponent } from 'svelte/legacy';+++
import App from './App.svelte'
---const app = new App({ target: document.getElementById("app") });---
+++const app = createClassComponent({ component: App, target: document.getElementById("app") });+++
export default app;
```
If this component is not under your control, you can use the `compatibility.componentApi` compiler option for auto-applied backwards compatibility, which means code using `new Component(...)` keeps working without adjustments (note that this adds a bit of overhead to each component). This will also add `$set` and `$on` methods for all component instances you get through `bind:this`.
```js
/// svelte.config.js
export default {
compilerOptions: {
compatibility: {
componentApi: 4
}
}
};
```
Note that `mount` and `hydrate` are _not_ synchronous, so things like `onMount` won't have been called by the time the function returns and the pending block of promises will not have been rendered yet (because `#await` waits a microtask to wait for a potentially immediately-resolved promise). If you need that guarantee, call `flushSync` (import from `'svelte'`) after calling `mount/hydrate`.
### Server API changes
Similarly, components no longer have a `render` method when compiled for server side rendering. Instead, pass the function to `render` from `svelte/server`:
```js
+++import { render } from 'svelte/server';+++
import App from './App.svelte';
---const { html, head } = App.render({ props: { message: 'hello' }});---
+++const { html, head } = render(App, { props: { message: 'hello' }});+++
```
In Svelte 4, rendering a component to a string also returned the CSS of all components. In Svelte 5, this is no longer the case by default because most of the time you're using a tooling chain that takes care of it in other ways (like SvelteKit). If you need CSS to be returned from `render`, you can set the `css` compiler option to `'injected'` and it will add `<style>` elements to the `head`.
### Component typing changes
The change from classes towards functions is also reflected in the typings: `SvelteComponent`, the base class from Svelte 4, is deprecated in favour of the new `Component` type which defines the function shape of a Svelte component. To manually define a component shape in a `d.ts` file:
```ts
import type { Component } from 'svelte';
export declare const MyComponent: Component<{
foo: string;
}>;
```
To declare that a component of a certain type is required:
```js
import { ComponentA, ComponentB } from 'component-library';
---import type { SvelteComponent } from 'svelte';---
+++import type { Component } from 'svelte';+++
---let C: typeof SvelteComponent<{ foo: string }> = $state(---
+++let C: Component<{ foo: string }> = $state(+++
Math.random() ? ComponentA : ComponentB
);
```
The two utility types `ComponentEvents` and `ComponentType` are also deprecated. `ComponentEvents` is obsolete because events are defined as callback props now, and `ComponentType` is obsolete because the new `Component` type is the component type already (i.e. `ComponentType<SvelteComponent<{ prop: string }>>` is equivalent to `Component<{ prop: string }>`).
### bind:this changes
Because components are no longer classes, using `bind:this` no longer returns a class instance with `$set`, `$on` and `$destroy` methods on it. It only returns the instance exports (`export function/const`) and, if you're using the `accessors` option, a getter/setter-pair for each property.
## `<svelte:component>` is no longer necessary
In Svelte 4, components are _static_ — if you render `<Thing>`, and the value of `Thing` changes, [nothing happens](/playground/7f1fa24f0ab44c1089dcbb03568f8dfa?version=4.2.18). To make it dynamic you had to use `<svelte:component>`.
This is no longer true in Svelte 5:
```svelte
<script>
import A from './A.svelte';
import B from './B.svelte';
let Thing = $state();
</script>
<select bind:value={Thing}>
<option value={A}>A</option>
<option value={B}>B</option>
</select>
<!-- these are equivalent -->
<Thing />
<svelte:component this={Thing} />
```
While migrating, keep in mind that your component's name should be capitalized (`Thing`) to distinguish it from elements, unless using dot notation.
### Dot notation indicates a component
In Svelte 4, `<foo.bar>` would create an element with a tag name of `"foo.bar"`. In Svelte 5, `foo.bar` is treated as a component instead. This is particularly useful inside `each` blocks:
```svelte
{#each items as item}
<item.component {...item.props} />
{/each}
```
## Whitespace handling changed
Previously, Svelte employed a very complicated algorithm to determine if whitespace should be kept or not. Svelte 5 simplifies this which makes it easier to reason about as a developer. The rules are:
- Whitespace between nodes is collapsed to one whitespace
- Whitespace at the start and end of a tag is removed completely
- Certain exceptions apply such as keeping whitespace inside `pre` tags
As before, you can disable whitespace trimming by setting the `preserveWhitespace` option in your compiler settings or on a per-component basis in `<svelte:options>`.
## Modern browser required
Svelte 5 requires a modern browser (in other words, not Internet Explorer) for various reasons:
- it uses [`Proxies`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy)
- elements with `clientWidth`/`clientHeight`/`offsetWidth`/`offsetHeight` bindings use a [`ResizeObserver`](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserver) rather than a convoluted `<iframe>` hack
- `<input type="range" bind:value={...} />` only uses an `input` event listener, rather than also listening for `change` events as a fallback
The `legacy` compiler option, which generated bulkier but IE-friendly code, no longer exists.
## Changes to compiler options
- The `false`/`true` (already deprecated previously) and the `"none"` values were removed as valid values from the `css` option
- The `legacy` option was repurposed
- The `hydratable` option has been removed. Svelte components are always hydratable now
- The `enableSourcemap` option has been removed. Source maps are always generated now, tooling can choose to ignore it
- The `tag` option was removed. Use `<svelte:options customElement="tag-name" />` inside the component instead
- The `loopGuardTimeout`, `format`, `sveltePath`, `errorMode` and `varsReport` options were removed
## The `children` prop is reserved
Content inside component tags becomes a snippet prop called `children`. You cannot have a separate prop by that name.
## Breaking changes in runes mode
Some breaking changes only apply once your component is in runes mode.
### Bindings to component exports are not allowed
Exports from runes mode components cannot be bound to directly. For example, having `export const foo = ...` in component `A` and then doing `<A bind:foo />` causes an error. Use `bind:this` instead — `<A bind:this={a} />` — and access the export as `a.foo`. This change makes things easier to reason about, as it enforces a clear separation between props and exports.
### Bindings need to be explicitly defined using `$bindable()`
In Svelte 4 syntax, every property (declared via `export let`) is bindable, meaning you can `bind:` to it. In runes mode, properties are not bindable by default: you need to denote bindable props with the `$bindable` rune.
If a bindable property has a default value (e.g. `let { foo = $bindable('bar') } = $props();`), you need to pass a non-`undefined` value to that property if you're binding to it. This prevents ambiguous behavior — the parent and child must have the same value — and results in better performance (in Svelte 4, the default value was reflected back to the parent, resulting in wasteful additional render cycles).
### `accessors` option is ignored
Setting the `accessors` option to `true` makes properties of a component directly accessible on the component instance. In runes mode, properties are never accessible on the component instance. You can use component exports instead if you need to expose them.
### `immutable` option is ignored
Setting the `immutable` option has no effect in runes mode. This concept is replaced by how `$state` and its variations work.
### Classes are no longer "auto-reactive"
In Svelte 4, doing the following triggered reactivity:
```svelte
<script>
let foo = new Foo();
</script>
<button on:click={() => (foo.value = 1)}>{foo.value}</button
>
```
This is because the Svelte compiler treated the assignment to `foo.value` as an instruction to update anything that referenced `foo`. In Svelte 5, reactivity is determined at runtime rather than compile time, so you should define `value` as a reactive `$state` field on the `Foo` class. Wrapping `new Foo()` with `$state(...)` will have no effect — only vanilla objects and arrays are made deeply reactive.
### Touch and wheel events are passive
When using `onwheel`, `onmousewheel`, `ontouchstart` and `ontouchmove` event attributes, the handlers are [passive](https://developer.mozilla.org/en-US/docs/Web/API/EventTarget/addEventListener#using_passive_listeners) to align with browser defaults. This greatly improves responsiveness by allowing the browser to scroll the document immediately, rather than waiting to see if the event handler calls `event.preventDefault()`.
In the very rare cases that you need to prevent these event defaults, you should use [`on`](/docs/svelte/svelte-events#on) instead (for example inside an action).
### Attribute/prop syntax is stricter
In Svelte 4, complex attribute values needn't be quoted:
<!-- prettier-ignore -->
```svelte
<Component prop=this{is}valid />
```
This is a footgun. In runes mode, if you want to concatenate stuff you must wrap the value in quotes:
```svelte
<Component prop="this{is}valid" />
```
Note that Svelte 5 will also warn if you have a single expression wrapped in quotes, like `answer="{42}"` — in Svelte 6, that will cause the value to be converted to a string, rather than passed as a number.
### HTML structure is stricter
In Svelte 4, you were allowed to write HTML code that would be repaired by the browser when server side rendering it. For example you could write this...
```svelte
<table>
<tr>
<td>hi</td>
</tr>
</table>
```
... and the browser would auto-insert a `<tbody>` element:
```svelte
<table>
<tbody>
<tr>
<td>hi</td>
</tr>
</tbody>
</table>
```
Svelte 5 is more strict about the HTML structure and will throw a compiler error in cases where the browser would repair the DOM.
## Other breaking changes
### Stricter `@const` assignment validation
Assignments to destructured parts of a `@const` declaration are no longer allowed. It was an oversight that this was ever allowed.
### :is(...) and :where(...) are scoped
Previously, Svelte did not analyse selectors inside `:is(...)` and `:where(...)`, effectively treating them as global. Svelte 5 analyses them in the context of the current component. As such, some selectors may now be treated as unused if they were relying on this treatment. To fix this, use `:global(...)` inside the `:is(...)/:where(...)` selectors.
When using Tailwind's `@apply` directive, add a `:global` selector to preserve rules that use Tailwind-generated `:is(...)` selectors:
<!-- prettier-ignore -->
```css
main +++:global+++ {
@apply bg-blue-100 dark:bg-blue-900;
}
```
### CSS hash position no longer deterministic
Previously Svelte would always insert the CSS hash last. This is no longer guaranteed in Svelte 5. This is only breaking if you [have very weird css selectors](https://stackoverflow.com/questions/15670631/does-the-order-of-classes-listed-on-an-item-affect-the-css).
### Scoped CSS uses :where(...)
To avoid issues caused by unpredictable specificity changes, scoped CSS selectors now use `:where(.svelte-xyz123)` selector modifiers alongside `.svelte-xyz123` (where `xyz123` is, as previously, a hash of the `<style>` contents). You can read more detail [here](https://github.com/sveltejs/svelte/pull/10443).
In the event that you need to support ancient browsers that don't implement `:where`, you can manually alter the emitted CSS, at the cost of unpredictable specificity changes:
```js
// @errors: 2552
css = css.replace(/:where\((.+?)\)/, '$1');
```
### Error/warning codes have been renamed
Error and warning codes have been renamed. Previously they used dashes to separate the words, they now use underscores (e.g. foo-bar becomes foo_bar). Additionally, a handful of codes have been reworded slightly.
### Reduced number of namespaces
The number of valid namespaces you can pass to the compiler option `namespace` has been reduced to `html` (the default), `mathml` and `svg`.
The `foreign` namespace was only useful for Svelte Native, which we're planning to support differently in a 5.x minor.
### beforeUpdate/afterUpdate changes
`beforeUpdate` no longer runs twice on initial render if it modifies a variable referenced in the template.
`afterUpdate` callbacks in a parent component will now run after `afterUpdate` callbacks in any child components.
`beforeUpdate/afterUpdate` no longer run when the component contains a `<slot>` and its content is updated.
Both functions are disallowed in runes mode — use `$effect.pre(...)` and `$effect(...)` instead.
### `contenteditable` behavior change
If you have a `contenteditable` node with a corresponding binding _and_ a reactive value inside it (example: `<div contenteditable=true bind:textContent>count is {count}</div>`), then the value inside the contenteditable will not be updated by updates to `count` because the binding takes full control over the content immediately and it should only be updated through it.
### `oneventname` attributes no longer accept string values
In Svelte 4, it was possible to specify event attributes on HTML elements as a string:
```svelte
<button onclick="alert('hello')">...</button>
```
This is not recommended, and is no longer possible in Svelte 5, where properties like `onclick` replace `on:click` as the mechanism for adding event handlers.
### `null` and `undefined` become the empty string
In Svelte 4, `null` and `undefined` were printed as the corresponding string. In 99 out of 100 cases you want this to become the empty string instead, which is also what most other frameworks out there do. Therefore, in Svelte 5, `null` and `undefined` become the empty string.
### `bind:files` values can only be `null`, `undefined` or `FileList`
`bind:files` is now a two-way binding. As such, when setting a value, it needs to be either falsy (`null` or `undefined`) or of type `FileList`.
### Bindings now react to form resets
Previously, bindings did not take into account `reset` event of forms, and therefore values could get out of sync with the DOM. Svelte 5 fixes this by placing a `reset` listener on the document and invoking bindings where necessary.
### `walk` no longer exported
`svelte/compiler` reexported `walk` from `estree-walker` for convenience. This is no longer true in Svelte 5, import it directly from that package instead in case you need it.
### Content inside `svelte:options` is forbidden
In Svelte 4 you could have content inside a `<svelte:options />` tag. It was ignored, but you could write something in there. In Svelte 5, content inside that tag is a compiler error.
### `<slot>` elements in declarative shadow roots are preserved
Svelte 4 replaced the `<slot />` tag in all places with its own version of slots. Svelte 5 preserves them in the case they are a child of a `<template shadowrootmode="...">` element.
### `<svelte:element>` tag must be an expression
In Svelte 4, `<svelte:element this="div">` is valid code. This makes little sense — you should just do `<div>`. In the vanishingly rare case that you _do_ need to use a literal value for some reason, you can do this:
```svelte
<svelte:element this=+++{+++"div"+++}+++>
```
Note that whereas Svelte 4 would treat `<svelte:element this="input">` (for example) identically to `<input>` for the purposes of determining which `bind:` directives could be applied, Svelte 5 does not.
### `mount` plays transitions by default
The `mount` function used to render a component tree plays transitions by default unless the `intro` option is set to `false`. This is different from legacy class components which, when manually instantiated, didn't play transitions by default.
### `<img src={...}>` and `{@html ...}` hydration mismatches are not repaired
In Svelte 4, if the value of a `src` attribute or `{@html ...}` tag differ between server and client (a.k.a. a hydration mismatch), the mismatch is repaired. This is very costly: setting a `src` attribute (even if it evaluates to the same thing) causes images and iframes to be reloaded, and reinserting a large blob of HTML is slow.
Since these mismatches are extremely rare, Svelte 5 assumes that the values are unchanged, but in development will warn you if they are not. To force an update you can do something like this:
```svelte
<script>
let { markup, src } = $props();
if (typeof window !== 'undefined') {
// stash the values...
const initial = { markup, src };
// unset them...
markup = src = undefined;
$effect(() => {
// ...and reset after we've mounted
markup = initial.markup;
src = initial.src;
});
}
</script>
{@html markup}
<img {src} />
```
### Hydration works differently
Svelte 5 makes use of comments during server side rendering which are used for more robust and efficient hydration on the client. As such, you shouldn't remove comments from your HTML output if you intend to hydrate it, and if you manually authored HTML to be hydrated by a Svelte component, you need to adjust that HTML to include said comments at the correct positions.
### `onevent` attributes are delegated
Event attributes replace event directives: Instead of `on:click={handler}` you write `onclick={handler}`. For backwards compatibility the `on:event` syntax is still supported and behaves the same as in Svelte 4. Some of the `onevent` attributes however are delegated, which means you need to take care to not stop event propagation on those manually, as they then might never reach the listener for this event type at the root.
### `--style-props` uses a different element
Svelte 5 uses an extra `<svelte-css-wrapper>` element instead of a `<div>` to wrap the component when using CSS custom properties.
<!-- TODO in final docs, add link to corresponding section for more details -->
# Frequently asked questions
## I'm new to Svelte. Where should I start?
We think the best way to get started is playing through the interactive [tutorial](/tutorial). Each step there is mainly focused on one specific aspect and is easy to follow. You'll be editing and running real Svelte components right in your browser.
Five to ten minutes should be enough to get you up and running. An hour and a half should get you through the entire tutorial.
## Where can I get support?
If your question is about certain syntax, the [reference docs](/docs/svelte) are a good place to start.
Stack Overflow is a popular forum to ask code-level questions or if you’re stuck with a specific error. Read through the existing questions tagged with [Svelte](https://stackoverflow.com/questions/tagged/svelte+or+svelte-3) or [ask your own](https://stackoverflow.com/questions/ask?tags=svelte)!
There are online forums and chats which are a great place for discussion about best practices, application architecture or just to get to know fellow Svelte users. [Our Discord](/chat) or [the Reddit channel](https://www.reddit.com/r/sveltejs/) are examples of that. If you have an answerable code-level question, Stack Overflow is usually a better fit.
## Are there any third-party resources?
Svelte Society maintains a [list of books and videos](https://sveltesociety.dev/resources).
## How can I get VS Code to syntax-highlight my .svelte files?
There is an [official VS Code extension for Svelte](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode).
## Is there a tool to automatically format my .svelte files?
You can use prettier with the [prettier-plugin-svelte](https://www.npmjs.com/package/prettier-plugin-svelte) plugin.
## How do I document my components?
In editors which use the Svelte Language Server you can document Components, functions and exports using specially formatted comments.
````svelte
<script>
/** What should we call the user? */
export let name = 'world';
</script>
<!--
@component
Here's some documentation for this component.
It will show up on hover.
- You can use markdown here.
- You can also use code blocks here.
- Usage:
```tsx
<main name="Arethra">
```
-->
<main>
<h1>
Hello, {name}
</h1>
</main>
````
Note: The `@component` is necessary in the HTML comment which describes your component.
## Does Svelte scale?
There will be a blog post about this eventually, but in the meantime, check out [this issue](https://github.com/sveltejs/svelte/issues/2546).
## Is there a UI component library?
There are several UI component libraries as well as standalone components. Find them under the [design systems section of the components page](https://sveltesociety.dev/packages?category=design-system) on the Svelte Society website.
## How do I test Svelte apps?
How your application is structured and where logic is defined will determine the best way to ensure it is properly tested. It is important to note that not all logic belongs within a component - this includes concerns such as data transformation, cross-component state management, and logging, among others. Remember that the Svelte library has its own test suite, so you do not need to write tests to validate implementation details provided by Svelte.
A Svelte application will typically have three different types of tests: Unit, Component, and End-to-End (E2E).
_Unit Tests_: Focus on testing business logic in isolation. Often this is validating individual functions and edge cases. By minimizing the surface area of these tests they can be kept lean and fast, and by extracting as much logic as possible from your Svelte components more of your application can be covered using them. When creating a new SvelteKit project, you will be asked whether you would like to setup [Vitest](https://vitest.dev/) for unit testing. There are a number of other test runners that could be used as well.
_Component Tests_: Validating that a Svelte component mounts and interacts as expected throughout its lifecycle requires a tool that provides a Document Object Model (DOM). Components can be compiled (since Svelte is a compiler and not a normal library) and mounted to allow asserting against element structure, listeners, state, and all the other capabilities provided by a Svelte component. Tools for component testing range from an in-memory implementation like jsdom paired with a test runner like [Vitest](https://vitest.dev/) to solutions that leverage an actual browser to provide a visual testing capability such as [Playwright](https://playwright.dev/docs/test-components) or [Cypress](https://www.cypress.io/).
_End-to-End Tests_: To ensure your users are able to interact with your application it is necessary to test it as a whole in a manner as close to production as possible. This is done by writing end-to-end (E2E) tests which load and interact with a deployed version of your application in order to simulate how the user will interact with your application. When creating a new SvelteKit project, you will be asked whether you would like to setup [Playwright](https://playwright.dev/) for end-to-end testing. There are many other E2E test libraries available for use as well.
Some resources for getting started with testing:
- [Svelte Testing Library](https://testing-library.com/docs/svelte-testing-library/example/)
- [Svelte Component Testing in Cypress](https://docs.cypress.io/guides/component-testing/svelte/overview)
- [Example using vitest](https://github.com/vitest-dev/vitest/tree/main/examples/sveltekit)
- [Example using uvu test runner with JSDOM](https://github.com/lukeed/uvu/tree/master/examples/svelte)
- [Test Svelte components using Vitest & Playwright](https://davipon.hashnode.dev/test-svelte-component-using-vitest-playwright)
- [Component testing with WebdriverIO](https://webdriver.io/docs/component-testing/svelte)
## Is there a router?
The official routing library is [SvelteKit](/docs/kit). SvelteKit provides a filesystem router, server-side rendering (SSR), and hot module reloading (HMR) in one easy-to-use package. It shares similarities with Next.js for React.
However, you can use any router library. A lot of people use [page.js](https://github.com/visionmedia/page.js). There's also [navaid](https://github.com/lukeed/navaid), which is very similar. And [universal-router](https://github.com/kriasoft/universal-router), which is isomorphic with child routes, but without built-in history support.
If you prefer a declarative HTML approach, there's the isomorphic [svelte-routing](https://github.com/EmilTholin/svelte-routing) library and a fork of it called [svelte-navigator](https://github.com/mefechoel/svelte-navigator) containing some additional functionality.
If you need hash-based routing on the client side, check out [svelte-spa-router](https://github.com/ItalyPaleAle/svelte-spa-router) or [abstract-state-router](https://github.com/TehShrike/abstract-state-router/).
[Routify](https://routify.dev) is another filesystem-based router, similar to SvelteKit's router. Version 3 supports Svelte's native SSR.
You can see a [community-maintained list of routers on sveltesociety.dev](https://sveltesociety.dev/packages?category=routers).
## How do I write a mobile app with Svelte?
While most mobile apps are written without using JavaScript, if you'd like to leverage your existing Svelte components and knowledge of Svelte when building mobile apps, you can turn a [SvelteKit SPA](https://kit.svelte.dev/docs/single-page-apps) into a mobile app with [Tauri](https://v2.tauri.app/start/frontend/sveltekit/) or [Capacitor](https://capacitorjs.com/solution/svelte). Mobile features like the camera, geolocation, and push notifications are available via plugins for both platforms.
Svelte Native was an option available for Svelte 4, but note that Svelte 5 does not currently support it. Svelte Native lets you write NativeScript apps using Svelte components that contain [NativeScript UI components](https://docs.nativescript.org/ui/) rather than DOM elements, which may be familiar for users coming from React Native.
## Can I tell Svelte not to remove my unused styles?
No. Svelte removes the styles from the component and warns you about them in order to prevent issues that would otherwise arise.
Svelte's component style scoping works by generating a class unique to the given component, adding it to the relevant elements in the component that are under Svelte's control, and then adding it to each of the selectors in that component's styles. When the compiler can't see what elements a style selector applies to, there would be two bad options for keeping it:
- If it keeps the selector and adds the scoping class to it, the selector will likely not match the expected elements in the component, and they definitely won't if they were created by a child component or `{@html ...}`.
- If it keeps the selector without adding the scoping class to it, the given style will become a global style, affecting your entire page.
If you need to style something that Svelte can't identify at compile time, you will need to explicitly opt into global styles by using `:global(...)`. But also keep in mind that you can wrap `:global(...)` around only part of a selector. `.foo :global(.bar) { ... }` will style any `.bar` elements that appear within the component's `.foo` elements. As long as there's some parent element in the current component to start from, partially global selectors like this will almost always be able to get you what you want.
## Is Svelte v2 still available?
New features aren't being added to it, and bugs will probably only be fixed if they are extremely nasty or present some sort of security vulnerability.
The documentation is still available [here](https://v2.svelte.dev/guide).
## How do I do hot module reloading?
We recommend using [SvelteKit](/docs/kit), which supports HMR out of the box and is built on top of [Vite](https://vitejs.dev/) and [svelte-hmr](https://github.com/sveltejs/svelte-hmr). There are also community plugins for [rollup](https://github.com/rixo/rollup-plugin-svelte-hot) and [webpack](https://github.com/sveltejs/svelte-loader).
# Reactivity in depth
- how to think about Runes ("just JavaScript" with added reactivity, what this means for keeping reactivity alive across boundaries)
- signals
# svelte
```js
// @noErrors
import {
SvelteComponent,
SvelteComponentTyped,
afterUpdate,
beforeUpdate,
createEventDispatcher,
createRawSnippet,
flushSync,
getAllContexts,
getContext,
hasContext,
hydrate,
mount,
onDestroy,
onMount,
setContext,
tick,
unmount,
untrack
} from 'svelte';
```
## SvelteComponent
This was the base class for Svelte components in Svelte 4. Svelte 5+ components
are completely different under the hood. For typing, use `Component` instead.
To instantiate components, use `mount` instead.
See [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more info.
<div class="ts-block">
```dts
class SvelteComponent<
Props extends Record<string, any> = Record<string, any>,
Events extends Record<string, any> = any,
Slots extends Record<string, any> = any
> {/*…*/}
```
<div class="ts-block-property">
```dts
static element?: typeof HTMLElement;
```
<div class="ts-block-property-details">
The custom element version of the component. Only present if compiled with the `customElement` compiler option
</div>
</div>
<div class="ts-block-property">
```dts
[prop: string]: any;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
constructor(options: ComponentConstructorOptions<Properties<Props, Slots>>);
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span> This constructor only exists when using the `asClassComponent` compatibility helper, which
is a stop-gap solution. Migrate towards using `mount` instead. See
[migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more info.
</div>
</div>
</div>
<div class="ts-block-property">
```dts
$destroy(): void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span> This method only exists when using one of the legacy compatibility helpers, which
is a stop-gap solution. See [migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
for more info.
</div>
</div>
</div>
<div class="ts-block-property">
```dts
$on<K extends Extract<keyof Events, string>>(
type: K,
callback: (e: Events[K]) => void
): () => void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span> This method only exists when using one of the legacy compatibility helpers, which
is a stop-gap solution. See [migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
for more info.
</div>
</div>
</div>
<div class="ts-block-property">
```dts
$set(props: Partial<Props>): void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span> This method only exists when using one of the legacy compatibility helpers, which
is a stop-gap solution. See [migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
for more info.
</div>
</div>
</div></div>
## SvelteComponentTyped
<blockquote class="tag deprecated note">
Use `Component` instead. See [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information.
</blockquote>
<div class="ts-block">
```dts
class SvelteComponentTyped<
Props extends Record<string, any> = Record<string, any>,
Events extends Record<string, any> = any,
Slots extends Record<string, any> = any
> extends SvelteComponent<Props, Events, Slots> {}
```
</div>
## afterUpdate
<blockquote class="tag deprecated note">
Use [`$effect`](/docs/svelte/$effect) instead
</blockquote>
Schedules a callback to run immediately after the component has been updated.
The first time the callback runs will be after the initial `onMount`.
In runes mode use `$effect` instead.
<div class="ts-block">
```dts
function afterUpdate(fn: () => void): void;
```
</div>
## beforeUpdate
<blockquote class="tag deprecated note">
Use [`$effect.pre`](/docs/svelte/$effect#$effect.pre) instead
</blockquote>
Schedules a callback to run immediately before the component is updated after any state change.
The first time the callback runs will be before the initial `onMount`.
In runes mode use `$effect.pre` instead.
<div class="ts-block">
```dts
function beforeUpdate(fn: () => void): void;
```
</div>
## createEventDispatcher
<blockquote class="tag deprecated note">
Use callback props and/or the `$host()` rune instead — see [migration guide](/docs/svelte/v5-migration-guide#Event-changes-Component-events)
</blockquote>
Creates an event dispatcher that can be used to dispatch [component events](/docs/svelte/legacy-on#Component-events).
Event dispatchers are functions that can take two arguments: `name` and `detail`.
Component events created with `createEventDispatcher` create a
[CustomEvent](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent).
These events do not [bubble](https://developer.mozilla.org/en-US/docs/Learn/JavaScript/Building_blocks/Events#Event_bubbling_and_capture).
The `detail` argument corresponds to the [CustomEvent.detail](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent/detail)
property and can contain any type of data.
The event dispatcher can be typed to narrow the allowed event names and the type of the `detail` argument:
```ts
const dispatch = createEventDispatcher<{
loaded: never; // does not take a detail argument
change: string; // takes a detail argument of type string, which is required
optional: number | null; // takes an optional detail argument of type number
}>();
```
<div class="ts-block">
```dts
function createEventDispatcher<
EventMap extends Record<string, any> = any
>(): EventDispatcher<EventMap>;
```
</div>
## createRawSnippet
Create a snippet programmatically
<div class="ts-block">
```dts
function createRawSnippet<Params extends unknown[]>(
fn: (...params: Getters<Params>) => {
render: () => string;
setup?: (element: Element) => void | (() => void);
}
): Snippet<Params>;
```
</div>
## flushSync
Synchronously flushes any pending state changes and those that result from it.
<div class="ts-block">
```dts
function flushSync(fn?: (() => void) | undefined): void;
```
</div>
## getAllContexts
Retrieves the whole context map that belongs to the closest parent component.
Must be called during component initialisation. Useful, for example, if you
programmatically create a component and want to pass the existing context to it.
<div class="ts-block">
```dts
function getAllContexts<
T extends Map<any, any> = Map<any, any>
>(): T;
```
</div>
## getContext
Retrieves the context that belongs to the closest parent component with the specified `key`.
Must be called during component initialisation.
<div class="ts-block">
```dts
function getContext<T>(key: any): T;
```
</div>
## hasContext
Checks whether a given `key` has been set in the context of a parent component.
Must be called during component initialisation.
<div class="ts-block">
```dts
function hasContext(key: any): boolean;
```
</div>
## hydrate
Hydrates a component on the given target and returns the exports and potentially the props (if compiled with `accessors: true`) of the component
<div class="ts-block">
```dts
function hydrate<
Props extends Record<string, any>,
Exports extends Record<string, any>
>(
component:
| ComponentType<SvelteComponent<Props>>
| Component<Props, Exports, any>,
options: {} extends Props
? {
target: Document | Element | ShadowRoot;
props?: Props;
events?: Record<string, (e: any) => any>;
context?: Map<any, any>;
intro?: boolean;
recover?: boolean;
}
: {
target: Document | Element | ShadowRoot;
props: Props;
events?: Record<string, (e: any) => any>;
context?: Map<any, any>;
intro?: boolean;
recover?: boolean;
}
): Exports;
```
</div>
## mount
Mounts a component to the given target and returns the exports and potentially the props (if compiled with `accessors: true`) of the component.
Transitions will play during the initial render unless the `intro` option is set to `false`.
<div class="ts-block">
```dts
function mount<
Props extends Record<string, any>,
Exports extends Record<string, any>
>(
component:
| ComponentType<SvelteComponent<Props>>
| Component<Props, Exports, any>,
options: MountOptions<Props>
): Exports;
```
</div>
## onDestroy
Schedules a callback to run immediately before the component is unmounted.
Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the
only one that runs inside a server-side component.
<div class="ts-block">
```dts
function onDestroy(fn: () => any): void;
```
</div>
## onMount
The `onMount` function schedules a callback to run as soon as the component has been mounted to the DOM.
It must be called during the component's initialisation (but doesn't need to live *inside* the component;
it can be called from an external module).
If a function is returned _synchronously_ from `onMount`, it will be called when the component is unmounted.
`onMount` does not run inside [server-side components](/docs/svelte/svelte-server#render).
<div class="ts-block">
```dts
function onMount<T>(
fn: () =>
| NotFunction<T>
| Promise<NotFunction<T>>
| (() => any)
): void;
```
</div>
## setContext
Associates an arbitrary `context` object with the current component and the specified `key`
and returns that object. The context is then available to children of the component
(including slotted content) with `getContext`.
Like lifecycle functions, this must be called during component initialisation.
<div class="ts-block">
```dts
function setContext<T>(key: any, context: T): T;
```
</div>
## tick
Returns a promise that resolves once any pending state changes have been applied.
<div class="ts-block">
```dts
function tick(): Promise<void>;
```
</div>
## unmount
Unmounts a component that was previously mounted using `mount` or `hydrate`.
Since 5.13.0, if `options.outro` is `true`, [transitions](/docs/svelte/transition) will play before the component is removed from the DOM.
Returns a `Promise` that resolves after transitions have completed if `options.outro` is true, or immediately otherwise (prior to 5.13.0, returns `void`).
```js
// @errors: 7031
import { mount, unmount } from 'svelte';
import App from './App.svelte';
const app = mount(App, { target: document.body });
// later...
unmount(app, { outro: true });
```
<div class="ts-block">
```dts
function unmount(
component: Record<string, any>,
options?:
| {
outro?: boolean;
}
| undefined
): Promise<void>;
```
</div>
## untrack
When used inside a [`$derived`](/docs/svelte/$derived) or [`$effect`](/docs/svelte/$effect),
any state read inside `fn` will not be treated as a dependency.
```ts
$effect(() => {
// this will run when `data` changes, but not when `time` changes
save(data, {
timestamp: untrack(() => time)
});
});
```
<div class="ts-block">
```dts
function untrack<T>(fn: () => T): T;
```
</div>
## Component
Can be used to create strongly typed Svelte components.
#### Example:
You have component library on npm called `component-library`, from which
you export a component called `MyComponent`. For Svelte+TypeScript users,
you want to provide typings. Therefore you create a `index.d.ts`:
```ts
import type { Component } from 'svelte';
export declare const MyComponent: Component<{ foo: string }> {}
```
Typing this makes it possible for IDEs like VS Code with the Svelte extension
to provide intellisense and to use the component like this in a Svelte file
with TypeScript:
```svelte
<script lang="ts">
import { MyComponent } from "component-library";
</script>
<MyComponent foo={'bar'} />
```
<div class="ts-block">
```dts
interface Component<
Props extends Record<string, any> = {},
Exports extends Record<string, any> = {},
Bindings extends keyof Props | '' = string
> {/*…*/}
```
<div class="ts-block-property">
```dts
(
this: void,
internals: ComponentInternals,
props: Props
): {
/**
* @deprecated This method only exists when using one of the legacy compatibility helpers, which
* is a stop-gap solution. See [migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
* for more info.
*/
$on?(type: string, callback: (e: any) => void): () => void;
/**
* @deprecated This method only exists when using one of the legacy compatibility helpers, which
* is a stop-gap solution. See [migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
* for more info.
*/
$set?(props: Partial<Props>): void;
} & Exports;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `internal` An internal object used by Svelte. Do not use or modify.
- `props` The props passed to the component.
</div>
</div>
</div>
<div class="ts-block-property">
```dts
element?: typeof HTMLElement;
```
<div class="ts-block-property-details">
The custom element version of the component. Only present if compiled with the `customElement` compiler option
</div>
</div></div>
## ComponentConstructorOptions
<blockquote class="tag deprecated note">
In Svelte 4, components are classes. In Svelte 5, they are functions.
Use `mount` instead to instantiate components.
See [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
for more info.
</blockquote>
<div class="ts-block">
```dts
interface ComponentConstructorOptions<
Props extends Record<string, any> = Record<string, any>
> {/*…*/}
```
<div class="ts-block-property">
```dts
target: Element | Document | ShadowRoot;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
anchor?: Element;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
props?: Props;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
context?: Map<any, any>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
hydrate?: boolean;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
intro?: boolean;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
recover?: boolean;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
sync?: boolean;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
$$inline?: boolean;
```
<div class="ts-block-property-details"></div>
</div></div>
## ComponentEvents
<blockquote class="tag deprecated note">
The new `Component` type does not have a dedicated Events type. Use `ComponentProps` instead.
</blockquote>
<div class="ts-block">
```dts
type ComponentEvents<Comp extends SvelteComponent> =
Comp extends SvelteComponent<any, infer Events>
? Events
: never;
```
</div>
## ComponentInternals
Internal implementation details that vary between environments
<div class="ts-block">
```dts
type ComponentInternals = Branded<{}, 'ComponentInternals'>;
```
</div>
## ComponentProps
Convenience type to get the props the given component expects.
Example: Ensure a variable contains the props expected by `MyComponent`:
```ts
import type { ComponentProps } from 'svelte';
import MyComponent from './MyComponent.svelte';
// Errors if these aren't the correct props expected by MyComponent.
const props: ComponentProps<typeof MyComponent> = { foo: 'bar' };
```
> [!NOTE] In Svelte 4, you would do `ComponentProps<MyComponent>` because `MyComponent` was a class.
Example: A generic function that accepts some component and infers the type of its props:
```ts
import type { Component, ComponentProps } from 'svelte';
import MyComponent from './MyComponent.svelte';
function withProps<TComponent extends Component<any>>(
component: TComponent,
props: ComponentProps<TComponent>
) {};
// Errors if the second argument is not the correct props expected by the component in the first argument.
withProps(MyComponent, { foo: 'bar' });
```
<div class="ts-block">
```dts
type ComponentProps<
Comp extends SvelteComponent | Component<any, any>
> =
Comp extends SvelteComponent<infer Props>
? Props
: Comp extends Component<infer Props, any>
? Props
: never;
```
</div>
## ComponentType
<blockquote class="tag deprecated note">
This type is obsolete when working with the new `Component` type.
</blockquote>
<div class="ts-block">
```dts
type ComponentType<
Comp extends SvelteComponent = SvelteComponent
> = (new (
options: ComponentConstructorOptions<
Comp extends SvelteComponent<infer Props>
? Props
: Record<string, any>
>
) => Comp) & {
/** The custom element version of the component. Only present if compiled with the `customElement` compiler option */
element?: typeof HTMLElement;
};
```
</div>
## EventDispatcher
<div class="ts-block">
```dts
interface EventDispatcher<
EventMap extends Record<string, any>
> {/*…*/}
```
<div class="ts-block-property">
```dts
<Type extends keyof EventMap>(
...args: null extends EventMap[Type]
? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
: undefined extends EventMap[Type]
? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
: [type: Type, parameter: EventMap[Type], options?: DispatchOptions]
): boolean;
```
<div class="ts-block-property-details"></div>
</div></div>
## MountOptions
Defines the options accepted by the `mount()` function.
<div class="ts-block">
```dts
type MountOptions<
Props extends Record<string, any> = Record<string, any>
> = {
/**
* Target element where the component will be mounted.
*/
target: Document | Element | ShadowRoot;
/**
* Optional node inside `target`. When specified, it is used to render the component immediately before it.
*/
anchor?: Node;
/**
* Allows the specification of events.
* @deprecated Use callback props instead.
*/
events?: Record<string, (e: any) => any>;
/**
* Can be accessed via `getContext()` at the component level.
*/
context?: Map<any, any>;
/**
* Whether or not to play transitions on initial render.
* @default true
*/
intro?: boolean;
} & ({} extends Props
? {
/**
* Component properties.
*/
props?: Props;
}
: {
/**
* Component properties.
*/
props: Props;
});
```
</div>
## Snippet
The type of a `#snippet` block. You can use it to (for example) express that your component expects a snippet of a certain type:
```ts
let { banner }: { banner: Snippet<[{ text: string }]> } = $props();
```
You can only call a snippet through the `{@render ...}` tag.
See the [snippet documentation](/docs/svelte/snippet) for more info.
<div class="ts-block">
```dts
interface Snippet<Parameters extends unknown[] = []> {/*…*/}
```
<div class="ts-block-property">
```dts
(
this: void,
// this conditional allows tuples but not arrays. Arrays would indicate a
// rest parameter type, which is not supported. If rest parameters are added
// in the future, the condition can be removed.
...args: number extends Parameters['length'] ? never : Parameters
): {
'{@render ...} must be called with a Snippet': "import type { Snippet } from 'svelte'";
} & typeof SnippetReturn;
```
<div class="ts-block-property-details"></div>
</div></div>
# svelte/action
## Action
Actions are functions that are called when an element is created.
You can use this interface to type such actions.
The following example defines an action that only works on `<div>` elements
and optionally accepts a parameter which it has a default value for:
```ts
export const myAction: Action<HTMLDivElement, { someProperty: boolean } | undefined> = (node, param = { someProperty: true }) => {
// ...
}
```
`Action<HTMLDivElement>` and `Action<HTMLDivElement, undefined>` both signal that the action accepts no parameters.
You can return an object with methods `update` and `destroy` from the function and type which additional attributes and events it has.
See interface `ActionReturn` for more details.
<div class="ts-block">
```dts
interface Action<
Element = HTMLElement,
Parameter = undefined,
Attributes extends Record<string, any> = Record<
never,
any
>
> {/*…*/}
```
<div class="ts-block-property">
```dts
<Node extends Element>(
...args: undefined extends Parameter
? [node: Node, parameter?: Parameter]
: [node: Node, parameter: Parameter]
): void | ActionReturn<Parameter, Attributes>;
```
<div class="ts-block-property-details"></div>
</div></div>
## ActionReturn
Actions can return an object containing the two properties defined in this interface. Both are optional.
- update: An action can have a parameter. This method will be called whenever that parameter changes,
immediately after Svelte has applied updates to the markup. `ActionReturn` and `ActionReturn<undefined>` both
mean that the action accepts no parameters.
- destroy: Method that is called after the element is unmounted
Additionally, you can specify which additional attributes and events the action enables on the applied element.
This applies to TypeScript typings only and has no effect at runtime.
Example usage:
```ts
interface Attributes {
newprop?: string;
'on:event': (e: CustomEvent<boolean>) => void;
}
export function myAction(node: HTMLElement, parameter: Parameter): ActionReturn<Parameter, Attributes> {
// ...
return {
update: (updatedParameter) => {...},
destroy: () => {...}
};
}
```
<div class="ts-block">
```dts
interface ActionReturn<
Parameter = undefined,
Attributes extends Record<string, any> = Record<
never,
any
>
> {/*…*/}
```
<div class="ts-block-property">
```dts
update?: (parameter: Parameter) => void;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
destroy?: () => void;
```
<div class="ts-block-property-details"></div>
</div></div>
# svelte/animate
```js
// @noErrors
import { flip } from 'svelte/animate';
```
## flip
The flip function calculates the start and end position of an element and animates between them, translating the x and y values.
`flip` stands for [First, Last, Invert, Play](https://aerotwist.com/blog/flip-your-animations/).
<div class="ts-block">
```dts
function flip(
node: Element,
{
from,
to
}: {
from: DOMRect;
to: DOMRect;
},
params?: FlipParams
): AnimationConfig;
```
</div>
## AnimationConfig
<div class="ts-block">
```dts
interface AnimationConfig {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: (t: number) => number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
css?: (t: number, u: number) => string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
tick?: (t: number, u: number) => void;
```
<div class="ts-block-property-details"></div>
</div></div>
## FlipParams
<div class="ts-block">
```dts
interface FlipParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number | ((len: number) => number);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: (t: number) => number;
```
<div class="ts-block-property-details"></div>
</div></div>
# svelte/compiler
```js
// @noErrors
import {
VERSION,
compile,
compileModule,
migrate,
parse,
preprocess,
walk
} from 'svelte/compiler';
```
## VERSION
The current version, as set in package.json.
<div class="ts-block">
```dts
const VERSION: string;
```
</div>
## compile
`compile` converts your `.svelte` source code into a JavaScript module that exports a component
<div class="ts-block">
```dts
function compile(
source: string,
options: CompileOptions
): CompileResult;
```
</div>
## compileModule
`compileModule` takes your JavaScript source code containing runes, and turns it into a JavaScript module.
<div class="ts-block">
```dts
function compileModule(
source: string,
options: ModuleCompileOptions
): CompileResult;
```
</div>
## migrate
Does a best-effort migration of Svelte code towards using runes, event attributes and render tags.
May throw an error if the code is too complex to migrate automatically.
<div class="ts-block">
```dts
function migrate(
source: string,
{
filename,
use_ts
}?:
| {
filename?: string;
use_ts?: boolean;
}
| undefined
): {
code: string;
};
```
</div>
## parse
The parse function parses a component, returning only its abstract syntax tree.
The `modern` option (`false` by default in Svelte 5) makes the parser return a modern AST instead of the legacy AST.
`modern` will become `true` by default in Svelte 6, and the option will be removed in Svelte 7.
<div class="ts-block">
```dts
function parse(
source: string,
options: {
filename?: string;
modern: true;
loose?: boolean;
}
): AST.Root;
```
</div>
<div class="ts-block">
```dts
function parse(
source: string,
options?:
| {
filename?: string;
modern?: false;
loose?: boolean;
}
| undefined
): Record<string, any>;
```
</div>
## preprocess
The preprocess function provides convenient hooks for arbitrarily transforming component source code.
For example, it can be used to convert a `<style lang="sass">` block into vanilla CSS.
<div class="ts-block">
```dts
function preprocess(
source: string,
preprocessor: PreprocessorGroup | PreprocessorGroup[],
options?:
| {
filename?: string;
}
| undefined
): Promise<Processed>;
```
</div>
## walk
<blockquote class="tag deprecated note">
Replace this with `import { walk } from 'estree-walker'`
</blockquote>
<div class="ts-block">
```dts
function walk(): never;
```
</div>
## AST
<div class="ts-block">
```dts
namespace AST {
export interface BaseNode {
type: string;
start: number;
end: number;
}
export interface Fragment {
type: 'Fragment';
nodes: Array<
Text | Tag | ElementLike | Block | Comment
>;
}
export interface Root extends BaseNode {
type: 'Root';
/**
* Inline options provided by `<svelte:options>` — these override options passed to `compile(...)`
*/
options: SvelteOptions | null;
fragment: Fragment;
/** The parsed `<style>` element, if exists */
css: AST.CSS.StyleSheet | null;
/** The parsed `<script>` element, if exists */
instance: Script | null;
/** The parsed `<script module>` element, if exists */
module: Script | null;
}
export interface SvelteOptions {
// start/end info (needed for warnings and for our Prettier plugin)
start: number;
end: number;
// options
runes?: boolean;
immutable?: boolean;
accessors?: boolean;
preserveWhitespace?: boolean;
namespace?: Namespace;
css?: 'injected';
customElement?: {
tag?: string;
shadow?: 'open' | 'none';
props?: Record<
string,
{
attribute?: string;
reflect?: boolean;
type?:
| 'Array'
| 'Boolean'
| 'Number'
| 'Object'
| 'String';
}
>;
/**
* Is of type
* ```ts
* (ceClass: new () => HTMLElement) => new () => HTMLElement
* ```
*/
extend?: ArrowFunctionExpression | Identifier;
};
attributes: Attribute[];
}
/** Static text */
export interface Text extends BaseNode {
type: 'Text';
/** Text with decoded HTML entities */
data: string;
/** The original text, with undecoded HTML entities */
raw: string;
}
/** A (possibly reactive) template expression — `{...}` */
export interface ExpressionTag extends BaseNode {
type: 'ExpressionTag';
expression: Expression;
}
/** A (possibly reactive) HTML template expression — `{@html ...}` */
export interface HtmlTag extends BaseNode {
type: 'HtmlTag';
expression: Expression;
}
/** An HTML comment */
// TODO rename to disambiguate
export interface Comment extends BaseNode {
type: 'Comment';
/** the contents of the comment */
data: string;
}
/** A `{@const ...}` tag */
export interface ConstTag extends BaseNode {
type: 'ConstTag';
declaration: VariableDeclaration & {
declarations: [
VariableDeclarator & {
id: Pattern;
init: Expression;
}
];
};
}
/** A `{@debug ...}` tag */
export interface DebugTag extends BaseNode {
type: 'DebugTag';
identifiers: Identifier[];
}
/** A `{@render foo(...)} tag */
export interface RenderTag extends BaseNode {
type: 'RenderTag';
expression:
| SimpleCallExpression
| (ChainExpression & {
expression: SimpleCallExpression;
});
}
/** An `animate:` directive */
export interface AnimateDirective extends BaseNode {
type: 'AnimateDirective';
/** The 'x' in `animate:x` */
name: string;
/** The y in `animate:x={y}` */
expression: null | Expression;
}
/** A `bind:` directive */
export interface BindDirective extends BaseNode {
type: 'BindDirective';
/** The 'x' in `bind:x` */
name: string;
/** The y in `bind:x={y}` */
expression:
| Identifier
| MemberExpression
| SequenceExpression;
}
/** A `class:` directive */
export interface ClassDirective extends BaseNode {
type: 'ClassDirective';
/** The 'x' in `class:x` */
name: 'class';
/** The 'y' in `class:x={y}`, or the `x` in `class:x` */
expression: Expression;
}
/** A `let:` directive */
export interface LetDirective extends BaseNode {
type: 'LetDirective';
/** The 'x' in `let:x` */
name: string;
/** The 'y' in `let:x={y}` */
expression:
| null
| Identifier
| ArrayExpression
| ObjectExpression;
}
/** An `on:` directive */
export interface OnDirective extends BaseNode {
type: 'OnDirective';
/** The 'x' in `on:x` */
name: string;
/** The 'y' in `on:x={y}` */
expression: null | Expression;
modifiers: string[];
}
/** A `style:` directive */
export interface StyleDirective extends BaseNode {
type: 'StyleDirective';
/** The 'x' in `style:x` */
name: string;
/** The 'y' in `style:x={y}` */
value:
| true
| ExpressionTag
| Array<ExpressionTag | Text>;
modifiers: Array<'important'>;
}
// TODO have separate in/out/transition directives
/** A `transition:`, `in:` or `out:` directive */
export interface TransitionDirective extends BaseNode {
type: 'TransitionDirective';
/** The 'x' in `transition:x` */
name: string;
/** The 'y' in `transition:x={y}` */
expression: null | Expression;
modifiers: Array<'local' | 'global'>;
/** True if this is a `transition:` or `in:` directive */
intro: boolean;
/** True if this is a `transition:` or `out:` directive */
outro: boolean;
}
/** A `use:` directive */
export interface UseDirective extends BaseNode {
type: 'UseDirective';
/** The 'x' in `use:x` */
name: string;
/** The 'y' in `use:x={y}` */
expression: null | Expression;
}
interface BaseElement extends BaseNode {
name: string;
attributes: Array<
Attribute | SpreadAttribute | Directive
>;
fragment: Fragment;
}
export interface Component extends BaseElement {
type: 'Component';
}
export interface TitleElement extends BaseElement {
type: 'TitleElement';
name: 'title';
}
export interface SlotElement extends BaseElement {
type: 'SlotElement';
name: 'slot';
}
export interface RegularElement extends BaseElement {
type: 'RegularElement';
}
export interface SvelteBody extends BaseElement {
type: 'SvelteBody';
name: 'svelte:body';
}
export interface SvelteComponent extends BaseElement {
type: 'SvelteComponent';
name: 'svelte:component';
expression: Expression;
}
export interface SvelteDocument extends BaseElement {
type: 'SvelteDocument';
name: 'svelte:document';
}
export interface SvelteElement extends BaseElement {
type: 'SvelteElement';
name: 'svelte:element';
tag: Expression;
}
export interface SvelteFragment extends BaseElement {
type: 'SvelteFragment';
name: 'svelte:fragment';
}
export interface SvelteBoundary extends BaseElement {
type: 'SvelteBoundary';
name: 'svelte:boundary';
}
export interface SvelteHead extends BaseElement {
type: 'SvelteHead';
name: 'svelte:head';
}
/** This is only an intermediate representation while parsing, it doesn't exist in the final AST */
export interface SvelteOptionsRaw extends BaseElement {
type: 'SvelteOptions';
name: 'svelte:options';
}
export interface SvelteSelf extends BaseElement {
type: 'SvelteSelf';
name: 'svelte:self';
}
export interface SvelteWindow extends BaseElement {
type: 'SvelteWindow';
name: 'svelte:window';
}
/** An `{#each ...}` block */
export interface EachBlock extends BaseNode {
type: 'EachBlock';
expression: Expression;
/** The `entry` in `{#each item as entry}`. `null` if `as` part is omitted */
context: Pattern | null;
body: Fragment;
fallback?: Fragment;
index?: string;
key?: Expression;
}
/** An `{#if ...}` block */
export interface IfBlock extends BaseNode {
type: 'IfBlock';
elseif: boolean;
test: Expression;
consequent: Fragment;
alternate: Fragment | null;
}
/** An `{#await ...}` block */
export interface AwaitBlock extends BaseNode {
type: 'AwaitBlock';
expression: Expression;
// TODO can/should we move these inside the ThenBlock and CatchBlock?
/** The resolved value inside the `then` block */
value: Pattern | null;
/** The rejection reason inside the `catch` block */
error: Pattern | null;
pending: Fragment | null;
then: Fragment | null;
catch: Fragment | null;
}
export interface KeyBlock extends BaseNode {
type: 'KeyBlock';
expression: Expression;
fragment: Fragment;
}
export interface SnippetBlock extends BaseNode {
type: 'SnippetBlock';
expression: Identifier;
parameters: Pattern[];
body: Fragment;
}
export interface Attribute extends BaseNode {
type: 'Attribute';
name: string;
/**
* Quoted/string values are represented by an array, even if they contain a single expression like `"{x}"`
*/
value:
| true
| ExpressionTag
| Array<Text | ExpressionTag>;
}
export interface SpreadAttribute extends BaseNode {
type: 'SpreadAttribute';
expression: Expression;
}
export interface Script extends BaseNode {
type: 'Script';
context: 'default' | 'module';
content: Program;
attributes: Attribute[];
}
export type AttributeLike =
| Attribute
| SpreadAttribute
| Directive;
export type Directive =
| AST.AnimateDirective
| AST.BindDirective
| AST.ClassDirective
| AST.LetDirective
| AST.OnDirective
| AST.StyleDirective
| AST.TransitionDirective
| AST.UseDirective;
export type Block =
| AST.EachBlock
| AST.IfBlock
| AST.AwaitBlock
| AST.KeyBlock
| AST.SnippetBlock;
export type ElementLike =
| AST.Component
| AST.TitleElement
| AST.SlotElement
| AST.RegularElement
| AST.SvelteBody
| AST.SvelteBoundary
| AST.SvelteComponent
| AST.SvelteDocument
| AST.SvelteElement
| AST.SvelteFragment
| AST.SvelteHead
| AST.SvelteOptionsRaw
| AST.SvelteSelf
| AST.SvelteWindow
| AST.SvelteBoundary;
export type Tag =
| AST.ExpressionTag
| AST.HtmlTag
| AST.ConstTag
| AST.DebugTag
| AST.RenderTag;
export type TemplateNode =
| AST.Root
| AST.Text
| Tag
| ElementLike
| AST.Attribute
| AST.SpreadAttribute
| Directive
| AST.Comment
| Block;
export type SvelteNode =
| Node
| TemplateNode
| AST.Fragment
| _CSS.Node;
export type { _CSS as CSS };
}
```
</div>
## CompileError
<div class="ts-block">
```dts
interface CompileError extends ICompileDiagnostic {}
```
</div>
## CompileOptions
<div class="ts-block">
```dts
interface CompileOptions extends ModuleCompileOptions {/*…*/}
```
<div class="ts-block-property">
```dts
name?: string;
```
<div class="ts-block-property-details">
Sets the name of the resulting JavaScript class (though the compiler will rename it if it would otherwise conflict with other variables in scope).
If unspecified, will be inferred from `filename`
</div>
</div>
<div class="ts-block-property">
```dts
customElement?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
</div>
If `true`, tells the compiler to generate a custom element constructor instead of a regular Svelte component.
</div>
</div>
<div class="ts-block-property">
```dts
accessors?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
- <span class="tag deprecated">deprecated</span> This will have no effect in runes mode
</div>
If `true`, getters and setters will be created for the component's props. If `false`, they will only be created for readonly exported values (i.e. those declared with `const`, `class` and `function`). If compiling with `customElement: true` this option defaults to `true`.
</div>
</div>
<div class="ts-block-property">
```dts
namespace?: Namespace;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `'html'`
</div>
The namespace of the element; e.g., `"html"`, `"svg"`, `"mathml"`.
</div>
</div>
<div class="ts-block-property">
```dts
immutable?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
- <span class="tag deprecated">deprecated</span> This will have no effect in runes mode
</div>
If `true`, tells the compiler that you promise not to mutate any objects.
This allows it to be less conservative about checking whether values have changed.
</div>
</div>
<div class="ts-block-property">
```dts
css?: 'injected' | 'external';
```
<div class="ts-block-property-details">
- `'injected'`: styles will be included in the `head` when using `render(...)`, and injected into the document (if not already present) when the component mounts. For components compiled as custom elements, styles are injected to the shadow root.
- `'external'`: the CSS will only be returned in the `css` field of the compilation result. Most Svelte bundler plugins will set this to `'external'` and use the CSS that is statically generated for better performance, as it will result in smaller JavaScript bundles and the output can be served as cacheable `.css` files.
This is always `'injected'` when compiling with `customElement` mode.
</div>
</div>
<div class="ts-block-property">
```dts
cssHash?: CssHashGetter;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `undefined`
</div>
A function that takes a `{ hash, css, name, filename }` argument and returns the string that is used as a classname for scoped CSS.
It defaults to returning `svelte-${hash(css)}`.
</div>
</div>
<div class="ts-block-property">
```dts
preserveComments?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
</div>
If `true`, your HTML comments will be preserved in the output. By default, they are stripped out.
</div>
</div>
<div class="ts-block-property">
```dts
preserveWhitespace?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
</div>
If `true`, whitespace inside and between elements is kept as you typed it, rather than removed or collapsed to a single space where possible.
</div>
</div>
<div class="ts-block-property">
```dts
runes?: boolean | undefined;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `undefined`
</div>
Set to `true` to force the compiler into runes mode, even if there are no indications of runes usage.
Set to `false` to force the compiler into ignoring runes, even if there are indications of runes usage.
Set to `undefined` (the default) to infer runes mode from the component code.
Is always `true` for JS/TS modules compiled with Svelte.
Will be `true` by default in Svelte 6.
Note that setting this to `true` in your `svelte.config.js` will force runes mode for your entire project, including components in `node_modules`,
which is likely not what you want. If you're using Vite, consider using [dynamicCompileOptions](https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/config.md#dynamiccompileoptions) instead.
</div>
</div>
<div class="ts-block-property">
```dts
discloseVersion?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `true`
</div>
If `true`, exposes the Svelte major version in the browser by adding it to a `Set` stored in the global `window.__svelte.v`.
</div>
</div>
<div class="ts-block-property">
```dts
compatibility?: {/*…*/}
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span> Use these only as a temporary solution before migrating your code
</div>
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
componentApi?: 4 | 5;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `5`
</div>
Applies a transformation so that the default export of Svelte files can still be instantiated the same way as in Svelte 4 —
as a class when compiling for the browser (as though using `createClassComponent(MyComponent, {...})` from `svelte/legacy`)
or as an object with a `.render(...)` method when compiling for the server
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```dts
sourcemap?: object | string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `null`
</div>
An initial sourcemap that will be merged into the final output sourcemap.
This is usually the preprocessor sourcemap.
</div>
</div>
<div class="ts-block-property">
```dts
outputFilename?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `null`
</div>
Used for your JavaScript sourcemap.
</div>
</div>
<div class="ts-block-property">
```dts
cssOutputFilename?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `null`
</div>
Used for your CSS sourcemap.
</div>
</div>
<div class="ts-block-property">
```dts
hmr?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
</div>
If `true`, compiles components with hot reloading support.
</div>
</div>
<div class="ts-block-property">
```dts
modernAst?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
</div>
If `true`, returns the modern version of the AST.
Will become `true` by default in Svelte 6, and the option will be removed in Svelte 7.
</div>
</div></div>
## CompileResult
The return value of `compile` from `svelte/compiler`
<div class="ts-block">
```dts
interface CompileResult {/*…*/}
```
<div class="ts-block-property">
```dts
js: {/*…*/}
```
<div class="ts-block-property-details">
The compiled JavaScript
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
code: string;
```
<div class="ts-block-property-details">
The generated code
</div>
</div>
<div class="ts-block-property">
```dts
map: SourceMap;
```
<div class="ts-block-property-details">
A source map
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```dts
css: null | {
/** The generated code */
code: string;
/** A source map */
map: SourceMap;
};
```
<div class="ts-block-property-details">
The compiled CSS
</div>
</div>
<div class="ts-block-property">
```dts
warnings: Warning[];
```
<div class="ts-block-property-details">
An array of warning objects that were generated during compilation. Each warning has several properties:
- `code` is a string identifying the category of warning
- `message` describes the issue in human-readable terms
- `start` and `end`, if the warning relates to a specific location, are objects with `line`, `column` and `character` properties
</div>
</div>
<div class="ts-block-property">
```dts
metadata: {/*…*/}
```
<div class="ts-block-property-details">
Metadata about the compiled component
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
runes: boolean;
```
<div class="ts-block-property-details">
Whether the file was compiled in runes mode, either because of an explicit option or inferred from usage.
For `compileModule`, this is always `true`
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```dts
ast: any;
```
<div class="ts-block-property-details">
The AST
</div>
</div></div>
## MarkupPreprocessor
A markup preprocessor that takes a string of code and returns a processed version.
<div class="ts-block">
```dts
type MarkupPreprocessor = (options: {
/**
* The whole Svelte file content
*/
content: string;
/**
* The filename of the Svelte file
*/
filename?: string;
}) => Processed | void | Promise<Processed | void>;
```
</div>
## ModuleCompileOptions
<div class="ts-block">
```dts
interface ModuleCompileOptions {/*…*/}
```
<div class="ts-block-property">
```dts
dev?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
</div>
If `true`, causes extra code to be added that will perform runtime checks and provide debugging information during development.
</div>
</div>
<div class="ts-block-property">
```dts
generate?: 'client' | 'server' | false;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `'client'`
</div>
If `"client"`, Svelte emits code designed to run in the browser.
If `"server"`, Svelte emits code suitable for server-side rendering.
If `false`, nothing is generated. Useful for tooling that is only interested in warnings.
</div>
</div>
<div class="ts-block-property">
```dts
filename?: string;
```
<div class="ts-block-property-details">
Used for debugging hints and sourcemaps. Your bundler plugin will set it automatically.
</div>
</div>
<div class="ts-block-property">
```dts
rootDir?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `process.cwd() on node-like environments, undefined elsewhere`
</div>
Used for ensuring filenames don't leak filesystem information. Your bundler plugin will set it automatically.
</div>
</div>
<div class="ts-block-property">
```dts
warningFilter?: (warning: Warning) => boolean;
```
<div class="ts-block-property-details">
A function that gets a `Warning` as an argument and returns a boolean.
Use this to filter out warnings. Return `true` to keep the warning, `false` to discard it.
</div>
</div></div>
## Preprocessor
A script/style preprocessor that takes a string of code and returns a processed version.
<div class="ts-block">
```dts
type Preprocessor = (options: {
/**
* The script/style tag content
*/
content: string;
/**
* The attributes on the script/style tag
*/
attributes: Record<string, string | boolean>;
/**
* The whole Svelte file content
*/
markup: string;
/**
* The filename of the Svelte file
*/
filename?: string;
}) => Processed | void | Promise<Processed | void>;
```
</div>
## PreprocessorGroup
A preprocessor group is a set of preprocessors that are applied to a Svelte file.
<div class="ts-block">
```dts
interface PreprocessorGroup {/*…*/}
```
<div class="ts-block-property">
```dts
name?: string;
```
<div class="ts-block-property-details">
Name of the preprocessor. Will be a required option in the next major version
</div>
</div>
<div class="ts-block-property">
```dts
markup?: MarkupPreprocessor;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
style?: Preprocessor;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
script?: Preprocessor;
```
<div class="ts-block-property-details"></div>
</div></div>
## Processed
The result of a preprocessor run. If the preprocessor does not return a result, it is assumed that the code is unchanged.
<div class="ts-block">
```dts
interface Processed {/*…*/}
```
<div class="ts-block-property">
```dts
code: string;
```
<div class="ts-block-property-details">
The new code
</div>
</div>
<div class="ts-block-property">
```dts
map?: string | object;
```
<div class="ts-block-property-details">
A source map mapping back to the original code
</div>
</div>
<div class="ts-block-property">
```dts
dependencies?: string[];
```
<div class="ts-block-property-details">
A list of additional files to watch for changes
</div>
</div>
<div class="ts-block-property">
```dts
attributes?: Record<string, string | boolean>;
```
<div class="ts-block-property-details">
Only for script/style preprocessors: The updated attributes to set on the tag. If undefined, attributes stay unchanged.
</div>
</div>
<div class="ts-block-property">
```dts
toString?: () => string;
```
<div class="ts-block-property-details"></div>
</div></div>
## Warning
<div class="ts-block">
```dts
interface Warning extends ICompileDiagnostic {}
```
</div>
# svelte/easing
```js
// @noErrors
import {
backIn,
backInOut,
backOut,
bounceIn,
bounceInOut,
bounceOut,
circIn,
circInOut,
circOut,
cubicIn,
cubicInOut,
cubicOut,
elasticIn,
elasticInOut,
elasticOut,
expoIn,
expoInOut,
expoOut,
linear,
quadIn,
quadInOut,
quadOut,
quartIn,
quartInOut,
quartOut,
quintIn,
quintInOut,
quintOut,
sineIn,
sineInOut,
sineOut
} from 'svelte/easing';
```
## backIn
<div class="ts-block">
```dts
function backIn(t: number): number;
```
</div>
## backInOut
<div class="ts-block">
```dts
function backInOut(t: number): number;
```
</div>
## backOut
<div class="ts-block">
```dts
function backOut(t: number): number;
```
</div>
## bounceIn
<div class="ts-block">
```dts
function bounceIn(t: number): number;
```
</div>
## bounceInOut
<div class="ts-block">
```dts
function bounceInOut(t: number): number;
```
</div>
## bounceOut
<div class="ts-block">
```dts
function bounceOut(t: number): number;
```
</div>
## circIn
<div class="ts-block">
```dts
function circIn(t: number): number;
```
</div>
## circInOut
<div class="ts-block">
```dts
function circInOut(t: number): number;
```
</div>
## circOut
<div class="ts-block">
```dts
function circOut(t: number): number;
```
</div>
## cubicIn
<div class="ts-block">
```dts
function cubicIn(t: number): number;
```
</div>
## cubicInOut
<div class="ts-block">
```dts
function cubicInOut(t: number): number;
```
</div>
## cubicOut
<div class="ts-block">
```dts
function cubicOut(t: number): number;
```
</div>
## elasticIn
<div class="ts-block">
```dts
function elasticIn(t: number): number;
```
</div>
## elasticInOut
<div class="ts-block">
```dts
function elasticInOut(t: number): number;
```
</div>
## elasticOut
<div class="ts-block">
```dts
function elasticOut(t: number): number;
```
</div>
## expoIn
<div class="ts-block">
```dts
function expoIn(t: number): number;
```
</div>
## expoInOut
<div class="ts-block">
```dts
function expoInOut(t: number): number;
```
</div>
## expoOut
<div class="ts-block">
```dts
function expoOut(t: number): number;
```
</div>
## linear
<div class="ts-block">
```dts
function linear(t: number): number;
```
</div>
## quadIn
<div class="ts-block">
```dts
function quadIn(t: number): number;
```
</div>
## quadInOut
<div class="ts-block">
```dts
function quadInOut(t: number): number;
```
</div>
## quadOut
<div class="ts-block">
```dts
function quadOut(t: number): number;
```
</div>
## quartIn
<div class="ts-block">
```dts
function quartIn(t: number): number;
```
</div>
## quartInOut
<div class="ts-block">
```dts
function quartInOut(t: number): number;
```
</div>
## quartOut
<div class="ts-block">
```dts
function quartOut(t: number): number;
```
</div>
## quintIn
<div class="ts-block">
```dts
function quintIn(t: number): number;
```
</div>
## quintInOut
<div class="ts-block">
```dts
function quintInOut(t: number): number;
```
</div>
## quintOut
<div class="ts-block">
```dts
function quintOut(t: number): number;
```
</div>
## sineIn
<div class="ts-block">
```dts
function sineIn(t: number): number;
```
</div>
## sineInOut
<div class="ts-block">
```dts
function sineInOut(t: number): number;
```
</div>
## sineOut
<div class="ts-block">
```dts
function sineOut(t: number): number;
```
</div>
# svelte/events
```js
// @noErrors
import { on } from 'svelte/events';
```
## on
Attaches an event handler to the window and returns a function that removes the handler. Using this
rather than `addEventListener` will preserve the correct order relative to handlers added declaratively
(with attributes like `onclick`), which use event delegation for performance reasons
<div class="ts-block">
```dts
function on<Type extends keyof WindowEventMap>(
window: Window,
type: Type,
handler: (
this: Window,
event: WindowEventMap[Type]
) => any,
options?: AddEventListenerOptions | undefined
): () => void;
```
</div>
<div class="ts-block">
```dts
function on<Type extends keyof DocumentEventMap>(
document: Document,
type: Type,
handler: (
this: Document,
event: DocumentEventMap[Type]
) => any,
options?: AddEventListenerOptions | undefined
): () => void;
```
</div>
<div class="ts-block">
```dts
function on<
Element extends HTMLElement,
Type extends keyof HTMLElementEventMap
>(
element: Element,
type: Type,
handler: (
this: Element,
event: HTMLElementEventMap[Type]
) => any,
options?: AddEventListenerOptions | undefined
): () => void;
```
</div>
<div class="ts-block">
```dts
function on<
Element extends MediaQueryList,
Type extends keyof MediaQueryListEventMap
>(
element: Element,
type: Type,
handler: (
this: Element,
event: MediaQueryListEventMap[Type]
) => any,
options?: AddEventListenerOptions | undefined
): () => void;
```
</div>
<div class="ts-block">
```dts
function on(
element: EventTarget,
type: string,
handler: EventListener,
options?: AddEventListenerOptions | undefined
): () => void;
```
</div>
# svelte/legacy
This module provides various functions for use during the migration, since some features can't be replaced one to one with new features. All imports are marked as deprecated and should be migrated away from over time.
```js
// @noErrors
import {
asClassComponent,
createBubbler,
createClassComponent,
handlers,
nonpassive,
once,
passive,
preventDefault,
run,
self,
stopImmediatePropagation,
stopPropagation,
trusted
} from 'svelte/legacy';
```
## asClassComponent
<blockquote class="tag deprecated note">
Use this only as a temporary solution to migrate your imperative component code to Svelte 5.
</blockquote>
Takes the component function and returns a Svelte 4 compatible component constructor.
<div class="ts-block">
```dts
function asClassComponent<
Props extends Record<string, any>,
Exports extends Record<string, any>,
Events extends Record<string, any>,
Slots extends Record<string, any>
>(
component:
| SvelteComponent<Props, Events, Slots>
| Component<Props>
): ComponentType<
SvelteComponent<Props, Events, Slots> & Exports
>;
```
</div>
## createBubbler
<blockquote class="tag deprecated note">
Use this only as a temporary solution to migrate your automatically delegated events in Svelte 5.
</blockquote>
Function to create a `bubble` function that mimic the behavior of `on:click` without handler available in svelte 4.
<div class="ts-block">
```dts
function createBubbler(): (
type: string
) => (event: Event) => boolean;
```
</div>
## createClassComponent
<blockquote class="tag deprecated note">
Use this only as a temporary solution to migrate your imperative component code to Svelte 5.
</blockquote>
Takes the same options as a Svelte 4 component and the component function and returns a Svelte 4 compatible component.
<div class="ts-block">
```dts
function createClassComponent<
Props extends Record<string, any>,
Exports extends Record<string, any>,
Events extends Record<string, any>,
Slots extends Record<string, any>
>(
options: ComponentConstructorOptions<Props> & {
component:
| ComponentType<SvelteComponent<Props, Events, Slots>>
| Component<Props>;
}
): SvelteComponent<Props, Events, Slots> & Exports;
```
</div>
## handlers
Function to mimic the multiple listeners available in svelte 4
<div class="ts-block">
```dts
function handlers(
...handlers: EventListener[]
): EventListener;
```
</div>
## nonpassive
Substitute for the `nonpassive` event modifier, implemented as an action
<div class="ts-block">
```dts
function nonpassive(
node: HTMLElement,
[event, handler]: [
event: string,
handler: () => EventListener
]
): void;
```
</div>
## once
Substitute for the `once` event modifier
<div class="ts-block">
```dts
function once(
fn: (event: Event, ...args: Array<unknown>) => void
): (event: Event, ...args: unknown[]) => void;
```
</div>
## passive
Substitute for the `passive` event modifier, implemented as an action
<div class="ts-block">
```dts
function passive(
node: HTMLElement,
[event, handler]: [
event: string,
handler: () => EventListener
]
): void;
```
</div>
## preventDefault
Substitute for the `preventDefault` event modifier
<div class="ts-block">
```dts
function preventDefault(
fn: (event: Event, ...args: Array<unknown>) => void
): (event: Event, ...args: unknown[]) => void;
```
</div>
## run
<blockquote class="tag deprecated note">
Use this only as a temporary solution to migrate your component code to Svelte 5.
</blockquote>
Runs the given function once immediately on the server, and works like `$effect.pre` on the client.
<div class="ts-block">
```dts
function run(fn: () => void | (() => void)): void;
```
</div>
## self
Substitute for the `self` event modifier
<div class="ts-block">
```dts
function self(
fn: (event: Event, ...args: Array<unknown>) => void
): (event: Event, ...args: unknown[]) => void;
```
</div>
## stopImmediatePropagation
Substitute for the `stopImmediatePropagation` event modifier
<div class="ts-block">
```dts
function stopImmediatePropagation(
fn: (event: Event, ...args: Array<unknown>) => void
): (event: Event, ...args: unknown[]) => void;
```
</div>
## stopPropagation
Substitute for the `stopPropagation` event modifier
<div class="ts-block">
```dts
function stopPropagation(
fn: (event: Event, ...args: Array<unknown>) => void
): (event: Event, ...args: unknown[]) => void;
```
</div>
## trusted
Substitute for the `trusted` event modifier
<div class="ts-block">
```dts
function trusted(
fn: (event: Event, ...args: Array<unknown>) => void
): (event: Event, ...args: unknown[]) => void;
```
</div>
## LegacyComponentType
Support using the component as both a class and function during the transition period
<div class="ts-block">
```dts
type LegacyComponentType = {
new (o: ComponentConstructorOptions): SvelteComponent;
(
...args: Parameters<Component<Record<string, any>>>
): ReturnType<
Component<Record<string, any>, Record<string, any>>
>;
};
```
</div>
# svelte/motion
```js
// @noErrors
import {
Spring,
Tween,
prefersReducedMotion,
spring,
tweened
} from 'svelte/motion';
```
## Spring
<blockquote class="since note">
Available since 5.8.0
</blockquote>
A wrapper for a value that behaves in a spring-like fashion. Changes to `spring.target` will cause `spring.current` to
move towards it over time, taking account of the `spring.stiffness` and `spring.damping` parameters.
```svelte
<script>
import { Spring } from 'svelte/motion';
const spring = new Spring(0);
</script>
<input type="range" bind:value={spring.target} />
<input type="range" bind:value={spring.current} disabled />
```
<div class="ts-block">
```dts
class Spring<T> {/*…*/}
```
<div class="ts-block-property">
```dts
constructor(value: T, options?: SpringOpts);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
static of<U>(fn: () => U, options?: SpringOpts): Spring<U>;
```
<div class="ts-block-property-details">
Create a spring whose value is bound to the return value of `fn`. This must be called
inside an effect root (for example, during component initialisation).
```svelte
<script>
import { Spring } from 'svelte/motion';
let { number } = $props();
const spring = Spring.of(() => number);
</script>
```
</div>
</div>
<div class="ts-block-property">
```dts
set(value: T, options?: SpringUpdateOpts): Promise<void>;
```
<div class="ts-block-property-details">
Sets `spring.target` to `value` and returns a `Promise` that resolves if and when `spring.current` catches up to it.
If `options.instant` is `true`, `spring.current` immediately matches `spring.target`.
If `options.preserveMomentum` is provided, the spring will continue on its current trajectory for
the specified number of milliseconds. This is useful for things like 'fling' gestures.
</div>
</div>
<div class="ts-block-property">
```dts
damping: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
precision: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
stiffness: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
target: T;
```
<div class="ts-block-property-details">
The end value of the spring.
This property only exists on the `Spring` class, not the legacy `spring` store.
</div>
</div>
<div class="ts-block-property">
```dts
get current(): T;
```
<div class="ts-block-property-details">
The current value of the spring.
This property only exists on the `Spring` class, not the legacy `spring` store.
</div>
</div></div>
## Tween
<blockquote class="since note">
Available since 5.8.0
</blockquote>
A wrapper for a value that tweens smoothly to its target value. Changes to `tween.target` will cause `tween.current` to
move towards it over time, taking account of the `delay`, `duration` and `easing` options.
```svelte
<script>
import { Tween } from 'svelte/motion';
const tween = new Tween(0);
</script>
<input type="range" bind:value={tween.target} />
<input type="range" bind:value={tween.current} disabled />
```
<div class="ts-block">
```dts
class Tween<T> {/*…*/}
```
<div class="ts-block-property">
```dts
static of<U>(fn: () => U, options?: TweenedOptions<U> | undefined): Tween<U>;
```
<div class="ts-block-property-details">
Create a tween whose value is bound to the return value of `fn`. This must be called
inside an effect root (for example, during component initialisation).
```svelte
<script>
import { Tween } from 'svelte/motion';
let { number } = $props();
const tween = Tween.of(() => number);
</script>
```
</div>
</div>
<div class="ts-block-property">
```dts
constructor(value: T, options?: TweenedOptions<T>);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
set(value: T, options?: TweenedOptions<T> | undefined): Promise<void>;
```
<div class="ts-block-property-details">
Sets `tween.target` to `value` and returns a `Promise` that resolves if and when `tween.current` catches up to it.
If `options` are provided, they will override the tween's defaults.
</div>
</div>
<div class="ts-block-property">
```dts
get current(): T;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
set target(v: T);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
get target(): T;
```
<div class="ts-block-property-details"></div>
</div></div>
## prefersReducedMotion
<blockquote class="since note">
Available since 5.7.0
</blockquote>
A [media query](/docs/svelte/svelte-reactivity#MediaQuery) that matches if the user [prefers reduced motion](https://developer.mozilla.org/en-US/docs/Web/CSS/@media/prefers-reduced-motion).
```svelte
<script>
import { prefersReducedMotion } from 'svelte/motion';
import { fly } from 'svelte/transition';
let visible = $state(false);
</script>
<button onclick={() => visible = !visible}>
toggle
</button>
{#if visible}
<p transition:fly={{ y: prefersReducedMotion.current ? 0 : 200 }}>
flies in, unless the user prefers reduced motion
</p>
{/if}
```
<div class="ts-block">
```dts
const prefersReducedMotion: MediaQuery;
```
</div>
## spring
<blockquote class="tag deprecated note">
Use [`Spring`](/docs/svelte/svelte-motion#Spring) instead
</blockquote>
The spring function in Svelte creates a store whose value is animated, with a motion that simulates the behavior of a spring. This means when the value changes, instead of transitioning at a steady rate, it "bounces" like a spring would, depending on the physics parameters provided. This adds a level of realism to the transitions and can enhance the user experience.
<div class="ts-block">
```dts
function spring<T = any>(
value?: T | undefined,
opts?: SpringOpts | undefined
): Spring<T>;
```
</div>
## tweened
<blockquote class="tag deprecated note">
Use [`Tween`](/docs/svelte/svelte-motion#Tween) instead
</blockquote>
A tweened store in Svelte is a special type of store that provides smooth transitions between state values over time.
<div class="ts-block">
```dts
function tweened<T>(
value?: T | undefined,
defaults?: TweenedOptions<T> | undefined
): Tweened<T>;
```
</div>
## Spring
<div class="ts-block">
```dts
interface Spring<T> extends Readable<T> {/*…*/}
```
<div class="ts-block-property">
```dts
set(new_value: T, opts?: SpringUpdateOpts): Promise<void>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span> Only exists on the legacy `spring` store, not the `Spring` class
</div>
</div>
</div>
<div class="ts-block-property">
```dts
subscribe(fn: (value: T) => void): Unsubscriber;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span> Only exists on the legacy `spring` store, not the `Spring` class
</div>
</div>
</div>
<div class="ts-block-property">
```dts
precision: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
damping: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
stiffness: number;
```
<div class="ts-block-property-details"></div>
</div></div>
## Tweened
<div class="ts-block">
```dts
interface Tweened<T> extends Readable<T> {/*…*/}
```
<div class="ts-block-property">
```dts
set(value: T, opts?: TweenedOptions<T>): Promise<void>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
update(updater: Updater<T>, opts?: TweenedOptions<T>): Promise<void>;
```
<div class="ts-block-property-details"></div>
</div></div>
# svelte/reactivity/window
This module exports reactive versions of various `window` values, each of which has a reactive `current` property that you can reference in reactive contexts (templates, [deriveds]($derived) and [effects]($effect)) without using [`<svelte:window>`](svelte-window) bindings or manually creating your own event listeners.
```svelte
<script>
import { innerWidth, innerHeight } from 'svelte/reactivity/window';
</script>
<p>{innerWidth.current}x{innerHeight.current}</p>
```
```js
// @noErrors
import {
devicePixelRatio,
innerHeight,
innerWidth,
online,
outerHeight,
outerWidth,
screenLeft,
screenTop,
scrollX,
scrollY
} from 'svelte/reactivity/window';
```
## devicePixelRatio
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`devicePixelRatio.current` is a reactive view of `window.devicePixelRatio`. On the server it is `undefined`.
Note that behaviour differs between browsers — on Chrome it will respond to the current zoom level,
on Firefox and Safari it won't.
<div class="ts-block">
```dts
const devicePixelRatio: {
get current(): number | undefined;
};
```
</div>
## innerHeight
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`innerHeight.current` is a reactive view of `window.innerHeight`. On the server it is `undefined`.
<div class="ts-block">
```dts
const innerHeight: ReactiveValue<number | undefined>;
```
</div>
## innerWidth
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`innerWidth.current` is a reactive view of `window.innerWidth`. On the server it is `undefined`.
<div class="ts-block">
```dts
const innerWidth: ReactiveValue<number | undefined>;
```
</div>
## online
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`online.current` is a reactive view of `navigator.onLine`. On the server it is `undefined`.
<div class="ts-block">
```dts
const online: ReactiveValue<boolean | undefined>;
```
</div>
## outerHeight
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`outerHeight.current` is a reactive view of `window.outerHeight`. On the server it is `undefined`.
<div class="ts-block">
```dts
const outerHeight: ReactiveValue<number | undefined>;
```
</div>
## outerWidth
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`outerWidth.current` is a reactive view of `window.outerWidth`. On the server it is `undefined`.
<div class="ts-block">
```dts
const outerWidth: ReactiveValue<number | undefined>;
```
</div>
## screenLeft
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`screenLeft.current` is a reactive view of `window.screenLeft`. It is updated inside a `requestAnimationFrame` callback. On the server it is `undefined`.
<div class="ts-block">
```dts
const screenLeft: ReactiveValue<number | undefined>;
```
</div>
## screenTop
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`screenTop.current` is a reactive view of `window.screenTop`. It is updated inside a `requestAnimationFrame` callback. On the server it is `undefined`.
<div class="ts-block">
```dts
const screenTop: ReactiveValue<number | undefined>;
```
</div>
## scrollX
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`scrollX.current` is a reactive view of `window.scrollX`. On the server it is `undefined`.
<div class="ts-block">
```dts
const scrollX: ReactiveValue<number | undefined>;
```
</div>
## scrollY
<blockquote class="since note">
Available since 5.11.0
</blockquote>
`scrollY.current` is a reactive view of `window.scrollY`. On the server it is `undefined`.
<div class="ts-block">
```dts
const scrollY: ReactiveValue<number | undefined>;
```
</div>
# svelte/reactivity
Svelte provides reactive versions of various built-ins like `SvelteMap`, `SvelteSet` and `SvelteURL`. These can be imported from `svelte/reactivity` and used just like their native counterparts.
```svelte
<script>
import { SvelteURL } from 'svelte/reactivity';
const url = new SvelteURL('https://example.com/path');
</script>
<!-- changes to these... -->
<input bind:value={url.protocol} />
<input bind:value={url.hostname} />
<input bind:value={url.pathname} />
<hr />
<!-- will update `href` and vice versa -->
<input bind:value={url.href} />
```
```js
// @noErrors
import {
MediaQuery,
SvelteDate,
SvelteMap,
SvelteSet,
SvelteURL,
SvelteURLSearchParams,
createSubscriber
} from 'svelte/reactivity';
```
## MediaQuery
<blockquote class="since note">
Available since 5.7.0
</blockquote>
Creates a media query and provides a `current` property that reflects whether or not it matches.
Use it carefully — during server-side rendering, there is no way to know what the correct value should be, potentially causing content to change upon hydration.
If you can use the media query in CSS to achieve the same effect, do that.
```svelte
<script>
import { MediaQuery } from 'svelte/reactivity';
const large = new MediaQuery('min-width: 800px');
</script>
<h1>{large.current ? 'large screen' : 'small screen'}</h1>
```
<div class="ts-block">
```dts
class MediaQuery extends ReactiveValue<boolean> {/*…*/}
```
<div class="ts-block-property">
```dts
constructor(query: string, fallback?: boolean | undefined);
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `query` A media query string
- `fallback` Fallback value for the server
</div>
</div>
</div></div>
## SvelteDate
<div class="ts-block">
```dts
class SvelteDate extends Date {/*…*/}
```
<div class="ts-block-property">
```dts
constructor(...params: any[]);
```
<div class="ts-block-property-details"></div>
</div></div>
## SvelteMap
<div class="ts-block">
```dts
class SvelteMap<K, V> extends Map<K, V> {/*…*/}
```
<div class="ts-block-property">
```dts
constructor(value?: Iterable<readonly [K, V]> | null | undefined);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
set(key: K, value: V): this;
```
<div class="ts-block-property-details"></div>
</div></div>
## SvelteSet
<div class="ts-block">
```dts
class SvelteSet<T> extends Set<T> {/*…*/}
```
<div class="ts-block-property">
```dts
constructor(value?: Iterable<T> | null | undefined);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
add(value: T): this;
```
<div class="ts-block-property-details"></div>
</div></div>
## SvelteURL
<div class="ts-block">
```dts
class SvelteURL extends URL {/*…*/}
```
<div class="ts-block-property">
```dts
get searchParams(): SvelteURLSearchParams;
```
<div class="ts-block-property-details"></div>
</div></div>
## SvelteURLSearchParams
<div class="ts-block">
```dts
class SvelteURLSearchParams extends URLSearchParams {/*…*/}
```
<div class="ts-block-property">
```dts
[REPLACE](params: URLSearchParams): void;
```
<div class="ts-block-property-details"></div>
</div></div>
## createSubscriber
<blockquote class="since note">
Available since 5.7.0
</blockquote>
Returns a `subscribe` function that, if called in an effect (including expressions in the template),
calls its `start` callback with an `update` function. Whenever `update` is called, the effect re-runs.
If `start` returns a function, it will be called when the effect is destroyed.
If `subscribe` is called in multiple effects, `start` will only be called once as long as the effects
are active, and the returned teardown function will only be called when all effects are destroyed.
It's best understood with an example. Here's an implementation of [`MediaQuery`](/docs/svelte/svelte-reactivity#MediaQuery):
```js
// @errors: 7031
import { createSubscriber } from 'svelte/reactivity';
import { on } from 'svelte/events';
export class MediaQuery {
#query;
#subscribe;
constructor(query) {
this.#query = window.matchMedia(`(${query})`);
this.#subscribe = createSubscriber((update) => {
// when the `change` event occurs, re-run any effects that read `this.current`
const off = on(this.#query, 'change', update);
// stop listening when all the effects are destroyed
return () => off();
});
}
get current() {
this.#subscribe();
// Return the current state of the query, whether or not we're in an effect
return this.#query.matches;
}
}
```
<div class="ts-block">
```dts
function createSubscriber(
start: (update: () => void) => (() => void) | void
): () => void;
```
</div>
# svelte/server
```js
// @noErrors
import { render } from 'svelte/server';
```
## render
Only available on the server and when compiling with the `server` option.
Takes a component and returns an object with `body` and `head` properties on it, which you can use to populate the HTML when server-rendering your app.
<div class="ts-block">
```dts
function render<
Comp extends SvelteComponent<any> | Component<any>,
Props extends ComponentProps<Comp> = ComponentProps<Comp>
>(
...args: {} extends Props
? [
component: Comp extends SvelteComponent<any>
? ComponentType<Comp>
: Comp,
options?: {
props?: Omit<Props, '$$slots' | '$$events'>;
context?: Map<any, any>;
}
]
: [
component: Comp extends SvelteComponent<any>
? ComponentType<Comp>
: Comp,
options: {
props: Omit<Props, '$$slots' | '$$events'>;
context?: Map<any, any>;
}
]
): RenderOutput;
```
</div>
# svelte/store
```js
// @noErrors
import {
derived,
fromStore,
get,
readable,
readonly,
toStore,
writable
} from 'svelte/store';
```
## derived
Derived value store by synchronizing one or more readable stores and
applying an aggregation function over its input values.
<div class="ts-block">
```dts
function derived<S extends Stores, T>(
stores: S,
fn: (
values: StoresValues<S>,
set: (value: T) => void,
update: (fn: Updater<T>) => void
) => Unsubscriber | void,
initial_value?: T | undefined
): Readable<T>;
```
</div>
<div class="ts-block">
```dts
function derived<S extends Stores, T>(
stores: S,
fn: (values: StoresValues<S>) => T,
initial_value?: T | undefined
): Readable<T>;
```
</div>
## fromStore
<div class="ts-block">
```dts
function fromStore<V>(store: Writable<V>): {
current: V;
};
```
</div>
<div class="ts-block">
```dts
function fromStore<V>(store: Readable<V>): {
readonly current: V;
};
```
</div>
## get
Get the current value from a store by subscribing and immediately unsubscribing.
<div class="ts-block">
```dts
function get<T>(store: Readable<T>): T;
```
</div>
## readable
Creates a `Readable` store that allows reading by subscription.
<div class="ts-block">
```dts
function readable<T>(
value?: T | undefined,
start?: StartStopNotifier<T> | undefined
): Readable<T>;
```
</div>
## readonly
Takes a store and returns a new one derived from the old one that is readable.
<div class="ts-block">
```dts
function readonly<T>(store: Readable<T>): Readable<T>;
```
</div>
## toStore
<div class="ts-block">
```dts
function toStore<V>(
get: () => V,
set: (v: V) => void
): Writable<V>;
```
</div>
<div class="ts-block">
```dts
function toStore<V>(get: () => V): Readable<V>;
```
</div>
## writable
Create a `Writable` store that allows both updating and reading by subscription.
<div class="ts-block">
```dts
function writable<T>(
value?: T | undefined,
start?: StartStopNotifier<T> | undefined
): Writable<T>;
```
</div>
## Readable
Readable interface for subscribing.
<div class="ts-block">
```dts
interface Readable<T> {/*…*/}
```
<div class="ts-block-property">
```dts
subscribe(this: void, run: Subscriber<T>, invalidate?: () => void): Unsubscriber;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `run` subscription callback
- `invalidate` cleanup callback
</div>
Subscribe on value changes.
</div>
</div></div>
## StartStopNotifier
Start and stop notification callbacks.
This function is called when the first subscriber subscribes.
<div class="ts-block">
```dts
type StartStopNotifier<T> = (
set: (value: T) => void,
update: (fn: Updater<T>) => void
) => void | (() => void);
```
</div>
## Subscriber
Callback to inform of a value updates.
<div class="ts-block">
```dts
type Subscriber<T> = (value: T) => void;
```
</div>
## Unsubscriber
Unsubscribes from value updates.
<div class="ts-block">
```dts
type Unsubscriber = () => void;
```
</div>
## Updater
Callback to update a value.
<div class="ts-block">
```dts
type Updater<T> = (value: T) => T;
```
</div>
## Writable
Writable interface for both updating and subscribing.
<div class="ts-block">
```dts
interface Writable<T> extends Readable<T> {/*…*/}
```
<div class="ts-block-property">
```dts
set(this: void, value: T): void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `value` to set
</div>
Set value and inform subscribers.
</div>
</div>
<div class="ts-block-property">
```dts
update(this: void, updater: Updater<T>): void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `updater` callback
</div>
Update value using callback and inform subscribers.
</div>
</div></div>
# svelte/transition
```js
// @noErrors
import {
blur,
crossfade,
draw,
fade,
fly,
scale,
slide
} from 'svelte/transition';
```
## blur
Animates a `blur` filter alongside an element's opacity.
<div class="ts-block">
```dts
function blur(
node: Element,
{
delay,
duration,
easing,
amount,
opacity
}?: BlurParams | undefined
): TransitionConfig;
```
</div>
## crossfade
The `crossfade` function creates a pair of [transitions](/docs/svelte/transition) called `send` and `receive`. When an element is 'sent', it looks for a corresponding element being 'received', and generates a transition that transforms the element to its counterpart's position and fades it out. When an element is 'received', the reverse happens. If there is no counterpart, the `fallback` transition is used.
<div class="ts-block">
```dts
function crossfade({
fallback,
...defaults
}: CrossfadeParams & {
fallback?: (
node: Element,
params: CrossfadeParams,
intro: boolean
) => TransitionConfig;
}): [
(
node: any,
params: CrossfadeParams & {
key: any;
}
) => () => TransitionConfig,
(
node: any,
params: CrossfadeParams & {
key: any;
}
) => () => TransitionConfig
];
```
</div>
## draw
Animates the stroke of an SVG element, like a snake in a tube. `in` transitions begin with the path invisible and draw the path to the screen over time. `out` transitions start in a visible state and gradually erase the path. `draw` only works with elements that have a `getTotalLength` method, like `<path>` and `<polyline>`.
<div class="ts-block">
```dts
function draw(
node: SVGElement & {
getTotalLength(): number;
},
{
delay,
speed,
duration,
easing
}?: DrawParams | undefined
): TransitionConfig;
```
</div>
## fade
Animates the opacity of an element from 0 to the current opacity for `in` transitions and from the current opacity to 0 for `out` transitions.
<div class="ts-block">
```dts
function fade(
node: Element,
{ delay, duration, easing }?: FadeParams | undefined
): TransitionConfig;
```
</div>
## fly
Animates the x and y positions and the opacity of an element. `in` transitions animate from the provided values, passed as parameters to the element's default values. `out` transitions animate from the element's default values to the provided values.
<div class="ts-block">
```dts
function fly(
node: Element,
{
delay,
duration,
easing,
x,
y,
opacity
}?: FlyParams | undefined
): TransitionConfig;
```
</div>
## scale
Animates the opacity and scale of an element. `in` transitions animate from the provided values, passed as parameters, to an element's current (default) values. `out` transitions animate from an element's default values to the provided values.
<div class="ts-block">
```dts
function scale(
node: Element,
{
delay,
duration,
easing,
start,
opacity
}?: ScaleParams | undefined
): TransitionConfig;
```
</div>
## slide
Slides an element in and out.
<div class="ts-block">
```dts
function slide(
node: Element,
{
delay,
duration,
easing,
axis
}?: SlideParams | undefined
): TransitionConfig;
```
</div>
## BlurParams
<div class="ts-block">
```dts
interface BlurParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
amount?: number | string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
opacity?: number;
```
<div class="ts-block-property-details"></div>
</div></div>
## CrossfadeParams
<div class="ts-block">
```dts
interface CrossfadeParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number | ((len: number) => number);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div></div>
## DrawParams
<div class="ts-block">
```dts
interface DrawParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
speed?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number | ((len: number) => number);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div></div>
## EasingFunction
<div class="ts-block">
```dts
type EasingFunction = (t: number) => number;
```
</div>
## FadeParams
<div class="ts-block">
```dts
interface FadeParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div></div>
## FlyParams
<div class="ts-block">
```dts
interface FlyParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
x?: number | string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
y?: number | string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
opacity?: number;
```
<div class="ts-block-property-details"></div>
</div></div>
## ScaleParams
<div class="ts-block">
```dts
interface ScaleParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
start?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
opacity?: number;
```
<div class="ts-block-property-details"></div>
</div></div>
## SlideParams
<div class="ts-block">
```dts
interface SlideParams {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
axis?: 'x' | 'y';
```
<div class="ts-block-property-details"></div>
</div></div>
## TransitionConfig
<div class="ts-block">
```dts
interface TransitionConfig {/*…*/}
```
<div class="ts-block-property">
```dts
delay?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
duration?: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
easing?: EasingFunction;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
css?: (t: number, u: number) => string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
tick?: (t: number, u: number) => void;
```
<div class="ts-block-property-details"></div>
</div></div>
# Compiler errors
<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
### animation_duplicate
```
An element can only have one 'animate' directive
```
### animation_invalid_placement
```
An element that uses the `animate:` directive must be the only child of a keyed `{#each ...}` block
```
### animation_missing_key
```
An element that uses the `animate:` directive must be the only child of a keyed `{#each ...}` block. Did you forget to add a key to your each block?
```
### attribute_contenteditable_dynamic
```
'contenteditable' attribute cannot be dynamic if element uses two-way binding
```
### attribute_contenteditable_missing
```
'contenteditable' attribute is required for textContent, innerHTML and innerText two-way bindings
```
### attribute_duplicate
```
Attributes need to be unique
```
### attribute_empty_shorthand
```
Attribute shorthand cannot be empty
```
### attribute_invalid_event_handler
```
Event attribute must be a JavaScript expression, not a string
```
### attribute_invalid_multiple
```
'multiple' attribute must be static if select uses two-way binding
```
### attribute_invalid_name
```
'%name%' is not a valid attribute name
```
### attribute_invalid_sequence_expression
```
Sequence expressions are not allowed as attribute/directive values in runes mode, unless wrapped in parentheses
```
### attribute_invalid_type
```
'type' attribute must be a static text value if input uses two-way binding
```
### attribute_unquoted_sequence
```
Attribute values containing `{...}` must be enclosed in quote marks, unless the value only contains the expression
```
### bind_group_invalid_expression
```
`bind:group` can only bind to an Identifier or MemberExpression
```
### bind_invalid_expression
```
Can only bind to an Identifier or MemberExpression or a `{get, set}` pair
```
### bind_invalid_name
```
`bind:%name%` is not a valid binding
```
```
`bind:%name%` is not a valid binding. %explanation%
```
### bind_invalid_parens
```
`bind:%name%={get, set}` must not have surrounding parentheses
```
### bind_invalid_target
```
`bind:%name%` can only be used with %elements%
```
### bind_invalid_value
```
Can only bind to state or props
```
### bindable_invalid_location
```
`$bindable()` can only be used inside a `$props()` declaration
```
### block_duplicate_clause
```
%name% cannot appear more than once within a block
```
### block_invalid_continuation_placement
```
{:...} block is invalid at this position (did you forget to close the preceding element or block?)
```
### block_invalid_elseif
```
'elseif' should be 'else if'
```
### block_invalid_placement
```
{#%name% ...} block cannot be %location%
```
### block_unclosed
```
Block was left open
```
### block_unexpected_character
```
Expected a `%character%` character immediately following the opening bracket
```
### block_unexpected_close
```
Unexpected block closing tag
```
### component_invalid_directive
```
This type of directive is not valid on components
```
### const_tag_cycle
```
Cyclical dependency detected: %cycle%
```
### const_tag_invalid_expression
```
{@const ...} must consist of a single variable declaration
```
### const_tag_invalid_placement
```
`{@const}` must be the immediate child of `{#snippet}`, `{#if}`, `{:else if}`, `{:else}`, `{#each}`, `{:then}`, `{:catch}`, `<svelte:fragment>`, `<svelte:boundary` or `<Component>`
```
### constant_assignment
```
Cannot assign to %thing%
```
### constant_binding
```
Cannot bind to %thing%
```
### css_empty_declaration
```
Declaration cannot be empty
```
### css_expected_identifier
```
Expected a valid CSS identifier
```
### css_global_block_invalid_combinator
```
A `:global` selector cannot follow a `%name%` combinator
```
### css_global_block_invalid_declaration
```
A top-level `:global {...}` block can only contain rules, not declarations
```
### css_global_block_invalid_list
```
A `:global` selector cannot be part of a selector list with more than one item
```
### css_global_block_invalid_modifier
```
A `:global` selector cannot modify an existing selector
```
### css_global_block_invalid_modifier_start
```
A `:global` selector can only be modified if it is a descendant of other selectors
```
### css_global_invalid_placement
```
`:global(...)` can be at the start or end of a selector sequence, but not in the middle
```
### css_global_invalid_selector
```
`:global(...)` must contain exactly one selector
```
### css_global_invalid_selector_list
```
`:global(...)` must not contain type or universal selectors when used in a compound selector
```
### css_nesting_selector_invalid_placement
```
Nesting selectors can only be used inside a rule or as the first selector inside a lone `:global(...)`
```
### css_selector_invalid
```
Invalid selector
```
### css_type_selector_invalid_placement
```
`:global(...)` must not be followed by a type selector
```
### debug_tag_invalid_arguments
```
{@debug ...} arguments must be identifiers, not arbitrary expressions
```
### declaration_duplicate
```
`%name%` has already been declared
```
### declaration_duplicate_module_import
```
Cannot declare a variable with the same name as an import inside `<script module>`
```
### derived_invalid_export
```
Cannot export derived state from a module. To expose the current derived value, export a function returning its value
```
### directive_invalid_value
```
Directive value must be a JavaScript expression enclosed in curly braces
```
### directive_missing_name
```
`%type%` name cannot be empty
```
### dollar_binding_invalid
```
The $ name is reserved, and cannot be used for variables and imports
```
### dollar_prefix_invalid
```
The $ prefix is reserved, and cannot be used for variables and imports
```
### each_item_invalid_assignment
```
Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. `array[i] = value` instead of `entry = value`, or `bind:value={array[i]}` instead of `bind:value={entry}`)
```
In legacy mode, it was possible to reassign or bind to the each block argument itself:
```svelte
<script>
let array = [1, 2, 3];
</script>
{#each array as entry}
<!-- reassignment -->
<button on:click={() => entry = 4}>change</button>
<!-- binding -->
<input bind:value={entry}>
{/each}
```
This turned out to be buggy and unpredictable, particularly when working with derived values (such as `array.map(...)`), and as such is forbidden in runes mode. You can achieve the same outcome by using the index instead:
```svelte
<script>
let array = $state([1, 2, 3]);
</script>
{#each array as entry, i}
<!-- reassignment -->
<button onclick={() => array[i] = 4}>change</button>
<!-- binding -->
<input bind:value={array[i]}>
{/each}
```
### effect_invalid_placement
```
`$effect()` can only be used as an expression statement
```
### element_invalid_closing_tag
```
`</%name%>` attempted to close an element that was not open
```
### element_invalid_closing_tag_autoclosed
```
`</%name%>` attempted to close element that was already automatically closed by `<%reason%>` (cannot nest `<%reason%>` inside `<%name%>`)
```
### element_unclosed
```
`<%name%>` was left open
```
### event_handler_invalid_component_modifier
```
Event modifiers other than 'once' can only be used on DOM elements
```
### event_handler_invalid_modifier
```
Valid event modifiers are %list%
```
### event_handler_invalid_modifier_combination
```
The '%modifier1%' and '%modifier2%' modifiers cannot be used together
```
### expected_attribute_value
```
Expected attribute value
```
### expected_block_type
```
Expected 'if', 'each', 'await', 'key' or 'snippet'
```
### expected_identifier
```
Expected an identifier
```
### expected_pattern
```
Expected identifier or destructure pattern
```
### expected_token
```
Expected token %token%
```
### expected_whitespace
```
Expected whitespace
```
### export_undefined
```
`%name%` is not defined
```
### global_reference_invalid
```
`%name%` is an illegal variable name. To reference a global variable called `%name%`, use `globalThis.%name%`
```
### host_invalid_placement
```
`$host()` can only be used inside custom element component instances
```
### illegal_element_attribute
```
`<%name%>` does not support non-event attributes or spread attributes
```
### import_svelte_internal_forbidden
```
Imports of `svelte/internal/*` are forbidden. It contains private runtime code which is subject to change without notice. If you're importing from `svelte/internal/*` to work around a limitation of Svelte, please open an issue at https://github.com/sveltejs/svelte and explain your use case
```
### inspect_trace_generator
```
`$inspect.trace(...)` cannot be used inside a generator function
```
### inspect_trace_invalid_placement
```
`$inspect.trace(...)` must be the first statement of a function body
```
### invalid_arguments_usage
```
The arguments keyword cannot be used within the template or at the top level of a component
```
### js_parse_error
```
%message%
```
### legacy_export_invalid
```
Cannot use `export let` in runes mode — use `$props()` instead
```
### legacy_props_invalid
```
Cannot use `$$props` in runes mode
```
### legacy_reactive_statement_invalid
```
`$:` is not allowed in runes mode, use `$derived` or `$effect` instead
```
### legacy_rest_props_invalid
```
Cannot use `$$restProps` in runes mode
```
### let_directive_invalid_placement
```
`let:` directive at invalid position
```
### mixed_event_handler_syntaxes
```
Mixing old (on:%name%) and new syntaxes for event handling is not allowed. Use only the on%name% syntax
```
### module_illegal_default_export
```
A component cannot have a default export
```
### node_invalid_placement
```
%message%. The browser will 'repair' the HTML (by moving, removing, or inserting elements) which breaks Svelte's assumptions about the structure of your components.
```
HTML restricts where certain elements can appear. In case of a violation the browser will 'repair' the HTML in a way that breaks Svelte's assumptions about the structure of your components. Some examples:
- `<p>hello <div>world</div></p>` will result in `<p>hello </p><div>world</div><p></p>` (the `<div>` autoclosed the `<p>` because `<p>` cannot contain block-level elements)
- `<option><div>option a</div></option>` will result in `<option>option a</option>` (the `<div>` is removed)
- `<table><tr><td>cell</td></tr></table>` will result in `<table><tbody><tr><td>cell</td></tr></tbody></table>` (a `<tbody>` is auto-inserted)
### options_invalid_value
```
Invalid compiler option: %details%
```
### options_removed
```
Invalid compiler option: %details%
```
### options_unrecognised
```
Unrecognised compiler option %keypath%
```
### props_duplicate
```
Cannot use `%rune%()` more than once
```
### props_id_invalid_placement
```
`$props.id()` can only be used at the top level of components as a variable declaration initializer
```
### props_illegal_name
```
Declaring or accessing a prop starting with `$$` is illegal (they are reserved for Svelte internals)
```
### props_invalid_identifier
```
`$props()` can only be used with an object destructuring pattern
```
### props_invalid_pattern
```
`$props()` assignment must not contain nested properties or computed keys
```
### props_invalid_placement
```
`$props()` can only be used at the top level of components as a variable declaration initializer
```
### reactive_declaration_cycle
```
Cyclical dependency detected: %cycle%
```
### render_tag_invalid_call_expression
```
Calling a snippet function using apply, bind or call is not allowed
```
### render_tag_invalid_expression
```
`{@render ...}` tags can only contain call expressions
```
### render_tag_invalid_spread_argument
```
cannot use spread arguments in `{@render ...}` tags
```
### rune_invalid_arguments
```
`%rune%` cannot be called with arguments
```
### rune_invalid_arguments_length
```
`%rune%` must be called with %args%
```
### rune_invalid_computed_property
```
Cannot access a computed property of a rune
```
### rune_invalid_name
```
`%name%` is not a valid rune
```
### rune_invalid_usage
```
Cannot use `%rune%` rune in non-runes mode
```
### rune_missing_parentheses
```
Cannot use rune without parentheses
```
### rune_removed
```
The `%name%` rune has been removed
```
### rune_renamed
```
`%name%` is now `%replacement%`
```
### runes_mode_invalid_import
```
%name% cannot be used in runes mode
```
### script_duplicate
```
A component can have a single top-level `<script>` element and/or a single top-level `<script module>` element
```
### script_invalid_attribute_value
```
If the `%name%` attribute is supplied, it must be a boolean attribute
```
### script_invalid_context
```
If the context attribute is supplied, its value must be "module"
```
### script_reserved_attribute
```
The `%name%` attribute is reserved and cannot be used
```
### slot_attribute_duplicate
```
Duplicate slot name '%name%' in <%component%>
```
### slot_attribute_invalid
```
slot attribute must be a static value
```
### slot_attribute_invalid_placement
```
Element with a slot='...' attribute must be a child of a component or a descendant of a custom element
```
### slot_default_duplicate
```
Found default slot content alongside an explicit slot="default"
```
### slot_element_invalid_attribute
```
`<slot>` can only receive attributes and (optionally) let directives
```
### slot_element_invalid_name
```
slot attribute must be a static value
```
### slot_element_invalid_name_default
```
`default` is a reserved word — it cannot be used as a slot name
```
### slot_snippet_conflict
```
Cannot use `<slot>` syntax and `{@render ...}` tags in the same component. Migrate towards `{@render ...}` tags completely
```
### snippet_conflict
```
Cannot use explicit children snippet at the same time as implicit children content. Remove either the non-whitespace content or the children snippet block
```
### snippet_invalid_export
```
An exported snippet can only reference things declared in a `<script module>`, or other exportable snippets
```
It's possible to export a snippet from a `<script module>` block, but only if it doesn't reference anything defined inside a non-module-level `<script>`. For example you can't do this...
```svelte
<script module>
export { greeting };
</script>
<script>
let message = 'hello';
</script>
{#snippet greeting(name)}
<p>{message} {name}!</p>
{/snippet}
```
...because `greeting` references `message`, which is defined in the second `<script>`.
### snippet_invalid_rest_parameter
```
Snippets do not support rest parameters; use an array instead
```
### snippet_parameter_assignment
```
Cannot reassign or bind to snippet parameter
```
### snippet_shadowing_prop
```
This snippet is shadowing the prop `%prop%` with the same name
```
### state_invalid_export
```
Cannot export state from a module if it is reassigned. Either export a function returning the state value or only mutate the state value's properties
```
### state_invalid_placement
```
`%rune%(...)` can only be used as a variable declaration initializer or a class field
```
### store_invalid_scoped_subscription
```
Cannot subscribe to stores that are not declared at the top level of the component
```
### store_invalid_subscription
```
Cannot reference store value inside `<script module>`
```
### store_invalid_subscription_module
```
Cannot reference store value outside a `.svelte` file
```
Using a `$` prefix to refer to the value of a store is only possible inside `.svelte` files, where Svelte can automatically create subscriptions when a component is mounted and unsubscribe when the component is unmounted. Consider migrating to runes instead.
### style_directive_invalid_modifier
```
`style:` directive can only use the `important` modifier
```
### style_duplicate
```
A component can have a single top-level `<style>` element
```
### svelte_body_illegal_attribute
```
`<svelte:body>` does not support non-event attributes or spread attributes
```
### svelte_boundary_invalid_attribute
```
Valid attributes on `<svelte:boundary>` are `onerror` and `failed`
```
### svelte_boundary_invalid_attribute_value
```
Attribute value must be a non-string expression
```
### svelte_component_invalid_this
```
Invalid component definition — must be an `{expression}`
```
### svelte_component_missing_this
```
`<svelte:component>` must have a 'this' attribute
```
### svelte_element_missing_this
```
`<svelte:element>` must have a 'this' attribute with a value
```
### svelte_fragment_invalid_attribute
```
`<svelte:fragment>` can only have a slot attribute and (optionally) a let: directive
```
### svelte_fragment_invalid_placement
```
`<svelte:fragment>` must be the direct child of a component
```
### svelte_head_illegal_attribute
```
`<svelte:head>` cannot have attributes nor directives
```
### svelte_meta_duplicate
```
A component can only have one `<%name%>` element
```
### svelte_meta_invalid_content
```
<%name%> cannot have children
```
### svelte_meta_invalid_placement
```
`<%name%>` tags cannot be inside elements or blocks
```
### svelte_meta_invalid_tag
```
Valid `<svelte:...>` tag names are %list%
```
### svelte_options_deprecated_tag
```
"tag" option is deprecated — use "customElement" instead
```
### svelte_options_invalid_attribute
```
`<svelte:options>` can only receive static attributes
```
### svelte_options_invalid_attribute_value
```
Value must be %list%, if specified
```
### svelte_options_invalid_customelement
```
"customElement" must be a string literal defining a valid custom element name or an object of the form { tag?: string; shadow?: "open" | "none"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }
```
### svelte_options_invalid_customelement_props
```
"props" must be a statically analyzable object literal of the form "{ [key: string]: { attribute?: string; reflect?: boolean; type?: "String" | "Boolean" | "Number" | "Array" | "Object" }"
```
### svelte_options_invalid_customelement_shadow
```
"shadow" must be either "open" or "none"
```
### svelte_options_invalid_tagname
```
Tag name must be lowercase and hyphenated
```
See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names
### svelte_options_reserved_tagname
```
Tag name is reserved
```
See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names
### svelte_options_unknown_attribute
```
`<svelte:options>` unknown attribute '%name%'
```
### svelte_self_invalid_placement
```
`<svelte:self>` components can only exist inside `{#if}` blocks, `{#each}` blocks, `{#snippet}` blocks or slots passed to components
```
### tag_invalid_name
```
Expected a valid element or component name. Components must have a valid variable name or dot notation expression
```
### tag_invalid_placement
```
{@%name% ...} tag cannot be %location%
```
### textarea_invalid_content
```
A `<textarea>` can have either a value attribute or (equivalently) child content, but not both
```
### title_illegal_attribute
```
`<title>` cannot have attributes nor directives
```
### title_invalid_content
```
`<title>` can only contain text and {tags}
```
### transition_conflict
```
Cannot use `%type%:` alongside existing `%existing%:` directive
```
### transition_duplicate
```
Cannot use multiple `%type%:` directives on a single element
```
### typescript_invalid_feature
```
TypeScript language features like %feature% are not natively supported, and their use is generally discouraged. Outside of `<script>` tags, these features are not supported. For use within `<script>` tags, you will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler. If you are using `vitePreprocess`, make sure to specifically enable preprocessing script tags (`vitePreprocess({ script: true })`)
```
### unexpected_eof
```
Unexpected end of input
```
### unexpected_reserved_word
```
'%word%' is a reserved word in JavaScript and cannot be used here
```
### unterminated_string_constant
```
Unterminated string constant
```
### void_element_invalid_content
```
Void elements cannot have children or closing tags
```
# Compiler warnings
Svelte warns you at compile time if it catches potential mistakes, such as writing inaccessible markup.
Some warnings may be incorrect in your concrete use case. You can disable such false positives by placing a `<!-- svelte-ignore <code> -->` comment above the line that causes the warning. Example:
```svelte
<!-- svelte-ignore a11y_autofocus -->
<input autofocus />
```
You can list multiple rules in a single comment (separated by commas), and add an explanatory note (in parentheses) alongside them:
```svelte
<!-- svelte-ignore a11y_click_events_have_key_events, a11y_no_static_element_interactions (because of reasons) -->
<div onclick>...</div>
```
<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
### a11y_accesskey
```
Avoid using accesskey
```
Enforce no `accesskey` on element. Access keys are HTML attributes that allow web developers to assign keyboard shortcuts to elements. Inconsistencies between keyboard shortcuts and keyboard commands used by screen reader and keyboard-only users create accessibility complications. To avoid complications, access keys should not be used.
<!-- prettier-ignore -->
```svelte
<!-- A11y: Avoid using accesskey -->
<div accesskey="z"></div>
```
### a11y_aria_activedescendant_has_tabindex
```
An element with an aria-activedescendant attribute should have a tabindex value
```
An element with `aria-activedescendant` must be tabbable, so it must either have an inherent `tabindex` or declare `tabindex` as an attribute.
```svelte
<!-- A11y: Elements with attribute aria-activedescendant should have tabindex value -->
<div aria-activedescendant="some-id"></div>
```
### a11y_aria_attributes
```
`<%name%>` should not have aria-* attributes
```
Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example `meta`, `html`, `script`, `style`. This rule enforces that these DOM elements do not contain the `aria-*` props.
```svelte
<!-- A11y: <meta> should not have aria-* attributes -->
<meta aria-hidden="false" />
```
### a11y_autocomplete_valid
```
'%value%' is an invalid value for 'autocomplete' on `<input type="%type%">`
```
### a11y_autofocus
```
Avoid using autofocus
```
Enforce that `autofocus` is not used on elements. Autofocusing elements can cause usability issues for sighted and non-sighted users alike.
```svelte
<!-- A11y: Avoid using autofocus -->
<input autofocus />
```
### a11y_click_events_have_key_events
```
Visible, non-interactive elements with a click event must be accompanied by a keyboard event handler. Consider whether an interactive element such as `<button type="button">` or `<a>` might be more appropriate
```
Enforce that visible, non-interactive elements with an `onclick` event are accompanied by a keyboard event handler.
Users should first consider whether an interactive element might be more appropriate such as a `<button type="button">` element for actions or `<a>` element for navigations. These elements are more semantically meaningful and will have built-in key handling. E.g. `Space` and `Enter` will trigger a `<button>` and `Enter` will trigger an `<a>` element.
If a non-interactive element is required then `onclick` should be accompanied by an `onkeyup` or `onkeydown` handler that enables the user to perform equivalent actions via the keyboard. In order for the user to be able to trigger a key press, the element will also need to be focusable by adding a [`tabindex`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex). While an `onkeypress` handler will also silence this warning, it should be noted that the `keypress` event is deprecated.
```svelte
<!-- A11y: visible, non-interactive elements with an onclick event must be accompanied by a keyboard event handler. -->
<div onclick={() => {}}></div>
```
Coding for the keyboard is important for users with physical disabilities who cannot use a mouse, AT compatibility, and screenreader users.
### a11y_consider_explicit_label
```
Buttons and links should either contain text or have an `aria-label` or `aria-labelledby` attribute
```
### a11y_distracting_elements
```
Avoid `<%name%>` elements
```
Enforces that no distracting elements are used. Elements that can be visually distracting can cause accessibility issues with visually impaired users. Such elements are most likely deprecated, and should be avoided.
The following elements are visually distracting: `<marquee>` and `<blink>`.
```svelte
<!-- A11y: Avoid <marquee> elements -->
<marquee></marquee>
```
### a11y_figcaption_index
```
`<figcaption>` must be first or last child of `<figure>`
```
### a11y_figcaption_parent
```
`<figcaption>` must be an immediate child of `<figure>`
```
Enforce that certain DOM elements have the correct structure.
```svelte
<!-- A11y: <figcaption> must be an immediate child of <figure> -->
<div>
<figcaption>Image caption</figcaption>
</div>
```
### a11y_hidden
```
`<%name%>` element should not be hidden
```
Certain DOM elements are useful for screen reader navigation and should not be hidden.
<!-- prettier-ignore -->
```svelte
<!-- A11y: <h2> element should not be hidden -->
<h2 aria-hidden="true">invisible header</h2>
```
### a11y_img_redundant_alt
```
Screenreaders already announce `<img>` elements as an image
```
Enforce img alt attribute does not contain the word image, picture, or photo. Screen readers already announce `img` elements as an image. There is no need to use words such as _image_, _photo_, and/or _picture_.
```svelte
<img src="foo" alt="Foo eating a sandwich." />
<!-- aria-hidden, won't be announced by screen reader -->
<img src="bar" aria-hidden="true" alt="Picture of me taking a photo of an image" />
<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="foo" alt="Photo of foo being weird." />
<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="bar" alt="Image of me at a bar!" />
<!-- A11y: Screen readers already announce <img> elements as an image. -->
<img src="foo" alt="Picture of baz fixing a bug." />
```
### a11y_incorrect_aria_attribute_type
```
The value of '%attribute%' must be a %type%
```
Enforce that only the correct type of value is used for aria attributes. For example, `aria-hidden`
should only receive a boolean.
```svelte
<!-- A11y: The value of 'aria-hidden' must be exactly one of true or false -->
<div aria-hidden="yes"></div>
```
### a11y_incorrect_aria_attribute_type_boolean
```
The value of '%attribute%' must be either 'true' or 'false'. It cannot be empty
```
### a11y_incorrect_aria_attribute_type_id
```
The value of '%attribute%' must be a string that represents a DOM element ID
```
### a11y_incorrect_aria_attribute_type_idlist
```
The value of '%attribute%' must be a space-separated list of strings that represent DOM element IDs
```
### a11y_incorrect_aria_attribute_type_integer
```
The value of '%attribute%' must be an integer
```
### a11y_incorrect_aria_attribute_type_token
```
The value of '%attribute%' must be exactly one of %values%
```
### a11y_incorrect_aria_attribute_type_tokenlist
```
The value of '%attribute%' must be a space-separated list of one or more of %values%
```
### a11y_incorrect_aria_attribute_type_tristate
```
The value of '%attribute%' must be exactly one of true, false, or mixed
```
### a11y_interactive_supports_focus
```
Elements with the '%role%' interactive role must have a tabindex value
```
Enforce that elements with an interactive role and interactive handlers (mouse or key press) must be focusable or tabbable.
```svelte
<!-- A11y: Elements with the 'button' interactive role must have a tabindex value. -->
<div role="button" onkeypress={() => {}} />
```
### a11y_invalid_attribute
```
'%href_value%' is not a valid %href_attribute% attribute
```
Enforce that attributes important for accessibility have a valid value. For example, `href` should not be empty, `'#'`, or `javascript:`.
```svelte
<!-- A11y: '' is not a valid href attribute -->
<a href="">invalid</a>
```
### a11y_label_has_associated_control
```
A form label must be associated with a control
```
Enforce that a label tag has a text label and an associated control.
There are two supported ways to associate a label with a control:
- Wrapping a control in a label tag.
- Adding `for` to a label and assigning it the ID of an input on the page.
```svelte
<label for="id">B</label>
<label>C <input type="text" /></label>
<!-- A11y: A form label must be associated with a control. -->
<label>A</label>
```
### a11y_media_has_caption
```
`<video>` elements must have a `<track kind="captions">`
```
Providing captions for media is essential for deaf users to follow along. Captions should be a transcription or translation of the dialogue, sound effects, relevant musical cues, and other relevant audio information. Not only is this important for accessibility, but can also be useful for all users in the case that the media is unavailable (similar to `alt` text on an image when an image is unable to load).
The captions should contain all important and relevant information to understand the corresponding media. This may mean that the captions are not a 1:1 mapping of the dialogue in the media content. However, captions are not necessary for video components with the `muted` attribute.
```svelte
<video><track kind="captions" /></video>
<audio muted></audio>
<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
<video></video>
<!-- A11y: Media elements must have a <track kind=\"captions\"> -->
<video><track /></video>
```
### a11y_misplaced_role
```
`<%name%>` should not have role attribute
```
Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example `meta`, `html`, `script`, `style`. This rule enforces that these DOM elements do not contain the `role` props.
```svelte
<!-- A11y: <meta> should not have role attribute -->
<meta role="tooltip" />
```
### a11y_misplaced_scope
```
The scope attribute should only be used with `<th>` elements
```
The scope attribute should only be used on `<th>` elements.
<!-- prettier-ignore -->
```svelte
<!-- A11y: The scope attribute should only be used with <th> elements -->
<div scope="row" />
```
### a11y_missing_attribute
```
`<%name%>` element should have %article% %sequence% attribute
```
Enforce that attributes required for accessibility are present on an element. This includes the following checks:
- `<a>` should have an href (unless it's a [fragment-defining tag](https://github.com/sveltejs/svelte/issues/4697))
- `<area>` should have alt, aria-label, or aria-labelledby
- `<html>` should have lang
- `<iframe>` should have title
- `<img>` should have alt
- `<object>` should have title, aria-label, or aria-labelledby
- `<input type="image">` should have alt, aria-label, or aria-labelledby
```svelte
<!-- A11y: <input type=\"image\"> element should have an alt, aria-label or aria-labelledby attribute -->
<input type="image" />
<!-- A11y: <html> element should have a lang attribute -->
<html></html>
<!-- A11y: <a> element should have an href attribute -->
<a>text</a>
```
### a11y_missing_content
```
`<%name%>` element should contain text
```
Enforce that heading elements (`h1`, `h2`, etc.) and anchors have content and that the content is accessible to screen readers
```svelte
<!-- A11y: <a> element should have child content -->
<a href="/foo"></a>
<!-- A11y: <h1> element should have child content -->
<h1></h1>
```
### a11y_mouse_events_have_key_events
```
'%event%' event must be accompanied by '%accompanied_by%' event
```
Enforce that `onmouseover` and `onmouseout` are accompanied by `onfocus` and `onblur`, respectively. This helps to ensure that any functionality triggered by these mouse events is also accessible to keyboard users.
```svelte
<!-- A11y: onmouseover must be accompanied by onfocus -->
<div onmouseover={handleMouseover} />
<!-- A11y: onmouseout must be accompanied by onblur -->
<div onmouseout={handleMouseout} />
```
### a11y_no_abstract_role
```
Abstract role '%role%' is forbidden
```
### a11y_no_interactive_element_to_noninteractive_role
```
`<%element%>` cannot have role '%role%'
```
[WAI-ARIA](https://www.w3.org/TR/wai-aria-1.1/#usage_intro) roles should not be used to convert an interactive element to a non-interactive element. Non-interactive ARIA roles include `article`, `banner`, `complementary`, `img`, `listitem`, `main`, `region` and `tooltip`.
```svelte
<!-- A11y: <textarea> cannot have role 'listitem' -->
<textarea role="listitem"></textarea>
```
### a11y_no_noninteractive_element_interactions
```
Non-interactive element `<%element%>` should not be assigned mouse or keyboard event listeners
```
A non-interactive element does not support event handlers (mouse and key handlers). Non-interactive elements include `<main>`, `<area>`, `<h1>` (,`<h2>`, etc), `<p>`, `<img>`, `<li>`, `<ul>` and `<ol>`. Non-interactive [WAI-ARIA roles](https://www.w3.org/TR/wai-aria-1.1/#usage_intro) include `article`, `banner`, `complementary`, `img`, `listitem`, `main`, `region` and `tooltip`.
```sv
<!-- `A11y: Non-interactive element <li> should not be assigned mouse or keyboard event listeners.` -->
<li onclick={() => {}}></li>
<!-- `A11y: Non-interactive element <div> should not be assigned mouse or keyboard event listeners.` -->
<div role="listitem" onclick={() => {}}></div>
```
### a11y_no_noninteractive_element_to_interactive_role
```
Non-interactive element `<%element%>` cannot have interactive role '%role%'
```
[WAI-ARIA](https://www.w3.org/TR/wai-aria-1.1/#usage_intro) roles should not be used to convert a non-interactive element to an interactive element. Interactive ARIA roles include `button`, `link`, `checkbox`, `menuitem`, `menuitemcheckbox`, `menuitemradio`, `option`, `radio`, `searchbox`, `switch` and `textbox`.
```svelte
<!-- A11y: Non-interactive element <h3> cannot have interactive role 'searchbox' -->
<h3 role="searchbox">Button</h3>
```
### a11y_no_noninteractive_tabindex
```
noninteractive element cannot have nonnegative tabIndex value
```
Tab key navigation should be limited to elements on the page that can be interacted with.
<!-- prettier-ignore -->
```svelte
<!-- A11y: noninteractive element cannot have nonnegative tabIndex value -->
<div tabindex="0"></div>
```
### a11y_no_redundant_roles
```
Redundant role '%role%'
```
Some HTML elements have default ARIA roles. Giving these elements an ARIA role that is already set by the browser [has no effect](https://www.w3.org/TR/using-aria/#aria-does-nothing) and is redundant.
```svelte
<!-- A11y: Redundant role 'button' -->
<button role="button">...</button>
<!-- A11y: Redundant role 'img' -->
<img role="img" src="foo.jpg" />
```
### a11y_no_static_element_interactions
```
`<%element%>` with a %handler% handler must have an ARIA role
```
Elements like `<div>` with interactive handlers like `click` must have an ARIA role.
<!-- prettier-ignore -->
```svelte
<!-- A11y: <div> with click handler must have an ARIA role -->
<div onclick={() => ''}></div>
```
### a11y_positive_tabindex
```
Avoid tabindex values above zero
```
Avoid positive `tabindex` property values. This will move elements out of the expected tab order, creating a confusing experience for keyboard users.
<!-- prettier-ignore -->
```svelte
<!-- A11y: avoid tabindex values above zero -->
<div tabindex="1"></div>
```
### a11y_role_has_required_aria_props
```
Elements with the ARIA role "%role%" must have the following attributes defined: %props%
```
Elements with ARIA roles must have all required attributes for that role.
```svelte
<!-- A11y: A11y: Elements with the ARIA role "checkbox" must have the following attributes defined: "aria-checked" -->
<span role="checkbox" aria-labelledby="foo" tabindex="0"></span>
```
### a11y_role_supports_aria_props
```
The attribute '%attribute%' is not supported by the role '%role%'
```
Elements with explicit or implicit roles defined contain only `aria-*` properties supported by that role.
```svelte
<!-- A11y: The attribute 'aria-multiline' is not supported by the role 'link'. -->
<div role="link" aria-multiline></div>
<!-- A11y: The attribute 'aria-required' is not supported by the role 'listitem'. This role is implicit on the element <li>. -->
<li aria-required></li>
```
### a11y_role_supports_aria_props_implicit
```
The attribute '%attribute%' is not supported by the role '%role%'. This role is implicit on the element `<%name%>`
```
Elements with explicit or implicit roles defined contain only `aria-*` properties supported by that role.
```svelte
<!-- A11y: The attribute 'aria-multiline' is not supported by the role 'link'. -->
<div role="link" aria-multiline></div>
<!-- A11y: The attribute 'aria-required' is not supported by the role 'listitem'. This role is implicit on the element <li>. -->
<li aria-required></li>
```
### a11y_unknown_aria_attribute
```
Unknown aria attribute 'aria-%attribute%'
```
```
Unknown aria attribute 'aria-%attribute%'. Did you mean '%suggestion%'?
```
Enforce that only known ARIA attributes are used. This is based on the [WAI-ARIA States and Properties spec](https://www.w3.org/WAI/PF/aria-1.1/states_and_properties).
```svelte
<!-- A11y: Unknown aria attribute 'aria-labeledby' (did you mean 'labelledby'?) -->
<input type="image" aria-labeledby="foo" />
```
### a11y_unknown_role
```
Unknown role '%role%'
```
```
Unknown role '%role%'. Did you mean '%suggestion%'?
```
Elements with ARIA roles must use a valid, non-abstract ARIA role. A reference to role definitions can be found at [WAI-ARIA](https://www.w3.org/TR/wai-aria/#role_definitions) site.
<!-- prettier-ignore -->
```svelte
<!-- A11y: Unknown role 'toooltip' (did you mean 'tooltip'?) -->
<div role="toooltip"></div>
```
### attribute_avoid_is
```
The "is" attribute is not supported cross-browser and should be avoided
```
### attribute_global_event_reference
```
You are referencing `globalThis.%name%`. Did you forget to declare a variable with that name?
```
### attribute_illegal_colon
```
Attributes should not contain ':' characters to prevent ambiguity with Svelte directives
```
### attribute_invalid_property_name
```
'%wrong%' is not a valid HTML attribute. Did you mean '%right%'?
```
### attribute_quoted
```
Quoted attributes on components and custom elements will be stringified in a future version of Svelte. If this isn't what you want, remove the quotes
```
### bind_invalid_each_rest
```
The rest operator (...) will create a new object and binding '%name%' with the original object will not work
```
### block_empty
```
Empty block
```
### component_name_lowercase
```
`<%name%>` will be treated as an HTML element unless it begins with a capital letter
```
### css_unused_selector
```
Unused CSS selector "%name%"
```
Svelte traverses both the template and the `<style>` tag to find out which of the CSS selectors are not used within the template, so it can remove them.
In some situations a selector may target an element that is not 'visible' to the compiler, for example because it is part of an `{@html ...}` tag or you're overriding styles in a child component. In these cases, use [`:global`](/docs/svelte/global-styles) to preserve the selector as-is:
```svelte
<div class="post">{@html content}</div>
<style>
.post :global {
p {...}
}
</style>
```
### element_invalid_self_closing_tag
```
Self-closing HTML tags for non-void elements are ambiguous — use `<%name% ...></%name%>` rather than `<%name% ... />`
```
In HTML, there's [no such thing as a self-closing tag](https://jakearchibald.com/2023/against-self-closing-tags-in-html/). While this _looks_ like a self-contained element with some text next to it...
```html
<div>
<span class="icon" /> some text!
</div>
```
...a spec-compliant HTML parser (such as a browser) will in fact parse it like this, with the text _inside_ the icon:
```html
<div>
<span class="icon"> some text! </span>
</div>
```
Some templating languages (including Svelte) will 'fix' HTML by turning `<span />` into `<span></span>`. Others adhere to the spec. Both result in ambiguity and confusion when copy-pasting code between different contexts, and as such Svelte prompts you to resolve the ambiguity directly by having an explicit closing tag.
To automate this, run the dedicated migration:
```bash
npx sv migrate self-closing-tags
```
In a future version of Svelte, self-closing tags may be upgraded from a warning to an error.
### event_directive_deprecated
```
Using `on:%name%` to listen to the %name% event is deprecated. Use the event attribute `on%name%` instead
```
See [the migration guide](v5-migration-guide#Event-changes) for more info.
### export_let_unused
```
Component has unused export property '%name%'. If it is for external reference only, please consider using `export const %name%`
```
### legacy_code
```
`%code%` is no longer valid — please use `%suggestion%` instead
```
### legacy_component_creation
```
Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.
```
See the [migration guide](v5-migration-guide#Components-are-no-longer-classes) for more info.
### node_invalid_placement_ssr
```
%message%. When rendering this component on the server, the resulting HTML will be modified by the browser (by moving, removing, or inserting elements), likely resulting in a `hydration_mismatch` warning
```
HTML restricts where certain elements can appear. In case of a violation the browser will 'repair' the HTML in a way that breaks Svelte's assumptions about the structure of your components. Some examples:
- `<p>hello <div>world</div></p>` will result in `<p>hello </p><div>world</div><p></p>` (the `<div>` autoclosed the `<p>` because `<p>` cannot contain block-level elements)
- `<option><div>option a</div></option>` will result in `<option>option a</option>` (the `<div>` is removed)
- `<table><tr><td>cell</td></tr></table>` will result in `<table><tbody><tr><td>cell</td></tr></tbody></table>` (a `<tbody>` is auto-inserted)
This code will work when the component is rendered on the client (which is why this is a warning rather than an error), but if you use server rendering it will cause hydration to fail.
### non_reactive_update
```
`%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates
```
This warning is thrown when the compiler detects the following:
- a variable was declared without `$state` or `$state.raw`
- the variable is reassigned
- the variable is read in a reactive context
In this case, changing the value will not correctly trigger updates. Example:
```svelte
<script>
let reactive = $state('reactive');
let stale = 'stale';
</script>
<p>This value updates: {reactive}</p>
<p>This value does not update: {stale}</p>
<button onclick={() => {
stale = 'updated';
reactive = 'updated';
}}>update</button>
```
To fix this, wrap your variable declaration with `$state`.
### options_deprecated_accessors
```
The `accessors` option has been deprecated. It will have no effect in runes mode
```
### options_deprecated_immutable
```
The `immutable` option has been deprecated. It will have no effect in runes mode
```
### options_missing_custom_element
```
The `customElement` option is used when generating a custom element. Did you forget the `customElement: true` compile option?
```
### options_removed_enable_sourcemap
```
The `enableSourcemap` option has been removed. Source maps are always generated now, and tooling can choose to ignore them
```
### options_removed_hydratable
```
The `hydratable` option has been removed. Svelte components are always hydratable now
```
### options_removed_loop_guard_timeout
```
The `loopGuardTimeout` option has been removed
```
### options_renamed_ssr_dom
```
`generate: "dom"` and `generate: "ssr"` options have been renamed to "client" and "server" respectively
```
### perf_avoid_inline_class
```
Avoid 'new class' — instead, declare the class at the top level scope
```
### perf_avoid_nested_class
```
Avoid declaring classes below the top level scope
```
### reactive_declaration_invalid_placement
```
Reactive declarations only exist at the top level of the instance script
```
### reactive_declaration_module_script_dependency
```
Reassignments of module-level declarations will not cause reactive statements to update
```
### script_context_deprecated
```
`context="module"` is deprecated, use the `module` attribute instead
```
```svelte
<script ---context="module"--- +++module+++>
let foo = 'bar';
</script>
```
### script_unknown_attribute
```
Unrecognized attribute — should be one of `generics`, `lang` or `module`. If this exists for a preprocessor, ensure that the preprocessor removes it
```
### slot_element_deprecated
```
Using `<slot>` to render parent content is deprecated. Use `{@render ...}` tags instead
```
See [the migration guide](v5-migration-guide#Snippets-instead-of-slots) for more info.
### state_referenced_locally
```
State referenced in its own scope will never update. Did you mean to reference it inside a closure?
```
This warning is thrown when the compiler detects the following:
- A reactive variable is declared
- the variable is reassigned
- the variable is referenced inside the same scope it is declared and it is a non-reactive context
In this case, the state reassignment will not be noticed by whatever you passed it to. For example, if you pass the state to a function, that function will not notice the updates:
```svelte
<!--- file: Parent.svelte --->
<script>
import { setContext } from 'svelte';
let count = $state(0);
// warning: state_referenced_locally
setContext('count', count);
</script>
<button onclick={() => count++}>
increment
</button>
```
```svelte
<!--- file: Child.svelte --->
<script>
import { getContext } from 'svelte';
const count = getContext('count');
</script>
<!-- This will never update -->
<p>The count is {count}</p>
```
To fix this, reference the variable such that it is lazily evaluated. For the above example, this can be achieved by wrapping `count` in a function:
```svelte
<!--- file: Parent.svelte --->
<script>
import { setContext } from 'svelte';
let count = $state(0);
setContext('count', +++() => count+++);
</script>
<button onclick={() => count++}>
increment
</button>
```
```svelte
<!--- file: Child.svelte --->
<script>
import { getContext } from 'svelte';
const count = getContext('count');
</script>
<!-- This will update -->
<p>The count is {+++count()+++}</p>
```
For more info, see [Passing state into functions]($state#Passing-state-into-functions).
### store_rune_conflict
```
It looks like you're using the `$%name%` rune, but there is a local binding called `%name%`. Referencing a local variable with a `$` prefix will create a store subscription. Please rename `%name%` to avoid the ambiguity
```
### svelte_component_deprecated
```
`<svelte:component>` is deprecated in runes mode — components are dynamic by default
```
In previous versions of Svelte, the component constructor was fixed when the component was rendered. In other words, if you wanted `<X>` to re-render when `X` changed, you would either have to use `<svelte:component this={X}>` or put the component inside a `{#key X}...{/key}` block.
In Svelte 5 this is no longer true — if `X` changes, `<X>` re-renders.
In some cases `<object.property>` syntax can be used as a replacement; a lowercased variable with property access is recognized as a component in Svelte 5.
For complex component resolution logic, an intermediary, capitalized variable may be necessary. E.g. in places where `@const` can be used:
<!-- prettier-ignore -->
```svelte
{#each items as item}
---<svelte:component this={item.condition ? Y : Z} />---
+++{@const Component = item.condition ? Y : Z}+++
+++<Component />+++
{/each}
```
A derived value may be used in other contexts:
<!-- prettier-ignore -->
```svelte
<script>
// ...
let condition = $state(false);
+++const Component = $derived(condition ? Y : Z);+++
</script>
---<svelte:component this={condition ? Y : Z} />---
+++<Component />+++
```
### svelte_element_invalid_this
```
`this` should be an `{expression}`. Using a string attribute value will cause an error in future versions of Svelte
```
### svelte_self_deprecated
```
`<svelte:self>` is deprecated — use self-imports (e.g. `import %name% from './%basename%'`) instead
```
See [the note in the docs](legacy-svelte-self) for more info.
### unknown_code
```
`%code%` is not a recognised code
```
```
`%code%` is not a recognised code (did you mean `%suggestion%`?)
```
# Runtime errors
## Client errors
<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
### bind_invalid_checkbox_value
```
Using `bind:value` together with a checkbox input is not allowed. Use `bind:checked` instead
```
### bind_invalid_export
```
Component %component% has an export named `%key%` that a consumer component is trying to access using `bind:%key%`, which is disallowed. Instead, use `bind:this` (e.g. `<%name% bind:this={component} />`) and then access the property on the bound component instance (e.g. `component.%key%`)
```
### bind_not_bindable
```
A component is attempting to bind to a non-bindable property `%key%` belonging to %component% (i.e. `<%name% bind:%key%={...}>`). To mark a property as bindable: `let { %key% = $bindable() } = $props()`
```
### component_api_changed
```
%parent% called `%method%` on an instance of %component%, which is no longer valid in Svelte 5
```
See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information.
### component_api_invalid_new
```
Attempted to instantiate %component% with `new %name%`, which is no longer valid in Svelte 5. If this component is not under your control, set the `compatibility.componentApi` compiler option to `4` to keep it working.
```
See the [migration guide](/docs/svelte/v5-migration-guide#Components-are-no-longer-classes) for more information.
### derived_references_self
```
A derived value cannot reference itself recursively
```
### each_key_duplicate
```
Keyed each block has duplicate key at indexes %a% and %b%
```
```
Keyed each block has duplicate key `%value%` at indexes %a% and %b%
```
### effect_in_teardown
```
`%rune%` cannot be used inside an effect cleanup function
```
### effect_in_unowned_derived
```
Effect cannot be created inside a `$derived` value that was not itself created inside an effect
```
### effect_orphan
```
`%rune%` can only be used inside an effect (e.g. during component initialisation)
```
### effect_update_depth_exceeded
```
Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops
```
### hydration_failed
```
Failed to hydrate the application
```
### invalid_snippet
```
Could not `{@render}` snippet due to the expression being `null` or `undefined`. Consider using optional chaining `{@render snippet?.()}`
```
### lifecycle_legacy_only
```
`%name%(...)` cannot be used in runes mode
```
### props_invalid_value
```
Cannot do `bind:%key%={undefined}` when `%key%` has a fallback value
```
### props_rest_readonly
```
Rest element properties of `$props()` such as `%property%` are readonly
```
### rune_outside_svelte
```
The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files
```
### state_descriptors_fixed
```
Property descriptors defined on `$state` objects must contain `value` and always be `enumerable`, `configurable` and `writable`.
```
### state_prototype_fixed
```
Cannot set prototype of `$state` object
```
### state_unsafe_local_read
```
Reading state that was created inside the same derived is forbidden. Consider using `untrack` to read locally created state
```
### state_unsafe_mutation
```
Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`
```
## Server errors
<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
### lifecycle_function_unavailable
```
`%name%(...)` is not available on the server
```
Certain methods such as `mount` cannot be invoked while running in a server context. Avoid calling them eagerly, i.e. not during render.
## Shared errors
<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
### invalid_default_snippet
```
Cannot use `{@render children(...)}` if the parent component uses `let:` directives. Consider using a named snippet instead
```
This error would be thrown in a setup like this:
```svelte
<!--- file: Parent.svelte --->
<List {items} let:entry>
<span>{entry}</span>
</List>
```
```svelte
<!--- file: List.svelte --->
<script>
let { items, children } = $props();
</script>
<ul>
{#each items as item}
<li>{@render children(item)}</li>
{/each}
</ul>
```
Here, `List.svelte` is using `{@render children(item)` which means it expects `Parent.svelte` to use snippets. Instead, `Parent.svelte` uses the deprecated `let:` directive. This combination of APIs is incompatible, hence the error.
### lifecycle_outside_component
```
`%name%(...)` can only be used during component initialisation
```
Certain lifecycle methods can only be used during component initialisation. To fix this, make sure you're invoking the method inside the _top level of the instance script_ of your component.
```svelte
<script>
import { onMount } from 'svelte';
function handleClick() {
// This is wrong
onMount(() => {})
}
// This is correct
onMount(() => {})
</script>
<button onclick={handleClick}>click me</button>
```
### store_invalid_shape
```
`%name%` is not a store with a `subscribe` method
```
### svelte_element_invalid_this_value
```
The `this` prop on `<svelte:element>` must be a string, if defined
```
# Runtime warnings
## Client warnings
<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
### assignment_value_stale
```
Assignment to `%property%` property (%location%) will evaluate to the right-hand side, not the value of `%property%` following the assignment. This may result in unexpected behaviour.
```
Given a case like this...
```svelte
<script>
let object = $state({ array: null });
function add() {
(object.array ??= []).push(object.array.length);
}
</script>
<button onclick={add}>add</button>
<p>items: {JSON.stringify(object.items)}</p>
```
...the array being pushed to when the button is first clicked is the `[]` on the right-hand side of the assignment, but the resulting value of `object.array` is an empty state proxy. As a result, the pushed value will be discarded.
You can fix this by separating it into two statements:
```js
let object = { array: [0] };
// ---cut---
function add() {
object.array ??= [];
object.array.push(object.array.length);
}
```
### binding_property_non_reactive
```
`%binding%` is binding to a non-reactive property
```
```
`%binding%` (%location%) is binding to a non-reactive property
```
### console_log_state
```
Your `console.%method%` contained `$state` proxies. Consider using `$inspect(...)` or `$state.snapshot(...)` instead
```
When logging a [proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy), browser devtools will log the proxy itself rather than the value it represents. In the case of Svelte, the 'target' of a `$state` proxy might not resemble its current value, which can be confusing.
The easiest way to log a value as it changes over time is to use the [`$inspect`](/docs/svelte/$inspect) rune. Alternatively, to log things on a one-off basis (for example, inside an event handler) you can use [`$state.snapshot`](/docs/svelte/$state#$state.snapshot) to take a snapshot of the current value.
### event_handler_invalid
```
%handler% should be a function. Did you mean to %suggestion%?
```
### hydration_attribute_changed
```
The `%attribute%` attribute on `%html%` changed its value between server and client renders. The client value, `%value%`, will be ignored in favour of the server value
```
Certain attributes like `src` on an `<img>` element will not be repaired during hydration, i.e. the server value will be kept. That's because updating these attributes can cause the image to be refetched (or in the case of an `<iframe>`, for the frame to be reloaded), even if they resolve to the same resource.
To fix this, either silence the warning with a [`svelte-ignore`](basic-markup#Comments) comment, or ensure that the value stays the same between server and client. If you really need the value to change on hydration, you can force an update like this:
```svelte
<script>
let { src } = $props();
if (typeof window !== 'undefined') {
// stash the value...
const initial = src;
// unset it...
src = undefined;
$effect(() => {
// ...and reset after we've mounted
src = initial;
});
}
</script>
<img {src} />
```
### hydration_html_changed
```
The value of an `{@html ...}` block changed between server and client renders. The client value will be ignored in favour of the server value
```
```
The value of an `{@html ...}` block %location% changed between server and client renders. The client value will be ignored in favour of the server value
```
If the `{@html ...}` value changes between the server and the client, it will not be repaired during hydration, i.e. the server value will be kept. That's because change detection during hydration is expensive and usually unnecessary.
To fix this, either silence the warning with a [`svelte-ignore`](basic-markup#Comments) comment, or ensure that the value stays the same between server and client. If you really need the value to change on hydration, you can force an update like this:
```svelte
<script>
let { markup } = $props();
if (typeof window !== 'undefined') {
// stash the value...
const initial = markup;
// unset it...
markup = undefined;
$effect(() => {
// ...and reset after we've mounted
markup = initial;
});
}
</script>
{@html markup}
```
### hydration_mismatch
```
Hydration failed because the initial UI does not match what was rendered on the server
```
```
Hydration failed because the initial UI does not match what was rendered on the server. The error occurred near %location%
```
This warning is thrown when Svelte encounters an error while hydrating the HTML from the server. During hydration, Svelte walks the DOM, expecting a certain structure. If that structure is different (for example because the HTML was repaired by the DOM because of invalid HTML), then Svelte will run into issues, resulting in this warning.
During development, this error is often preceeded by a `console.error` detailing the offending HTML, which needs fixing.
### invalid_raw_snippet_render
```
The `render` function passed to `createRawSnippet` should return HTML for a single element
```
### legacy_recursive_reactive_block
```
Detected a migrated `$:` reactive block in `%filename%` that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.
```
### lifecycle_double_unmount
```
Tried to unmount a component that was not mounted
```
### ownership_invalid_binding
```
%parent% passed a value to %child% with `bind:`, but the value is owned by %owner%. Consider creating a binding between %owner% and %parent%
```
Consider three components `GrandParent`, `Parent` and `Child`. If you do `<GrandParent bind:value>`, inside `GrandParent` pass on the variable via `<Parent {value} />` (note the missing `bind:`) and then do `<Child bind:value>` inside `Parent`, this warning is thrown.
To fix it, `bind:` to the value instead of just passing a property (i.e. in this example do `<Parent bind:value />`).
### ownership_invalid_mutation
```
Mutating a value outside the component that created it is strongly discouraged. Consider passing values to child components with `bind:`, or use a callback instead
```
```
%component% mutated a value owned by %owner%. This is strongly discouraged. Consider passing values to child components with `bind:`, or use a callback instead
```
Consider the following code:
```svelte
<!--- file: App.svelte --->
<script>
import Child from './Child.svelte';
let person = $state({ name: 'Florida', surname: 'Man' });
</script>
<Child {person} />
```
```svelte
<!--- file: Child.svelte --->
<script>
let { person } = $props();
</script>
<input bind:value={person.name}>
<input bind:value={person.surname}>
```
`Child` is mutating `person` which is owned by `App` without being explicitly "allowed" to do so. This is strongly discouraged since it can create code that is hard to reason about at scale ("who mutated this value?"), hence the warning.
To fix it, either create callback props to communicate changes, or mark `person` as [`$bindable`]($bindable).
### state_proxy_equality_mismatch
```
Reactive `$state(...)` proxies and the values they proxy have different identities. Because of this, comparisons with `%operator%` will produce unexpected results
```
`$state(...)` creates a [proxy](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Proxy) of the value it is passed. The proxy and the value have different identities, meaning equality checks will always return `false`:
```svelte
<script>
let value = { foo: 'bar' };
let proxy = $state(value);
value === proxy; // always false
</script>
```
To resolve this, ensure you're comparing values where both values were created with `$state(...)`, or neither were. Note that `$state.raw(...)` will _not_ create a state proxy.
### transition_slide_display
```
The `slide` transition does not work correctly for elements with `display: %value%`
```
The [slide](/docs/svelte/svelte-transition#slide) transition works by animating the `height` of the element, which requires a `display` style like `block`, `flex` or `grid`. It does not work for:
- `display: inline` (which is the default for elements like `<span>`), and its variants like `inline-block`, `inline-flex` and `inline-grid`
- `display: table` and `table-[name]`, which are the defaults for elements like `<table>` and `<tr>`
- `display: contents`
## Shared warnings
<!-- This file is generated by scripts/process-messages/index.js. Do not edit! -->
### dynamic_void_element_content
```
`<svelte:element this="%tag%">` is a void element — it cannot have content
```
Elements such as `<input>` cannot have content, any children passed to these elements will be ignored.
### state_snapshot_uncloneable
```
Value cannot be cloned with `$state.snapshot` — the original value was returned
```
```
The following properties cannot be cloned with `$state.snapshot` — the return value contains the originals:
%properties%
```
`$state.snapshot` tries to clone the given value in order to return a reference that no longer changes. Certain objects may not be cloneable, in which case the original value is returned. In the following example, `property` is cloned, but `window` is not, because DOM elements are uncloneable:
```js
const object = $state({ property: 'this is cloneable', window })
const snapshot = $state.snapshot(object);
```
# Overview
Svelte 5 introduced some significant changes to Svelte's API, including [runes](what-are-runes), [snippets](snippet) and event attributes. As a result, some Svelte 3/4 features are deprecated (though supported for now, unless otherwise specified) and will eventually be removed. We recommend that you incrementally [migrate your existing code](v5-migration-guide).
The following pages document these features for
- people still using Svelte 3/4
- people using Svelte 5, but with components that haven't yet been migrated
Since Svelte 3/4 syntax still works in Svelte 5, we will distinguish between _legacy mode_ and _runes mode_. Once a component is in runes mode (which you can opt into by using runes, or by explicitly setting the `runes: true` compiler option), legacy mode features are no longer available.
If you're exclusively interested in the Svelte 3/4 syntax, you can browse its documentation at [v4.svelte.dev](https://v4.svelte.dev).
# Reactive let/var declarations
In runes mode, reactive state is explicitly declared with the [`$state` rune]($state).
In legacy mode, variables declared at the top level of a component are automatically considered _reactive_. Reassigning or mutating these variables (`count += 1` or `object.x = y`) will cause the UI to update.
```svelte
<script>
let count = 0;
</script>
<button on:click={() => count += 1}>
clicks: {count}
</button>
```
Because Svelte's legacy mode reactivity is based on _assignments_, using array methods like `.push()` and `.splice()` won't automatically trigger updates. A subsequent assignment is required to 'tell' the compiler to update the UI:
```svelte
<script>
let numbers = [1, 2, 3, 4];
function addNumber() {
// this method call does not trigger an update
numbers.push(numbers.length + 1);
// this assignment will update anything
// that depends on `numbers`
numbers = numbers;
}
</script>
```
# Reactive $: statements
In runes mode, reactions to state updates are handled with the [`$derived`]($derived) and [`$effect`]($effect) runes.
In legacy mode, any top-level statement (i.e. not inside a block or a function) can be made reactive by prefixing it with a `$:` [label](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label). These statements run after other code in the `<script>` and before the component markup is rendered, then whenever the values that they depend on change.
```svelte
<script>
let a = 1;
let b = 2;
// this is a 'reactive statement', and it will re-run
// when `a`, `b` or `sum` change
$: console.log(`${a} + ${b} = ${sum}`);
// this is a 'reactive assignment' — `sum` will be
// recalculated when `a` or `b` change. It is
// not necessary to declare `sum` separately
$: sum = a + b;
</script>
```
Statements are ordered _topologically_ by their dependencies and their assignments: since the `console.log` statement depends on `sum`, `sum` is calculated first even though it appears later in the source.
Multiple statements can be combined by putting them in a block:
```js
// @noErrors
$: {
// recalculate `total` when `items` changes
total = 0;
for (const item of items) {
total += item.value;
}
}
```
The left-hand side of a reactive assignments can be an identifier, or it can be a destructuring assignment:
```js
// @noErrors
$: ({ larry, moe, curly } = stooges);
```
## Understanding dependencies
The dependencies of a `$:` statement are determined at compile time — they are whichever variables are referenced (but not assigned to) inside the statement.
In other words, a statement like this will _not_ re-run when `count` changes, because the compiler cannot 'see' the dependency:
```js
// @noErrors
let count = 0;
let double = () => count * 2;
$: doubled = double();
```
Similarly, topological ordering will fail if dependencies are referenced indirectly: `z` will never update, because `y` is not considered 'dirty' when the update occurs. Moving `$: z = y` below `$: setY(x)` will fix it:
```svelte
<script>
let x = 0;
let y = 0;
$: z = y;
$: setY(x);
function setY(value) {
y = value;
}
</script>
```
## Browser-only code
Reactive statements run during server-side rendering as well as in the browser. This means that any code that should only run in the browser must be wrapped in an `if` block:
```js
// @noErrors
$: if (browser) {
document.title = title;
}
```
# export let
In runes mode, [component props](basic-markup#Component-props) are declared with the [`$props`]($props) rune, allowing parent components to pass in data.
In legacy mode, props are marked with the `export` keyword, and can have a default value:
```svelte
<script>
export let foo;
export let bar = 'default value';
// Values that are passed in as props
// are immediately available
console.log({ foo });
</script>
```
The default value is used if it would otherwise be `undefined` when the component is created.
> [!NOTE] Unlike in runes mode, if the parent component changes a prop from a defined value to `undefined`, it does not revert to the initial value.
Props without default values are considered _required_, and Svelte will print a warning during development if no value is provided, which you can squelch by specifying `undefined` as the default value:
```js
export let foo +++= undefined;+++
```
## Component exports
An exported `const`, `class` or `function` declaration is _not_ considered a prop — instead, it becomes part of the component's API:
```svelte
<!--- file: Greeter.svelte--->
<script>
export function greet(name) {
alert(`hello ${name}!`);
}
</script>
```
```svelte
<!--- file: App.svelte --->
<script>
import Greeter from './Greeter.svelte';
let greeter;
</script>
<Greeter bind:this={greeter} />
<button on:click={() => greeter.greet('world')}>
greet
</button>
```
## Renaming props
The `export` keyword can appear separately from the declaration. This is useful for renaming props, for example in the case of a reserved word:
```svelte
<!--- file: App.svelte --->
<script>
/** @type {string} */
let className;
// creates a `class` property, even
// though it is a reserved word
export { className as class };
</script>
```
# $$props and $$restProps
In runes mode, getting an object containing all the props that were passed in is easy, using the [`$props`]($props) rune.
In legacy mode, we use `$$props` and `$$restProps`:
- `$$props` contains all the props that were passed in, including ones that are not individually declared with the `export` keyword
- `$$restProps` contains all the props that were passed in _except_ the ones that were individually declared
For example, a `<Button>` component might need to pass along all its props to its own `<button>` element, except the `variant` prop:
```svelte
<script>
export let variant;
</script>
<button {...$$restProps} class="variant-{variant} {$$props.class ?? ''}">
click me
</button>
<style>
.variant-danger {
background: red;
}
</style>
```
In Svelte 3/4 using `$$props` and `$$restProps` creates a modest performance penalty, so they should only be used when needed.
# on:
In runes mode, event handlers are just like any other attribute or prop.
In legacy mode, we use the `on:` directive:
```svelte
<!--- file: App.svelte --->
<script>
let count = 0;
/** @param {MouseEvent} event */
function handleClick(event) {
count += 1;
}
</script>
<button on:click={handleClick}>
count: {count}
</button>
```
Handlers can be declared inline with no performance penalty:
```svelte
<button on:click={() => (count += 1)}>
count: {count}
</button>
```
Add _modifiers_ to element event handlers with the `|` character.
```svelte
<form on:submit|preventDefault={handleSubmit}>
<!-- the `submit` event's default is prevented,
so the page won't reload -->
</form>
```
The following modifiers are available:
- `preventDefault` — calls `event.preventDefault()` before running the handler
- `stopPropagation` — calls `event.stopPropagation()`, preventing the event reaching the next element
- `stopImmediatePropagation` - calls `event.stopImmediatePropagation()`, preventing other listeners of the same event from being fired.
- `passive` — improves scrolling performance on touch/wheel events (Svelte will add it automatically where it's safe to do so)
- `nonpassive` — explicitly set `passive: false`
- `capture` — fires the handler during the _capture_ phase instead of the _bubbling_ phase
- `once` — remove the handler after the first time it runs
- `self` — only trigger handler if `event.target` is the element itself
- `trusted` — only trigger handler if `event.isTrusted` is `true`. I.e. if the event is triggered by a user action.
Modifiers can be chained together, e.g. `on:click|once|capture={...}`.
If the `on:` directive is used without a value, the component will _forward_ the event, meaning that a consumer of the component can listen for it.
```svelte
<button on:click>
The component itself will emit the click event
</button>
```
It's possible to have multiple event listeners for the same event:
```svelte
<!--- file: App.svelte --->
<script>
let count = 0;
function increment() {
count += 1;
}
/** @param {MouseEvent} event */
function log(event) {
console.log(event);
}
</script>
<button on:click={increment} on:click={log}>
clicks: {count}
</button>
```
## Component events
Components can dispatch events by creating a _dispatcher_ when they are initialised:
```svelte
<!--- file: Stepper.svelte -->
<script>
import { createEventDispatcher } from 'svelte';
const dispatch = createEventDispatcher();
</script>
<button on:click={() => dispatch('decrement')}>decrement</button>
<button on:click={() => dispatch('increment')}>increment</button>
```
`dispatch` creates a [`CustomEvent`](https://developer.mozilla.org/en-US/docs/Web/API/CustomEvent). If a second argument is provided, it becomes the `detail` property of the event object.
A consumer of this component can listen for the dispatched events:
```svelte
<script>
import Stepper from './Stepper.svelte';
let n = 0;
</script>
<Stepper
on:decrement={() => n -= 1}
on:increment={() => n += 1}
/>
<p>n: {n}</p>
```
Component events do not bubble — a parent component can only listen for events on its immediate children.
Other than `once`, modifiers are not valid on component event handlers.
> [!NOTE]
> If you're planning an eventual migration to Svelte 5, use callback props instead. This will make upgrading easier as `createEventDispatcher` is deprecated:
>
> ```svelte
> <!--- file: Stepper.svelte --->
> <script>
> export let decrement;
> export let increment;
> </script>
>
> <button on:click={decrement}>decrement</button>
> <button on:click={increment}>increment</button>
> ```
# <slot>
In Svelte 5, content can be passed to components in the form of [snippets](snippet) and rendered using [render tags](@render).
In legacy mode, content inside component tags is considered _slotted content_, which can be rendered by the component using a `<slot>` element:
```svelte
<!--- file: App.svelte --->
<script>
import Modal from './Modal.svelte';
</script>
<Modal>This is some slotted content</Modal>
```
```svelte
<!--- file: Modal.svelte --->
<div class="modal">
<slot></slot>
</div>
```
> [!NOTE] If you want to render a regular `<slot>` element, you can use `<svelte:element this={'slot'} />`.
## Named slots
A component can have _named_ slots in addition to the default slot. On the parent side, add a `slot="..."` attribute to an element, component or [`<svelte:fragment>`](legacy-svelte-fragment) directly inside the component tags.
```svelte
<!--- file: App.svelte --->
<script>
import Modal from './Modal.svelte';
let open = true;
</script>
{#if open}
<Modal>
This is some slotted content
+++<div slot="buttons">+++
<button on:click={() => open = false}>
close
</button>
+++</div>+++
</Modal>
{/if}
```
On the child side, add a corresponding `<slot name="...">` element:
```svelte
<!--- file: Modal.svelte --->
<div class="modal">
<slot></slot>
<hr>
+++<slot name="buttons"></slot>+++
</div>
```
## Fallback content
If no slotted content is provided, a component can define fallback content by putting it inside the `<slot>` element:
```svelte
<slot>
This will be rendered if no slotted content is provided
</slot>
```
## Passing data to slotted content
Slots can be rendered zero or more times and can pass values _back_ to the parent using props. The parent exposes the values to the slot template using the `let:` directive.
```svelte
<!--- file: FancyList.svelte --->
<ul>
{#each items as data}
<li class="fancy">
<!-- 'item' here... -->
<slot item={process(data)} />
</li>
{/each}
</ul>
```
```svelte
<!--- file: App.svelte --->
<!-- ...corresponds to 'item' here: -->
<FancyList {items} let:item={processed}>
<div>{processed.text}</div>
</FancyList>
```
The usual shorthand rules apply — `let:item` is equivalent to `let:item={item}`, and `<slot {item}>` is equivalent to `<slot item={item}>`.
Named slots can also expose values. The `let:` directive goes on the element with the `slot` attribute.
```svelte
<!--- file: FancyList.svelte --->
<ul>
{#each items as item}
<li class="fancy">
<slot name="item" item={process(data)} />
</li>
{/each}
</ul>
<slot name="footer" />
```
```svelte
<!--- file: App.svelte --->
<FancyList {items}>
<div slot="item" let:item>{item.text}</div>
<p slot="footer">Copyright (c) 2019 Svelte Industries</p>
</FancyList>
```
# $$slots
In runes mode, we know which [snippets](snippet) were provided to a component, as they're just normal props.
In legacy mode, the way to know if content was provided for a given slot is with the `$$slots` object, whose keys are the names of the slots passed into the component by the parent.
```svelte
<!--- file: Card.svelte --->
<div>
<slot name="title" />
{#if $$slots.description}
<!-- This <hr> and slot will render only if `slot="description"` is provided. -->
<hr />
<slot name="description" />
{/if}
</div>
```
```svelte
<!--- file: App.svelte --->
<Card>
<h1 slot="title">Blog Post Title</h1>
<!-- No slot named "description" was provided so the optional slot will not be rendered. -->
</Card>
```
# <svelte:fragment>
The `<svelte:fragment>` element allows you to place content in a [named slot](legacy-slots) without wrapping it in a container DOM element. This keeps the flow layout of your document intact.
```svelte
<!--- file: Widget.svelte --->
<div>
<slot name="header">No header was provided</slot>
<p>Some content between header and footer</p>
<slot name="footer" />
</div>
```
```svelte
<!--- file: App.svelte --->
<script>
import Widget from './Widget.svelte';
</script>
<Widget>
<h1 slot="header">Hello</h1>
<svelte:fragment slot="footer">
<p>All rights reserved.</p>
<p>Copyright (c) 2019 Svelte Industries</p>
</svelte:fragment>
</Widget>
```
> [!NOTE]
> In Svelte 5+, this concept is obsolete, as snippets don't create a wrapping element
# <svelte:component>
In runes mode, `<MyComponent>` will re-render if the value of `MyComponent` changes. See the [Svelte 5 migration guide](/docs/svelte/v5-migration-guide#svelte:component-is-no-longer-necessary) for an example.
In legacy mode, it won't — we must use `<svelte:component>`, which destroys and recreates the component instance when the value of its `this` expression changes:
```svelte
<svelte:component this={MyComponent} />
```
If `this` is falsy, no component is rendered.
# <svelte:self>
The `<svelte:self>` element allows a component to include itself, recursively.
It cannot appear at the top level of your markup; it must be inside an if or each block or passed to a component's slot to prevent an infinite loop.
```svelte
<script>
export let count;
</script>
{#if count > 0}
<p>counting down... {count}</p>
<svelte:self count={count - 1} />
{:else}
<p>lift-off!</p>
{/if}
```
> [!NOTE]
> This concept is obsolete, as components can import themselves:
> ```svelte
> <!--- file: App.svelte --->
> <script>
> import Self from './App.svelte'
> export let count;
> </script>
>
> {#if count > 0}
> <p>counting down... {count}</p>
> <Self count={count - 1} />
> {:else}
> <p>lift-off!</p>
> {/if}
> ```
# Imperative component API
In Svelte 3 and 4, the API for interacting with a component is different than in Svelte 5. Note that this page does _not_ apply to legacy mode components in a Svelte 5 application.
## Creating a component
```ts
// @noErrors
const component = new Component(options);
```
A client-side component — that is, a component compiled with `generate: 'dom'` (or the `generate` option left unspecified) is a JavaScript class.
```ts
// @noErrors
import App from './App.svelte';
const app = new App({
target: document.body,
props: {
// assuming App.svelte contains something like
// `export let answer`:
answer: 42
}
});
```
The following initialisation options can be provided:
| option | default | description |
| --------- | ----------- | ---------------------------------------------------------------------------------------------------- |
| `target` | **none** | An `HTMLElement` or `ShadowRoot` to render to. This option is required |
| `anchor` | `null` | A child of `target` to render the component immediately before |
| `props` | `{}` | An object of properties to supply to the component |
| `context` | `new Map()` | A `Map` of root-level context key-value pairs to supply to the component |
| `hydrate` | `false` | See below |
| `intro` | `false` | If `true`, will play transitions on initial render, rather than waiting for subsequent state changes |
Existing children of `target` are left where they are.
The `hydrate` option instructs Svelte to upgrade existing DOM (usually from server-side rendering) rather than creating new elements. It will only work if the component was compiled with the [`hydratable: true` option](/docs/svelte-compiler#compile). Hydration of `<head>` elements only works properly if the server-side rendering code was also compiled with `hydratable: true`, which adds a marker to each element in the `<head>` so that the component knows which elements it's responsible for removing during hydration.
Whereas children of `target` are normally left alone, `hydrate: true` will cause any children to be removed. For that reason, the `anchor` option cannot be used alongside `hydrate: true`.
The existing DOM doesn't need to match the component — Svelte will 'repair' the DOM as it goes.
```ts
/// file: index.js
// @noErrors
import App from './App.svelte';
const app = new App({
target: document.querySelector('#server-rendered-html'),
hydrate: true
});
```
> [!NOTE]
> In Svelte 5+, use [`mount`](svelte#mount) instead
## `$set`
```ts
// @noErrors
component.$set(props);
```
Programmatically sets props on an instance. `component.$set({ x: 1 })` is equivalent to `x = 1` inside the component's `<script>` block.
Calling this method schedules an update for the next microtask — the DOM is _not_ updated synchronously.
```ts
// @noErrors
component.$set({ answer: 42 });
```
> [!NOTE]
> In Svelte 5+, use `$state` instead to create a component props and update that
>
> ```js
> // @noErrors
> let props = $state({ answer: 42 });
> const component = mount(Component, { props });
> // ...
> props.answer = 24;
> ```
## `$on`
```ts
// @noErrors
component.$on(ev, callback);
```
Causes the `callback` function to be called whenever the component dispatches an `event`.
A function is returned that will remove the event listener when called.
```ts
// @noErrors
const off = component.$on('selected', (event) => {
console.log(event.detail.selection);
});
off();
```
> [!NOTE]
> In Svelte 5+, pass callback props instead
## `$destroy`
```js
// @noErrors
component.$destroy();
```
Removes a component from the DOM and triggers any `onDestroy` handlers.
> [!NOTE]
> In Svelte 5+, use [`unmount`](svelte#unmount) instead
## Component props
```js
// @noErrors
component.prop;
```
```js
// @noErrors
component.prop = value;
```
If a component is compiled with `accessors: true`, each instance will have getters and setters corresponding to each of the component's props. Setting a value will cause a _synchronous_ update, rather than the default async update caused by `component.$set(...)`.
By default, `accessors` is `false`, unless you're compiling as a custom element.
```js
// @noErrors
console.log(component.count);
component.count += 1;
```
> [!NOTE]
> In Svelte 5+, this concept is obsolete. If you want to make properties accessible from the outside, `export` them
## Server-side component API
```js
// @noErrors
const result = Component.render(...)
```
Unlike client-side components, server-side components don't have a lifespan after you render them — their whole job is to create some HTML and CSS. For that reason, the API is somewhat different.
A server-side component exposes a `render` method that can be called with optional props. It returns an object with `head`, `html`, and `css` properties, where `head` contains the contents of any `<svelte:head>` elements encountered.
You can import a Svelte component directly into Node using `svelte/register`.
```js
// @noErrors
require('svelte/register');
const App = require('./App.svelte').default;
const { head, html, css } = App.render({
answer: 42
});
```
The `.render()` method accepts the following parameters:
| parameter | default | description |
| --------- | ------- | -------------------------------------------------- |
| `props` | `{}` | An object of properties to supply to the component |
| `options` | `{}` | An object of options |
The `options` object takes in the following options:
| option | default | description |
| --------- | ----------- | ------------------------------------------------------------------------ |
| `context` | `new Map()` | A `Map` of root-level context key-value pairs to supply to the component |
```js
// @noErrors
const { head, html, css } = App.render(
// props
{ answer: 42 },
// options
{
context: new Map([['context-key', 'context-value']])
}
);
```
> [!NOTE]
> In Svelte 5+, use [`render`](svelte-server#render) instead
# Start of SvelteKit documentation
# Introduction
## Before we begin
> [!NOTE] If you're new to Svelte or SvelteKit we recommend checking out the [interactive tutorial](/tutorial/kit).
>
> If you get stuck, reach out for help in the [Discord chatroom](/chat).
## What is SvelteKit?
SvelteKit is a framework for rapidly developing robust, performant web applications using [Svelte](../svelte). If you're coming from React, SvelteKit is similar to Next. If you're coming from Vue, SvelteKit is similar to Nuxt.
To learn more about the kinds of applications you can build with SvelteKit, see the [FAQ](faq#What-can-I-make-with-SvelteKit).
## What is Svelte?
In short, Svelte is a way of writing user interface components — like a navigation bar, comment section, or contact form — that users see and interact with in their browsers. The Svelte compiler converts your components to JavaScript that can be run to render the HTML for the page and to CSS that styles the page. You don't need to know Svelte to understand the rest of this guide, but it will help. If you'd like to learn more, check out [the Svelte tutorial](/tutorial).
## SvelteKit vs Svelte
Svelte renders UI components. You can compose these components and render an entire page with just Svelte, but you need more than just Svelte to write an entire app.
SvelteKit helps you build web apps while following modern best practices and providing solutions to common development challenges. It offers everything from basic functionalities — like a [router](glossary#Routing) that updates your UI when a link is clicked — to more advanced capabilities. Its extensive list of features includes [build optimizations](https://vitejs.dev/guide/features.html#build-optimizations) to load only the minimal required code; [offline support](service-workers); [preloading](link-options#data-sveltekit-preload-data) pages before user navigation; [configurable rendering](page-options) to handle different parts of your app on the server via [SSR](glossary#SSR), in the browser through [client-side rendering](glossary#CSR), or at build-time with [prerendering](glossary#Prerendering); [image optimization](images); and much more. Building an app with all the modern best practices is fiendishly complicated, but SvelteKit does all the boring stuff for you so that you can get on with the creative part.
It reflects changes to your code in the browser instantly to provide a lightning-fast and feature-rich development experience by leveraging [Vite](https://vitejs.dev/) with a [Svelte plugin](https://github.com/sveltejs/vite-plugin-svelte) to do [Hot Module Replacement (HMR)](https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/config.md#hot).
# Creating a project
The easiest way to start building a SvelteKit app is to run `npx sv create`:
```bash
npx sv create my-app
cd my-app
npm install
npm run dev
```
The first command will scaffold a new project in the `my-app` directory asking you if you'd like to set up some basic tooling such as TypeScript. See [integrations](./integrations) for pointers on setting up additional tooling. The subsequent commands will then install its dependencies and start a server on [localhost:5173](http://localhost:5173).
There are two basic concepts:
- Each page of your app is a [Svelte](../svelte) component
- You create pages by adding files to the `src/routes` directory of your project. These will be server-rendered so that a user's first visit to your app is as fast as possible, then a client-side app takes over
Try editing the files to get a feel for how everything works.
## Editor setup
We recommend using [Visual Studio Code (aka VS Code)](https://code.visualstudio.com/download) with [the Svelte extension](https://marketplace.visualstudio.com/items?itemName=svelte.svelte-vscode), but [support also exists for numerous other editors](https://sveltesociety.dev/resources#editor-support).
# Project structure
A typical SvelteKit project looks like this:
```bash
my-project/
├ src/
│ ├ lib/
│ │ ├ server/
│ │ │ └ [your server-only lib files]
│ │ └ [your lib files]
│ ├ params/
│ │ └ [your param matchers]
│ ├ routes/
│ │ └ [your routes]
│ ├ app.html
│ ├ error.html
│ ├ hooks.client.js
│ ├ hooks.server.js
│ └ service-worker.js
├ static/
│ └ [your static assets]
├ tests/
│ └ [your tests]
├ package.json
├ svelte.config.js
├ tsconfig.json
└ vite.config.js
```
You'll also find common files like `.gitignore` and `.npmrc` (and `.prettierrc` and `eslint.config.js` and so on, if you chose those options when running `npx sv create`).
## Project files
### src
The `src` directory contains the meat of your project. Everything except `src/routes` and `src/app.html` is optional.
- `lib` contains your library code (utilities and components), which can be imported via the [`$lib`]($lib) alias, or packaged up for distribution using [`svelte-package`](packaging)
- `server` contains your server-only library code. It can be imported by using the [`$lib/server`](server-only-modules) alias. SvelteKit will prevent you from importing these in client code.
- `params` contains any [param matchers](advanced-routing#Matching) your app needs
- `routes` contains the [routes](routing) of your application. You can also colocate other components that are only used within a single route here
- `app.html` is your page template — an HTML document containing the following placeholders:
- `%sveltekit.head%` — `<link>` and `<script>` elements needed by the app, plus any `<svelte:head>` content
- `%sveltekit.body%` — the markup for a rendered page. This should live inside a `<div>` or other element, rather than directly inside `<body>`, to prevent bugs caused by browser extensions injecting elements that are then destroyed by the hydration process. SvelteKit will warn you in development if this is not the case
- `%sveltekit.assets%` — either [`paths.assets`](configuration#paths), if specified, or a relative path to [`paths.base`](configuration#paths)
- `%sveltekit.nonce%` — a [CSP](configuration#csp) nonce for manually included links and scripts, if used
- `%sveltekit.env.[NAME]%` - this will be replaced at render time with the `[NAME]` environment variable, which must begin with the [`publicPrefix`](configuration#env) (usually `PUBLIC_`). It will fallback to `''` if not matched.
- `error.html` is the page that is rendered when everything else fails. It can contain the following placeholders:
- `%sveltekit.status%` — the HTTP status
- `%sveltekit.error.message%` — the error message
- `hooks.client.js` contains your client [hooks](hooks)
- `hooks.server.js` contains your server [hooks](hooks)
- `service-worker.js` contains your [service worker](service-workers)
(Whether the project contains `.js` or `.ts` files depends on whether you opt to use TypeScript when you create your project. You can switch between JavaScript and TypeScript in the documentation using the toggle at the bottom of this page.)
If you added [Vitest](https://vitest.dev) when you set up your project, your unit tests will live in the `src` directory with a `.test.js` extension.
### static
Any static assets that should be served as-is, like `robots.txt` or `favicon.png`, go in here.
### tests
If you added [Playwright](https://playwright.dev/) for browser testing when you set up your project, the tests will live in this directory.
### package.json
Your `package.json` file must include `@sveltejs/kit`, `svelte` and `vite` as `devDependencies`.
When you create a project with `npx sv create`, you'll also notice that `package.json` includes `"type": "module"`. This means that `.js` files are interpreted as native JavaScript modules with `import` and `export` keywords. Legacy CommonJS files need a `.cjs` file extension.
### svelte.config.js
This file contains your Svelte and SvelteKit [configuration](configuration).
### tsconfig.json
This file (or `jsconfig.json`, if you prefer type-checked `.js` files over `.ts` files) configures TypeScript, if you added typechecking during `npx sv create`. Since SvelteKit relies on certain configuration being set a specific way, it generates its own `.svelte-kit/tsconfig.json` file which your own config `extends`.
### vite.config.js
A SvelteKit project is really just a [Vite](https://vitejs.dev) project that uses the [`@sveltejs/kit/vite`](@sveltejs-kit-vite) plugin, along with any other [Vite configuration](https://vitejs.dev/config/).
## Other files
### .svelte-kit
As you develop and build your project, SvelteKit will generate files in a `.svelte-kit` directory (configurable as [`outDir`](configuration#outDir)). You can ignore its contents, and delete them at any time (they will be regenerated when you next `dev` or `build`).
# Web standards
Throughout this documentation, you'll see references to the standard [Web APIs](https://developer.mozilla.org/en-US/docs/Web/API) that SvelteKit builds on top of. Rather than reinventing the wheel, we _use the platform_, which means your existing web development skills are applicable to SvelteKit. Conversely, time spent learning SvelteKit will help you be a better web developer elsewhere.
These APIs are available in all modern browsers and in many non-browser environments like Cloudflare Workers, Deno, and Vercel Functions. During development, and in [adapters](adapters) for Node-based environments (including AWS Lambda), they're made available via polyfills where necessary (for now, that is — Node is rapidly adding support for more web standards).
In particular, you'll get comfortable with the following:
## Fetch APIs
SvelteKit uses [`fetch`](https://developer.mozilla.org/en-US/docs/Web/API/fetch) for getting data from the network. It's available in [hooks](hooks) and [server routes](routing#server) as well as in the browser.
> [!NOTE] A special version of `fetch` is available in [`load`](load) functions, [server hooks](hooks#Server-hooks) and [API routes](routing#server) for invoking endpoints directly during server-side rendering, without making an HTTP call, while preserving credentials. (To make credentialled fetches in server-side code outside `load`, you must explicitly pass `cookie` and/or `authorization` headers.) It also allows you to make relative requests, whereas server-side `fetch` normally requires a fully qualified URL.
Besides `fetch` itself, the [Fetch API](https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API) includes the following interfaces:
### Request
An instance of [`Request`](https://developer.mozilla.org/en-US/docs/Web/API/Request) is accessible in [hooks](hooks) and [server routes](routing#server) as `event.request`. It contains useful methods like `request.json()` and `request.formData()` for getting data that was posted to an endpoint.
### Response
An instance of [`Response`](https://developer.mozilla.org/en-US/docs/Web/API/Response) is returned from `await fetch(...)` and handlers in `+server.js` files. Fundamentally, a SvelteKit app is a machine for turning a `Request` into a `Response`.
### Headers
The [`Headers`](https://developer.mozilla.org/en-US/docs/Web/API/Headers) interface allows you to read incoming `request.headers` and set outgoing `response.headers`. For example, you can get the `request.headers` as shown below, and use the [`json` convenience function](@sveltejs-kit#json) to send modified `response.headers`:
```js
// @errors: 2461
/// file: src/routes/what-is-my-user-agent/+server.js
import { json } from '@sveltejs/kit';
/** @type {import('./$types').RequestHandler} */
export function GET({ request }) {
// log all headers
console.log(...request.headers);
// create a JSON Response using a header we received
return json({
// retrieve a specific header
userAgent: request.headers.get('user-agent')
}, {
// set a header on the response
headers: { 'x-custom-header': 'potato' }
});
}
```
## FormData
When dealing with HTML native form submissions you'll be working with [`FormData`](https://developer.mozilla.org/en-US/docs/Web/API/FormData) objects.
```js
// @errors: 2461
/// file: src/routes/hello/+server.js
import { json } from '@sveltejs/kit';
/** @type {import('./$types').RequestHandler} */
export async function POST(event) {
const body = await event.request.formData();
// log all fields
console.log([...body]);
return json({
// get a specific field's value
name: body.get('name') ?? 'world'
});
}
```
## Stream APIs
Most of the time, your endpoints will return complete data, as in the `userAgent` example above. Sometimes, you may need to return a response that's too large to fit in memory in one go, or is delivered in chunks, and for this the platform provides [streams](https://developer.mozilla.org/en-US/docs/Web/API/Streams_API) — [ReadableStream](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream), [WritableStream](https://developer.mozilla.org/en-US/docs/Web/API/WritableStream) and [TransformStream](https://developer.mozilla.org/en-US/docs/Web/API/TransformStream).
## URL APIs
URLs are represented by the [`URL`](https://developer.mozilla.org/en-US/docs/Web/API/URL) interface, which includes useful properties like `origin` and `pathname` (and, in the browser, `hash`). This interface shows up in various places — `event.url` in [hooks](hooks) and [server routes](routing#server), [`page.url`]($app-state) in [pages](routing#page), `from` and `to` in [`beforeNavigate` and `afterNavigate`]($app-navigation) and so on.
### URLSearchParams
Wherever you encounter a URL, you can access query parameters via `url.searchParams`, which is an instance of [`URLSearchParams`](https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams):
```js
// @filename: ambient.d.ts
declare global {
const url: URL;
}
export {};
// @filename: index.js
// ---cut---
const foo = url.searchParams.get('foo');
```
## Web Crypto
The [Web Crypto API](https://developer.mozilla.org/en-US/docs/Web/API/Web_Crypto_API) is made available via the `crypto` global. It's used internally for [Content Security Policy](configuration#csp) headers, but you can also use it for things like generating UUIDs:
```js
const uuid = crypto.randomUUID();
```
# Routing
At the heart of SvelteKit is a _filesystem-based router_. The routes of your app — i.e. the URL paths that users can access — are defined by the directories in your codebase:
- `src/routes` is the root route
- `src/routes/about` creates an `/about` route
- `src/routes/blog/[slug]` creates a route with a _parameter_, `slug`, that can be used to load data dynamically when a user requests a page like `/blog/hello-world`
> [!NOTE] You can change `src/routes` to a different directory by editing the [project config](configuration).
Each route directory contains one or more _route files_, which can be identified by their `+` prefix.
We'll introduce these files in a moment in more detail, but here are a few simple rules to help you remember how SvelteKit's routing works:
* All files can run on the server
* All files run on the client except `+server` files
* `+layout` and `+error` files apply to subdirectories as well as the directory they live in
## +page
### +page.svelte
A `+page.svelte` component defines a page of your app. By default, pages are rendered both on the server ([SSR](glossary#SSR)) for the initial request and in the browser ([CSR](glossary#CSR)) for subsequent navigation.
```svelte
<!--- file: src/routes/+page.svelte --->
<h1>Hello and welcome to my site!</h1>
<a href="/about">About my site</a>
```
```svelte
<!--- file: src/routes/about/+page.svelte --->
<h1>About this site</h1>
<p>TODO...</p>
<a href="/">Home</a>
```
> [!NOTE] SvelteKit uses `<a>` elements to navigate between routes, rather than a framework-specific `<Link>` component.
Pages can receive data from `load` functions via the `data` prop.
```svelte
<!--- file: src/routes/blog/[slug]/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
</script>
<h1>{data.title}</h1>
<div>{@html data.content}</div>
```
> [!LEGACY]
> `PageProps` was added in 2.16.0. In earlier versions, you had to type the `data` property manually with `PageData` instead, see [$types](#\$types).
>
> In Svelte 4, you'd use `export let data` instead.
### +page.js
Often, a page will need to load some data before it can be rendered. For this, we add a `+page.js` module that exports a `load` function:
```js
/// file: src/routes/blog/[slug]/+page.js
import { error } from '@sveltejs/kit';
/** @type {import('./$types').PageLoad} */
export function load({ params }) {
if (params.slug === 'hello-world') {
return {
title: 'Hello world!',
content: 'Welcome to our blog. Lorem ipsum dolor sit amet...'
};
}
error(404, 'Not found');
}
```
This function runs alongside `+page.svelte`, which means it runs on the server during server-side rendering and in the browser during client-side navigation. See [`load`](load) for full details of the API.
As well as `load`, `+page.js` can export values that configure the page's behaviour:
- `export const prerender = true` or `false` or `'auto'`
- `export const ssr = true` or `false`
- `export const csr = true` or `false`
You can find more information about these in [page options](page-options).
### +page.server.js
If your `load` function can only run on the server — for example, if it needs to fetch data from a database or you need to access private [environment variables]($env-static-private) like API keys — then you can rename `+page.js` to `+page.server.js` and change the `PageLoad` type to `PageServerLoad`.
```js
/// file: src/routes/blog/[slug]/+page.server.js
// @filename: ambient.d.ts
declare global {
const getPostFromDatabase: (slug: string) => {
title: string;
content: string;
}
}
export {};
// @filename: index.js
// ---cut---
import { error } from '@sveltejs/kit';
/** @type {import('./$types').PageServerLoad} */
export async function load({ params }) {
const post = await getPostFromDatabase(params.slug);
if (post) {
return post;
}
error(404, 'Not found');
}
```
During client-side navigation, SvelteKit will load this data from the server, which means that the returned value must be serializable using [devalue](https://github.com/rich-harris/devalue). See [`load`](load) for full details of the API.
Like `+page.js`, `+page.server.js` can export [page options](page-options) — `prerender`, `ssr` and `csr`.
A `+page.server.js` file can also export _actions_. If `load` lets you read data from the server, `actions` let you write data _to_ the server using the `<form>` element. To learn how to use them, see the [form actions](form-actions) section.
## +error
If an error occurs during `load`, SvelteKit will render a default error page. You can customise this error page on a per-route basis by adding an `+error.svelte` file:
```svelte
<!--- file: src/routes/blog/[slug]/+error.svelte --->
<script>
import { page } from '$app/state';
</script>
<h1>{page.status}: {page.error.message}</h1>
```
> [!LEGACY]
> `$app/state` was added in SvelteKit 2.12. If you're using an earlier version or are using Svelte 4, use `$app/stores` instead.
SvelteKit will 'walk up the tree' looking for the closest error boundary — if the file above didn't exist it would try `src/routes/blog/+error.svelte` and then `src/routes/+error.svelte` before rendering the default error page. If _that_ fails (or if the error was thrown from the `load` function of the root `+layout`, which sits 'above' the root `+error`), SvelteKit will bail out and render a static fallback error page, which you can customise by creating a `src/error.html` file.
If the error occurs inside a `load` function in `+layout(.server).js`, the closest error boundary in the tree is an `+error.svelte` file _above_ that layout (not next to it).
If no route can be found (404), `src/routes/+error.svelte` (or the default error page, if that file does not exist) will be used.
> [!NOTE] `+error.svelte` is _not_ used when an error occurs inside [`handle`](hooks#Server-hooks-handle) or a [+server.js](#server) request handler.
You can read more about error handling [here](errors).
## +layout
So far, we've treated pages as entirely standalone components — upon navigation, the existing `+page.svelte` component will be destroyed, and a new one will take its place.
But in many apps, there are elements that should be visible on _every_ page, such as top-level navigation or a footer. Instead of repeating them in every `+page.svelte`, we can put them in _layouts_.
### +layout.svelte
To create a layout that applies to every page, make a file called `src/routes/+layout.svelte`. The default layout (the one that SvelteKit uses if you don't bring your own) looks like this...
```svelte
<script>
let { children } = $props();
</script>
{@render children()}
```
...but we can add whatever markup, styles and behaviour we want. The only requirement is that the component includes a `@render` tag for the page content. For example, let's add a nav bar:
```svelte
<!--- file: src/routes/+layout.svelte --->
<script>
let { children } = $props();
</script>
<nav>
<a href="/">Home</a>
<a href="/about">About</a>
<a href="/settings">Settings</a>
</nav>
{@render children()}
```
If we create pages for `/`, `/about` and `/settings`...
```html
/// file: src/routes/+page.svelte
<h1>Home</h1>
```
```html
/// file: src/routes/about/+page.svelte
<h1>About</h1>
```
```html
/// file: src/routes/settings/+page.svelte
<h1>Settings</h1>
```
...the nav will always be visible, and clicking between the three pages will only result in the `<h1>` being replaced.
Layouts can be _nested_. Suppose we don't just have a single `/settings` page, but instead have nested pages like `/settings/profile` and `/settings/notifications` with a shared submenu (for a real-life example, see [github.com/settings](https://github.com/settings)).
We can create a layout that only applies to pages below `/settings` (while inheriting the root layout with the top-level nav):
```svelte
<!--- file: src/routes/settings/+layout.svelte --->
<script>
/** @type {import('./$types').LayoutProps} */
let { data, children } = $props();
</script>
<h1>Settings</h1>
<div class="submenu">
{#each data.sections as section}
<a href="/settings/{section.slug}">{section.title}</a>
{/each}
</div>
{@render children()}
```
> [!LEGACY]
> `LayoutProps` was added in 2.16.0. In earlier versions, you had to [type the properties manually instead](#\$types).
You can see how `data` is populated by looking at the `+layout.js` example in the next section just below.
By default, each layout inherits the layout above it. Sometimes that isn't what you want - in this case, [advanced layouts](advanced-routing#Advanced-layouts) can help you.
### +layout.js
Just like `+page.svelte` loading data from `+page.js`, your `+layout.svelte` component can get data from a [`load`](load) function in `+layout.js`.
```js
/// file: src/routes/settings/+layout.js
/** @type {import('./$types').LayoutLoad} */
export function load() {
return {
sections: [
{ slug: 'profile', title: 'Profile' },
{ slug: 'notifications', title: 'Notifications' }
]
};
}
```
If a `+layout.js` exports [page options](page-options) — `prerender`, `ssr` and `csr` — they will be used as defaults for child pages.
Data returned from a layout's `load` function is also available to all its child pages:
```svelte
<!--- file: src/routes/settings/profile/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
console.log(data.sections); // [{ slug: 'profile', title: 'Profile' }, ...]
</script>
```
> [!NOTE] Often, layout data is unchanged when navigating between pages. SvelteKit will intelligently rerun [`load`](load) functions when necessary.
### +layout.server.js
To run your layout's `load` function on the server, move it to `+layout.server.js`, and change the `LayoutLoad` type to `LayoutServerLoad`.
Like `+layout.js`, `+layout.server.js` can export [page options](page-options) — `prerender`, `ssr` and `csr`.
## +server
As well as pages, you can define routes with a `+server.js` file (sometimes referred to as an 'API route' or an 'endpoint'), which gives you full control over the response. Your `+server.js` file exports functions corresponding to HTTP verbs like `GET`, `POST`, `PATCH`, `PUT`, `DELETE`, `OPTIONS`, and `HEAD` that take a `RequestEvent` argument and return a [`Response`](https://developer.mozilla.org/en-US/docs/Web/API/Response) object.
For example we could create an `/api/random-number` route with a `GET` handler:
```js
/// file: src/routes/api/random-number/+server.js
import { error } from '@sveltejs/kit';
/** @type {import('./$types').RequestHandler} */
export function GET({ url }) {
const min = Number(url.searchParams.get('min') ?? '0');
const max = Number(url.searchParams.get('max') ?? '1');
const d = max - min;
if (isNaN(d) || d < 0) {
error(400, 'min and max must be numbers, and min must be less than max');
}
const random = min + Math.random() * d;
return new Response(String(random));
}
```
The first argument to `Response` can be a [`ReadableStream`](https://developer.mozilla.org/en-US/docs/Web/API/ReadableStream), making it possible to stream large amounts of data or create server-sent events (unless deploying to platforms that buffer responses, like AWS Lambda).
You can use the [`error`](@sveltejs-kit#error), [`redirect`](@sveltejs-kit#redirect) and [`json`](@sveltejs-kit#json) methods from `@sveltejs/kit` for convenience (but you don't have to).
If an error is thrown (either `error(...)` or an unexpected error), the response will be a JSON representation of the error or a fallback error page — which can be customised via `src/error.html` — depending on the `Accept` header. The [`+error.svelte`](#error) component will _not_ be rendered in this case. You can read more about error handling [here](errors).
> [!NOTE] When creating an `OPTIONS` handler, note that Vite will inject `Access-Control-Allow-Origin` and `Access-Control-Allow-Methods` headers — these will not be present in production unless you add them.
> [!NOTE] `+layout` files have no effect on `+server.js` files. If you want to run some logic before each request, add it to the server [`handle`](hooks#Server-hooks-handle) hook.
### Receiving data
By exporting `POST`/`PUT`/`PATCH`/`DELETE`/`OPTIONS`/`HEAD` handlers, `+server.js` files can be used to create a complete API:
```svelte
<!--- file: src/routes/add/+page.svelte --->
<script>
let a = 0;
let b = 0;
let total = 0;
async function add() {
const response = await fetch('/api/add', {
method: 'POST',
body: JSON.stringify({ a, b }),
headers: {
'content-type': 'application/json'
}
});
total = await response.json();
}
</script>
<input type="number" bind:value={a}> +
<input type="number" bind:value={b}> =
{total}
<button onclick={add}>Calculate</button>
```
```js
/// file: src/routes/api/add/+server.js
import { json } from '@sveltejs/kit';
/** @type {import('./$types').RequestHandler} */
export async function POST({ request }) {
const { a, b } = await request.json();
return json(a + b);
}
```
> [!NOTE] In general, [form actions](form-actions) are a better way to submit data from the browser to the server.
> [!NOTE] If a `GET` handler is exported, a `HEAD` request will return the `content-length` of the `GET` handler's response body.
### Fallback method handler
Exporting the `fallback` handler will match any unhandled request methods, including methods like `MOVE` which have no dedicated export from `+server.js`.
```js
// @errors: 7031
/// file: src/routes/api/add/+server.js
import { json, text } from '@sveltejs/kit';
export async function POST({ request }) {
const { a, b } = await request.json();
return json(a + b);
}
// This handler will respond to PUT, PATCH, DELETE, etc.
/** @type {import('./$types').RequestHandler} */
export async function fallback({ request }) {
return text(`I caught your ${request.method} request!`);
}
```
> [!NOTE] For `HEAD` requests, the `GET` handler takes precedence over the `fallback` handler.
### Content negotiation
`+server.js` files can be placed in the same directory as `+page` files, allowing the same route to be either a page or an API endpoint. To determine which, SvelteKit applies the following rules:
- `PUT`/`PATCH`/`DELETE`/`OPTIONS` requests are always handled by `+server.js` since they do not apply to pages
- `GET`/`POST`/`HEAD` requests are treated as page requests if the `accept` header prioritises `text/html` (in other words, it's a browser page request), else they are handled by `+server.js`.
- Responses to `GET` requests will include a `Vary: Accept` header, so that proxies and browsers cache HTML and JSON responses separately.
## $types
Throughout the examples above, we've been importing types from a `$types.d.ts` file. This is a file SvelteKit creates for you in a hidden directory if you're using TypeScript (or JavaScript with JSDoc type annotations) to give you type safety when working with your root files.
For example, annotating `let { data } = $props()` with `PageProps` (or `LayoutProps`, for a `+layout.svelte` file) tells TypeScript that the type of `data` is whatever was returned from `load`:
```svelte
<!--- file: src/routes/blog/[slug]/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
</script>
```
> [!NOTE]
> The `PageProps` and `LayoutProps` types, added in 2.16.0, are a shortcut for typing the `data` prop as `PageData` or `LayoutData`, as well as other props, such as `form` for pages, or `children` for layouts. In earlier versions, you had to type these properties manually. For example, for a page:
>
> ```js
> /// file: +page.svelte
> /** @type {{ data: import('./$types').PageData, form: import('./$types').ActionData }} */
> let { data, form } = $props();
> ```
>
> Or, for a layout:
>
> ```js
> /// file: +layout.svelte
> /** @type {{ data: import('./$types').LayoutData, children: Snippet }} */
> let { data, children } = $props();
> ```
In turn, annotating the `load` function with `PageLoad`, `PageServerLoad`, `LayoutLoad` or `LayoutServerLoad` (for `+page.js`, `+page.server.js`, `+layout.js` and `+layout.server.js` respectively) ensures that `params` and the return value are correctly typed.
If you're using VS Code or any IDE that supports the language server protocol and TypeScript plugins then you can omit these types _entirely_! Svelte's IDE tooling will insert the correct types for you, so you'll get type checking without writing them yourself. It also works with our command line tool `svelte-check`.
You can read more about omitting `$types` in our [blog post](/blog/zero-config-type-safety) about it.
## Other files
Any other files inside a route directory are ignored by SvelteKit. This means you can colocate components and utility modules with the routes that need them.
If components and modules are needed by multiple routes, it's a good idea to put them in [`$lib`]($lib).
## Further reading
- [Tutorial: Routing](/tutorial/kit/pages)
- [Tutorial: API routes](/tutorial/kit/get-handlers)
- [Docs: Advanced routing](advanced-routing)
# Loading data
Before a [`+page.svelte`](routing#page-page.svelte) component (and its containing [`+layout.svelte`](routing#layout-layout.svelte) components) can be rendered, we often need to get some data. This is done by defining `load` functions.
## Page data
A `+page.svelte` file can have a sibling `+page.js` that exports a `load` function, the return value of which is available to the page via the `data` prop:
```js
/// file: src/routes/blog/[slug]/+page.js
/** @type {import('./$types').PageLoad} */
export function load({ params }) {
return {
post: {
title: `Title for ${params.slug} goes here`,
content: `Content for ${params.slug} goes here`
}
};
}
```
```svelte
<!--- file: src/routes/blog/[slug]/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
</script>
<h1>{data.post.title}</h1>
<div>{@html data.post.content}</div>
```
> [!LEGACY]
> Before version 2.16.0, the props of a page and layout had to be typed individually:
> ```js
> /// file: +page.svelte
> /** @type {{ data: import('./$types').PageData }} */
> let { data } = $props();
> ```
>
> In Svelte 4, you'd use `export let data` instead.
Thanks to the generated `$types` module, we get full type safety.
A `load` function in a `+page.js` file runs both on the server and in the browser (unless combined with `export const ssr = false`, in which case it will [only run in the browser](page-options#ssr)). If your `load` function should _always_ run on the server (because it uses private environment variables, for example, or accesses a database) then it would go in a `+page.server.js` instead.
A more realistic version of your blog post's `load` function, that only runs on the server and pulls data from a database, might look like this:
```js
/// file: src/routes/blog/[slug]/+page.server.js
// @filename: ambient.d.ts
declare module '$lib/server/database' {
export function getPost(slug: string): Promise<{ title: string, content: string }>
}
// @filename: index.js
// ---cut---
import * as db from '$lib/server/database';
/** @type {import('./$types').PageServerLoad} */
export async function load({ params }) {
return {
post: await db.getPost(params.slug)
};
}
```
Notice that the type changed from `PageLoad` to `PageServerLoad`, because server `load` functions can access additional arguments. To understand when to use `+page.js` and when to use `+page.server.js`, see [Universal vs server](load#Universal-vs-server).
## Layout data
Your `+layout.svelte` files can also load data, via `+layout.js` or `+layout.server.js`.
```js
/// file: src/routes/blog/[slug]/+layout.server.js
// @filename: ambient.d.ts
declare module '$lib/server/database' {
export function getPostSummaries(): Promise<Array<{ title: string, slug: string }>>
}
// @filename: index.js
// ---cut---
import * as db from '$lib/server/database';
/** @type {import('./$types').LayoutServerLoad} */
export async function load() {
return {
posts: await db.getPostSummaries()
};
}
```
```svelte
<!--- file: src/routes/blog/[slug]/+layout.svelte --->
<script>
/** @type {import('./$types').LayoutProps} */
let { data, children } = $props();
</script>
<main>
<!-- +page.svelte is `@render`ed here -->
{@render children()}
</main>
<aside>
<h2>More posts</h2>
<ul>
{#each data.posts as post}
<li>
<a href="/blog/{post.slug}">
{post.title}
</a>
</li>
{/each}
</ul>
</aside>
```
> [!LEGACY]
> `LayoutProps` was added in 2.16.0. In earlier versions, properties had to be typed individually:
> ```js
> /// file: +layout.svelte
> /** @type {{ data: import('./$types').LayoutData, children: Snippet }} */
> let { data, children } = $props();
> ```
Data returned from layout `load` functions is available to child `+layout.svelte` components and the `+page.svelte` component as well as the layout that it 'belongs' to.
```svelte
/// file: src/routes/blog/[slug]/+page.svelte
<script>
+++import { page } from '$app/state';+++
/** @type {import('./$types').PageProps} */
let { data } = $props();
+++ // we can access `data.posts` because it's returned from
// the parent layout `load` function
let index = $derived(data.posts.findIndex(post => post.slug === page.params.slug));
let next = $derived(data.posts[index + 1]);+++
</script>
<h1>{data.post.title}</h1>
<div>{@html data.post.content}</div>
+++{#if next}
<p>Next post: <a href="/blog/{next.slug}">{next.title}</a></p>
{/if}+++
```
> [!NOTE] If multiple `load` functions return data with the same key, the last one 'wins' — the result of a layout `load` returning `{ a: 1, b: 2 }` and a page `load` returning `{ b: 3, c: 4 }` would be `{ a: 1, b: 3, c: 4 }`.
## page.data
The `+page.svelte` component, and each `+layout.svelte` component above it, has access to its own data plus all the data from its parents.
In some cases, we might need the opposite — a parent layout might need to access page data or data from a child layout. For example, the root layout might want to access a `title` property returned from a `load` function in `+page.js` or `+page.server.js`. This can be done with `page.data`:
```svelte
<!--- file: src/routes/+layout.svelte --->
<script>
import { page } from '$app/state';
</script>
<svelte:head>
<title>{page.data.title}</title>
</svelte:head>
```
Type information for `page.data` is provided by `App.PageData`.
> [!LEGACY]
> `$app/state` was added in SvelteKit 2.12. If you're using an earlier version or are using Svelte 4, use `$app/stores` instead.
> It provides a `page` store with the same interface that you can subscribe to, e.g. `$page.data.title`.
## Universal vs server
As we've seen, there are two types of `load` function:
* `+page.js` and `+layout.js` files export _universal_ `load` functions that run both on the server and in the browser
* `+page.server.js` and `+layout.server.js` files export _server_ `load` functions that only run server-side
Conceptually, they're the same thing, but there are some important differences to be aware of.
### When does which load function run?
Server `load` functions _always_ run on the server.
By default, universal `load` functions run on the server during SSR when the user first visits your page. They will then run again during hydration, reusing any responses from [fetch requests](#Making-fetch-requests). All subsequent invocations of universal `load` functions happen in the browser. You can customize the behavior through [page options](page-options). If you disable [server side rendering](page-options#ssr), you'll get an SPA and universal `load` functions _always_ run on the client.
If a route contains both universal and server `load` functions, the server `load` runs first.
A `load` function is invoked at runtime, unless you [prerender](page-options#prerender) the page — in that case, it's invoked at build time.
### Input
Both universal and server `load` functions have access to properties describing the request (`params`, `route` and `url`) and various functions (`fetch`, `setHeaders`, `parent`, `depends` and `untrack`). These are described in the following sections.
Server `load` functions are called with a `ServerLoadEvent`, which inherits `clientAddress`, `cookies`, `locals`, `platform` and `request` from `RequestEvent`.
Universal `load` functions are called with a `LoadEvent`, which has a `data` property. If you have `load` functions in both `+page.js` and `+page.server.js` (or `+layout.js` and `+layout.server.js`), the return value of the server `load` function is the `data` property of the universal `load` function's argument.
### Output
A universal `load` function can return an object containing any values, including things like custom classes and component constructors.
A server `load` function must return data that can be serialized with [devalue](https://github.com/rich-harris/devalue) — anything that can be represented as JSON plus things like `BigInt`, `Date`, `Map`, `Set` and `RegExp`, or repeated/cyclical references — so that it can be transported over the network. Your data can include [promises](#Streaming-with-promises), in which case it will be streamed to browsers.
### When to use which
Server `load` functions are convenient when you need to access data directly from a database or filesystem, or need to use private environment variables.
Universal `load` functions are useful when you need to `fetch` data from an external API and don't need private credentials, since SvelteKit can get the data directly from the API rather than going via your server. They are also useful when you need to return something that can't be serialized, such as a Svelte component constructor.
In rare cases, you might need to use both together — for example, you might need to return an instance of a custom class that was initialised with data from your server. When using both, the server `load` return value is _not_ passed directly to the page, but to the universal `load` function (as the `data` property):
```js
/// file: src/routes/+page.server.js
/** @type {import('./$types').PageServerLoad} */
export async function load() {
return {
serverMessage: 'hello from server load function'
};
}
```
```js
/// file: src/routes/+page.js
// @errors: 18047
/** @type {import('./$types').PageLoad} */
export async function load({ data }) {
return {
serverMessage: data.serverMessage,
universalMessage: 'hello from universal load function'
};
}
```
## Using URL data
Often the `load` function depends on the URL in one way or another. For this, the `load` function provides you with `url`, `route` and `params`.
### url
An instance of [`URL`](https://developer.mozilla.org/en-US/docs/Web/API/URL), containing properties like the `origin`, `hostname`, `pathname` and `searchParams` (which contains the parsed query string as a [`URLSearchParams`](https://developer.mozilla.org/en-US/docs/Web/API/URLSearchParams) object). `url.hash` cannot be accessed during `load`, since it is unavailable on the server.
> [!NOTE] In some environments this is derived from request headers during server-side rendering. If you're using [adapter-node](adapter-node), for example, you may need to configure the adapter in order for the URL to be correct.
### route
Contains the name of the current route directory, relative to `src/routes`:
```js
/// file: src/routes/a/[b]/[...c]/+page.js
/** @type {import('./$types').PageLoad} */
export function load({ route }) {
console.log(route.id); // '/a/[b]/[...c]'
}
```
### params
`params` is derived from `url.pathname` and `route.id`.
Given a `route.id` of `/a/[b]/[...c]` and a `url.pathname` of `/a/x/y/z`, the `params` object would look like this:
```json
{
"b": "x",
"c": "y/z"
}
```
## Making fetch requests
To get data from an external API or a `+server.js` handler, you can use the provided `fetch` function, which behaves identically to the [native `fetch` web API](https://developer.mozilla.org/en-US/docs/Web/API/fetch) with a few additional features:
- It can be used to make credentialed requests on the server, as it inherits the `cookie` and `authorization` headers for the page request.
- It can make relative requests on the server (ordinarily, `fetch` requires a URL with an origin when used in a server context).
- Internal requests (e.g. for `+server.js` routes) go directly to the handler function when running on the server, without the overhead of an HTTP call.
- During server-side rendering, the response will be captured and inlined into the rendered HTML by hooking into the `text`, `json` and `arrayBuffer` methods of the `Response` object. Note that headers will _not_ be serialized, unless explicitly included via [`filterSerializedResponseHeaders`](hooks#Server-hooks-handle).
- During hydration, the response will be read from the HTML, guaranteeing consistency and preventing an additional network request - if you received a warning in your browser console when using the browser `fetch` instead of the `load` `fetch`, this is why.
```js
/// file: src/routes/items/[id]/+page.js
/** @type {import('./$types').PageLoad} */
export async function load({ fetch, params }) {
const res = await fetch(`/api/items/${params.id}`);
const item = await res.json();
return { item };
}
```
## Cookies
A server `load` function can get and set [`cookies`](@sveltejs-kit#Cookies).
```js
/// file: src/routes/+layout.server.js
// @filename: ambient.d.ts
declare module '$lib/server/database' {
export function getUser(sessionid: string | undefined): Promise<{ name: string, avatar: string }>
}
// @filename: index.js
// ---cut---
import * as db from '$lib/server/database';
/** @type {import('./$types').LayoutServerLoad} */
export async function load({ cookies }) {
const sessionid = cookies.get('sessionid');
return {
user: await db.getUser(sessionid)
};
}
```
Cookies will only be passed through the provided `fetch` function if the target host is the same as the SvelteKit application or a more specific subdomain of it.
For example, if SvelteKit is serving my.domain.com:
- domain.com WILL NOT receive cookies
- my.domain.com WILL receive cookies
- api.domain.com WILL NOT receive cookies
- sub.my.domain.com WILL receive cookies
Other cookies will not be passed when `credentials: 'include'` is set, because SvelteKit does not know which domain which cookie belongs to (the browser does not pass this information along), so it's not safe to forward any of them. Use the [handleFetch hook](hooks#Server-hooks-handleFetch) to work around it.
## Headers
Both server and universal `load` functions have access to a `setHeaders` function that, when running on the server, can set headers for the response. (When running in the browser, `setHeaders` has no effect.) This is useful if you want the page to be cached, for example:
```js
// @errors: 2322 1360
/// file: src/routes/products/+page.js
/** @type {import('./$types').PageLoad} */
export async function load({ fetch, setHeaders }) {
const url = `https://cms.example.com/products.json`;
const response = await fetch(url);
// Headers are only set during SSR, caching the page's HTML
// for the same length of time as the underlying data.
setHeaders({
age: response.headers.get('age'),
'cache-control': response.headers.get('cache-control')
});
return response.json();
}
```
Setting the same header multiple times (even in separate `load` functions) is an error. You can only set a given header once using the `setHeaders` function. You cannot add a `set-cookie` header with `setHeaders` — use `cookies.set(name, value, options)` instead.
## Using parent data
Occasionally it's useful for a `load` function to access data from a parent `load` function, which can be done with `await parent()`:
```js
/// file: src/routes/+layout.js
/** @type {import('./$types').LayoutLoad} */
export function load() {
return { a: 1 };
}
```
```js
/// file: src/routes/abc/+layout.js
/** @type {import('./$types').LayoutLoad} */
export async function load({ parent }) {
const { a } = await parent();
return { b: a + 1 };
}
```
```js
/// file: src/routes/abc/+page.js
/** @type {import('./$types').PageLoad} */
export async function load({ parent }) {
const { a, b } = await parent();
return { c: a + b };
}
```
```svelte
<!--- file: src/routes/abc/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
</script>
<!-- renders `1 + 2 = 3` -->
<p>{data.a} + {data.b} = {data.c}</p>
```
> [!NOTE] Notice that the `load` function in `+page.js` receives the merged data from both layout `load` functions, not just the immediate parent.
Inside `+page.server.js` and `+layout.server.js`, `parent` returns data from parent `+layout.server.js` files.
In `+page.js` or `+layout.js` it will return data from parent `+layout.js` files. However, a missing `+layout.js` is treated as a `({ data }) => data` function, meaning that it will also return data from parent `+layout.server.js` files that are not 'shadowed' by a `+layout.js` file
Take care not to introduce waterfalls when using `await parent()`. Here, for example, `getData(params)` does not depend on the result of calling `parent()`, so we should call it first to avoid a delayed render.
```js
/// file: +page.js
// @filename: ambient.d.ts
declare function getData(params: Record<string, string>): Promise<{ meta: any }>
// @filename: index.js
// ---cut---
/** @type {import('./$types').PageLoad} */
export async function load({ params, parent }) {
---const parentData = await parent();---
const data = await getData(params);
+++const parentData = await parent();+++
return {
...data,
meta: { ...parentData.meta, ...data.meta }
};
}
```
## Errors
If an error is thrown during `load`, the nearest [`+error.svelte`](routing#error) will be rendered. For [_expected_](errors#Expected-errors) errors, use the `error` helper from `@sveltejs/kit` to specify the HTTP status code and an optional message:
```js
/// file: src/routes/admin/+layout.server.js
// @filename: ambient.d.ts
declare namespace App {
interface Locals {
user?: {
name: string;
isAdmin: boolean;
}
}
}
// @filename: index.js
// ---cut---
import { error } from '@sveltejs/kit';
/** @type {import('./$types').LayoutServerLoad} */
export function load({ locals }) {
if (!locals.user) {
error(401, 'not logged in');
}
if (!locals.user.isAdmin) {
error(403, 'not an admin');
}
}
```
Calling `error(...)` will throw an exception, making it easy to stop execution from inside helper functions.
If an [_unexpected_](errors#Unexpected-errors) error is thrown, SvelteKit will invoke [`handleError`](hooks#Shared-hooks-handleError) and treat it as a 500 Internal Error.
> [!NOTE] [In SvelteKit 1.x](migrating-to-sveltekit-2#redirect-and-error-are-no-longer-thrown-by-you) you had to `throw` the error yourself
## Redirects
To redirect users, use the `redirect` helper from `@sveltejs/kit` to specify the location to which they should be redirected alongside a `3xx` status code. Like `error(...)`, calling `redirect(...)` will throw an exception, making it easy to stop execution from inside helper functions.
```js
/// file: src/routes/user/+layout.server.js
// @filename: ambient.d.ts
declare namespace App {
interface Locals {
user?: {
name: string;
}
}
}
// @filename: index.js
// ---cut---
import { redirect } from '@sveltejs/kit';
/** @type {import('./$types').LayoutServerLoad} */
export function load({ locals }) {
if (!locals.user) {
redirect(307, '/login');
}
}
```
> [!NOTE] Don't use `redirect()` inside a `try {...}` block, as the redirect will immediately trigger the catch statement.
In the browser, you can also navigate programmatically outside of a `load` function using [`goto`]($app-navigation#goto) from [`$app.navigation`]($app-navigation).
> [!NOTE] [In SvelteKit 1.x](migrating-to-sveltekit-2#redirect-and-error-are-no-longer-thrown-by-you) you had to `throw` the `redirect` yourself
## Streaming with promises
When using a server `load`, promises will be streamed to the browser as they resolve. This is useful if you have slow, non-essential data, since you can start rendering the page before all the data is available:
```js
/// file: src/routes/blog/[slug]/+page.server.js
// @filename: ambient.d.ts
declare global {
const loadPost: (slug: string) => Promise<{ title: string, content: string }>;
const loadComments: (slug: string) => Promise<{ content: string }>;
}
export {};
// @filename: index.js
// ---cut---
/** @type {import('./$types').PageServerLoad} */
export async function load({ params }) {
return {
// make sure the `await` happens at the end, otherwise we
// can't start loading comments until we've loaded the post
comments: loadComments(params.slug),
post: await loadPost(params.slug)
};
}
```
This is useful for creating skeleton loading states, for example:
```svelte
<!--- file: src/routes/blog/[slug]/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
</script>
<h1>{data.post.title}</h1>
<div>{@html data.post.content}</div>
{#await data.comments}
Loading comments...
{:then comments}
{#each comments as comment}
<p>{comment.content}</p>
{/each}
{:catch error}
<p>error loading comments: {error.message}</p>
{/await}
```
When streaming data, be careful to handle promise rejections correctly. More specifically, the server could crash with an "unhandled promise rejection" error if a lazy-loaded promise fails before rendering starts (at which point it's caught) and isn't handling the error in some way. When using SvelteKit's `fetch` directly in the `load` function, SvelteKit will handle this case for you. For other promises, it is enough to attach a noop-`catch` to the promise to mark it as handled.
```js
/// file: src/routes/+page.server.js
/** @type {import('./$types').PageServerLoad} */
export function load({ fetch }) {
const ok_manual = Promise.reject();
ok_manual.catch(() => {});
return {
ok_manual,
ok_fetch: fetch('/fetch/that/could/fail'),
dangerous_unhandled: Promise.reject()
};
}
```
> [!NOTE] On platforms that do not support streaming, such as AWS Lambda or Firebase, responses will be buffered. This means the page will only render once all promises resolve. If you are using a proxy (e.g. NGINX), make sure it does not buffer responses from the proxied server.
> [!NOTE] Streaming data will only work when JavaScript is enabled. You should avoid returning promises from a universal `load` function if the page is server rendered, as these are _not_ streamed — instead, the promise is recreated when the function reruns in the browser.
> [!NOTE] The headers and status code of a response cannot be changed once the response has started streaming, therefore you cannot `setHeaders` or throw redirects inside a streamed promise.
> [!NOTE] [In SvelteKit 1.x](migrating-to-sveltekit-2#Top-level-promises-are-no-longer-awaited) top-level promises were automatically awaited, only nested promises were streamed.
## Parallel loading
When rendering (or navigating to) a page, SvelteKit runs all `load` functions concurrently, avoiding a waterfall of requests. During client-side navigation, the result of calling multiple server `load` functions are grouped into a single response. Once all `load` functions have returned, the page is rendered.
## Rerunning load functions
SvelteKit tracks the dependencies of each `load` function to avoid rerunning it unnecessarily during navigation.
For example, given a pair of `load` functions like these...
```js
/// file: src/routes/blog/[slug]/+page.server.js
// @filename: ambient.d.ts
declare module '$lib/server/database' {
export function getPost(slug: string): Promise<{ title: string, content: string }>
}
// @filename: index.js
// ---cut---
import * as db from '$lib/server/database';
/** @type {import('./$types').PageServerLoad} */
export async function load({ params }) {
return {
post: await db.getPost(params.slug)
};
}
```
```js
/// file: src/routes/blog/[slug]/+layout.server.js
// @filename: ambient.d.ts
declare module '$lib/server/database' {
export function getPostSummaries(): Promise<Array<{ title: string, slug: string }>>
}
// @filename: index.js
// ---cut---
import * as db from '$lib/server/database';
/** @type {import('./$types').LayoutServerLoad} */
export async function load() {
return {
posts: await db.getPostSummaries()
};
}
```
...the one in `+page.server.js` will rerun if we navigate from `/blog/trying-the-raw-meat-diet` to `/blog/i-regret-my-choices` because `params.slug` has changed. The one in `+layout.server.js` will not, because the data is still valid. In other words, we won't call `db.getPostSummaries()` a second time.
A `load` function that calls `await parent()` will also rerun if a parent `load` function is rerun.
Dependency tracking does not apply _after_ the `load` function has returned — for example, accessing `params.x` inside a nested [promise](#Streaming-with-promises) will not cause the function to rerun when `params.x` changes. (Don't worry, you'll get a warning in development if you accidentally do this.) Instead, access the parameter in the main body of your `load` function.
Search parameters are tracked independently from the rest of the url. For example, accessing `event.url.searchParams.get("x")` inside a `load` function will make that `load` function re-run when navigating from `?x=1` to `?x=2`, but not when navigating from `?x=1&y=1` to `?x=1&y=2`.
### Untracking dependencies
In rare cases, you may wish to exclude something from the dependency tracking mechanism. You can do this with the provided `untrack` function:
```js
/// file: src/routes/+page.js
/** @type {import('./$types').PageLoad} */
export async function load({ untrack, url }) {
// Untrack url.pathname so that path changes don't trigger a rerun
if (untrack(() => url.pathname === '/')) {
return { message: 'Welcome!' };
}
}
```
### Manual invalidation
You can also rerun `load` functions that apply to the current page using [`invalidate(url)`]($app-navigation#invalidate), which reruns all `load` functions that depend on `url`, and [`invalidateAll()`]($app-navigation#invalidateAll), which reruns every `load` function. Server load functions will never automatically depend on a fetched `url` to avoid leaking secrets to the client.
A `load` function depends on `url` if it calls `fetch(url)` or `depends(url)`. Note that `url` can be a custom identifier that starts with `[a-z]:`:
```js
/// file: src/routes/random-number/+page.js
/** @type {import('./$types').PageLoad} */
export async function load({ fetch, depends }) {
// load reruns when `invalidate('https://api.example.com/random-number')` is called...
const response = await fetch('https://api.example.com/random-number');
// ...or when `invalidate('app:random')` is called
depends('app:random');
return {
number: await response.json()
};
}
```
```svelte
<!--- file: src/routes/random-number/+page.svelte --->
<script>
import { invalidate, invalidateAll } from '$app/navigation';
/** @type {import('./$types').PageProps} */
let { data } = $props();
function rerunLoadFunction() {
// any of these will cause the `load` function to rerun
invalidate('app:random');
invalidate('https://api.example.com/random-number');
invalidate(url => url.href.includes('random-number'));
invalidateAll();
}
</script>
<p>random number: {data.number}</p>
<button onclick={rerunLoadFunction}>Update random number</button>
```
### When do load functions rerun?
To summarize, a `load` function will rerun in the following situations:
- It references a property of `params` whose value has changed
- It references a property of `url` (such as `url.pathname` or `url.search`) whose value has changed. Properties in `request.url` are _not_ tracked
- It calls `url.searchParams.get(...)`, `url.searchParams.getAll(...)` or `url.searchParams.has(...)` and the parameter in question changes. Accessing other properties of `url.searchParams` will have the same effect as accessing `url.search`.
- It calls `await parent()` and a parent `load` function reran
- A child `load` function calls `await parent()` and is rerunning, and the parent is a server load function
- It declared a dependency on a specific URL via [`fetch`](#Making-fetch-requests) (universal load only) or [`depends`](@sveltejs-kit#LoadEvent), and that URL was marked invalid with [`invalidate(url)`]($app-navigation#invalidate)
- All active `load` functions were forcibly rerun with [`invalidateAll()`]($app-navigation#invalidateAll)
`params` and `url` can change in response to a `<a href="..">` link click, a [`<form>` interaction](form-actions#GET-vs-POST), a [`goto`]($app-navigation#goto) invocation, or a [`redirect`](@sveltejs-kit#redirect).
Note that rerunning a `load` function will update the `data` prop inside the corresponding `+layout.svelte` or `+page.svelte`; it does _not_ cause the component to be recreated. As a result, internal state is preserved. If this isn't what you want, you can reset whatever you need to reset inside an [`afterNavigate`]($app-navigation#afterNavigate) callback, and/or wrap your component in a [`{#key ...}`](../svelte/key) block.
## Implications for authentication
A couple features of loading data have important implications for auth checks:
- Layout `load` functions do not run on every request, such as during client side navigation between child routes. [(When do load functions rerun?)](load#Rerunning-load-functions-When-do-load-functions-rerun)
- Layout and page `load` functions run concurrently unless `await parent()` is called. If a layout `load` throws, the page `load` function runs, but the client will not receive the returned data.
There are a few possible strategies to ensure an auth check occurs before protected code.
To prevent data waterfalls and preserve layout `load` caches:
- Use [hooks](hooks) to protect multiple routes before any `load` functions run
- Use auth guards directly in `+page.server.js` `load` functions for route specific protection
Putting an auth guard in `+layout.server.js` requires all child pages to call `await parent()` before protected code. Unless every child page depends on returned data from `await parent()`, the other options will be more performant.
## Further reading
- [Tutorial: Loading data](/tutorial/kit/page-data)
- [Tutorial: Errors and redirects](/tutorial/kit/error-basics)
- [Tutorial: Advanced loading](/tutorial/kit/await-parent)
# Form actions
A `+page.server.js` file can export _actions_, which allow you to `POST` data to the server using the `<form>` element.
When using `<form>`, client-side JavaScript is optional, but you can easily _progressively enhance_ your form interactions with JavaScript to provide the best user experience.
## Default actions
In the simplest case, a page declares a `default` action:
```js
/// file: src/routes/login/+page.server.js
/** @satisfies {import('./$types').Actions} */
export const actions = {
default: async (event) => {
// TODO log the user in
}
};
```
To invoke this action from the `/login` page, just add a `<form>` — no JavaScript needed:
```svelte
<!--- file: src/routes/login/+page.svelte --->
<form method="POST">
<label>
Email
<input name="email" type="email">
</label>
<label>
Password
<input name="password" type="password">
</label>
<button>Log in</button>
</form>
```
If someone were to click the button, the browser would send the form data via `POST` request to the server, running the default action.
> [!NOTE] Actions always use `POST` requests, since `GET` requests should never have side-effects.
We can also invoke the action from other pages (for example if there's a login widget in the nav in the root layout) by adding the `action` attribute, pointing to the page:
```html
/// file: src/routes/+layout.svelte
<form method="POST" action="/login">
<!-- content -->
</form>
```
## Named actions
Instead of one `default` action, a page can have as many named actions as it needs:
```js
/// file: src/routes/login/+page.server.js
/** @satisfies {import('./$types').Actions} */
export const actions = {
--- default: async (event) => {---
+++ login: async (event) => {+++
// TODO log the user in
},
+++ register: async (event) => {
// TODO register the user
}+++
};
```
To invoke a named action, add a query parameter with the name prefixed by a `/` character:
```svelte
<!--- file: src/routes/login/+page.svelte --->
<form method="POST" action="?/register">
```
```svelte
<!--- file: src/routes/+layout.svelte --->
<form method="POST" action="/login?/register">
```
As well as the `action` attribute, we can use the `formaction` attribute on a button to `POST` the same form data to a different action than the parent `<form>`:
```svelte
/// file: src/routes/login/+page.svelte
<form method="POST" +++action="?/login"+++>
<label>
Email
<input name="email" type="email">
</label>
<label>
Password
<input name="password" type="password">
</label>
<button>Log in</button>
+++<button formaction="?/register">Register</button>+++
</form>
```
> [!NOTE] We can't have default actions next to named actions, because if you POST to a named action without a redirect, the query parameter is persisted in the URL, which means the next default POST would go through the named action from before.
## Anatomy of an action
Each action receives a `RequestEvent` object, allowing you to read the data with `request.formData()`. After processing the request (for example, logging the user in by setting a cookie), the action can respond with data that will be available through the `form` property on the corresponding page and through `page.form` app-wide until the next update.
```js
/// file: src/routes/login/+page.server.js
// @filename: ambient.d.ts
declare module '$lib/server/db';
// @filename: index.js
// ---cut---
import * as db from '$lib/server/db';
/** @type {import('./$types').PageServerLoad} */
export async function load({ cookies }) {
const user = await db.getUserFromSession(cookies.get('sessionid'));
return { user };
}
/** @satisfies {import('./$types').Actions} */
export const actions = {
login: async ({ cookies, request }) => {
const data = await request.formData();
const email = data.get('email');
const password = data.get('password');
const user = await db.getUser(email);
cookies.set('sessionid', await db.createSession(user), { path: '/' });
return { success: true };
},
register: async (event) => {
// TODO register the user
}
};
```
```svelte
<!--- file: src/routes/login/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data, form } = $props();
</script>
{#if form?.success}
<!-- this message is ephemeral; it exists because the page was rendered in
response to a form submission. it will vanish if the user reloads -->
<p>Successfully logged in! Welcome back, {data.user.name}</p>
{/if}
```
> [!LEGACY]
> `PageProps` was added in 2.16.0. In earlier versions, you had to type the `data` and `form` properties individually:
> ```js
> /// file: +page.svelte
> /** @type {{ data: import('./$types').PageData, form: import('./$types').ActionData }} */
> let { data, form } = $props();
> ```
>
> In Svelte 4, you'd use `export let data` and `export let form` instead to declare properties.
### Validation errors
If the request couldn't be processed because of invalid data, you can return validation errors — along with the previously submitted form values — back to the user so that they can try again. The `fail` function lets you return an HTTP status code (typically 400 or 422, in the case of validation errors) along with the data. The status code is available through `page.status` and the data through `form`:
```js
/// file: src/routes/login/+page.server.js
// @filename: ambient.d.ts
declare module '$lib/server/db';
// @filename: index.js
// ---cut---
+++import { fail } from '@sveltejs/kit';+++
import * as db from '$lib/server/db';
/** @satisfies {import('./$types').Actions} */
export const actions = {
login: async ({ cookies, request }) => {
const data = await request.formData();
const email = data.get('email');
const password = data.get('password');
+++ if (!email) {
return fail(400, { email, missing: true });
}+++
const user = await db.getUser(email);
+++ if (!user || user.password !== db.hash(password)) {
return fail(400, { email, incorrect: true });
}+++
cookies.set('sessionid', await db.createSession(user), { path: '/' });
return { success: true };
},
register: async (event) => {
// TODO register the user
}
};
```
> [!NOTE] Note that as a precaution, we only return the email back to the page — not the password.
```svelte
/// file: src/routes/login/+page.svelte
<form method="POST" action="?/login">
+++ {#if form?.missing}<p class="error">The email field is required</p>{/if}
{#if form?.incorrect}<p class="error">Invalid credentials!</p>{/if}+++
<label>
Email
<input name="email" type="email" +++value={form?.email ?? ''}+++>
</label>
<label>
Password
<input name="password" type="password">
</label>
<button>Log in</button>
<button formaction="?/register">Register</button>
</form>
```
The returned data must be serializable as JSON. Beyond that, the structure is entirely up to you. For example, if you had multiple forms on the page, you could distinguish which `<form>` the returned `form` data referred to with an `id` property or similar.
### Redirects
Redirects (and errors) work exactly the same as in [`load`](load#Redirects):
```js
// @errors: 2345
/// file: src/routes/login/+page.server.js
// @filename: ambient.d.ts
declare module '$lib/server/db';
// @filename: index.js
// ---cut---
import { fail, +++redirect+++ } from '@sveltejs/kit';
import * as db from '$lib/server/db';
/** @satisfies {import('./$types').Actions} */
export const actions = {
login: async ({ cookies, request, +++url+++ }) => {
const data = await request.formData();
const email = data.get('email');
const password = data.get('password');
const user = await db.getUser(email);
if (!user) {
return fail(400, { email, missing: true });
}
if (user.password !== db.hash(password)) {
return fail(400, { email, incorrect: true });
}
cookies.set('sessionid', await db.createSession(user), { path: '/' });
+++ if (url.searchParams.has('redirectTo')) {
redirect(303, url.searchParams.get('redirectTo'));
}+++
return { success: true };
},
register: async (event) => {
// TODO register the user
}
};
```
## Loading data
After an action runs, the page will be re-rendered (unless a redirect or an unexpected error occurs), with the action's return value available to the page as the `form` prop. This means that your page's `load` functions will run after the action completes.
Note that `handle` runs before the action is invoked, and does not rerun before the `load` functions. This means that if, for example, you use `handle` to populate `event.locals` based on a cookie, you must update `event.locals` when you set or delete the cookie in an action:
```js
/// file: src/hooks.server.js
// @filename: ambient.d.ts
declare namespace App {
interface Locals {
user: {
name: string;
} | null
}
}
// @filename: global.d.ts
declare global {
function getUser(sessionid: string | undefined): {
name: string;
};
}
export {};
// @filename: index.js
// ---cut---
/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
event.locals.user = await getUser(event.cookies.get('sessionid'));
return resolve(event);
}
```
```js
/// file: src/routes/account/+page.server.js
// @filename: ambient.d.ts
declare namespace App {
interface Locals {
user: {
name: string;
} | null
}
}
// @filename: index.js
// ---cut---
/** @type {import('./$types').PageServerLoad} */
export function load(event) {
return {
user: event.locals.user
};
}
/** @satisfies {import('./$types').Actions} */
export const actions = {
logout: async (event) => {
event.cookies.delete('sessionid', { path: '/' });
event.locals.user = null;
}
};
```
## Progressive enhancement
In the preceding sections we built a `/login` action that [works without client-side JavaScript](https://kryogenix.org/code/browser/everyonehasjs.html) — not a `fetch` in sight. That's great, but when JavaScript _is_ available we can progressively enhance our form interactions to provide a better user experience.
### use:enhance
The easiest way to progressively enhance a form is to add the `use:enhance` action:
```svelte
/// file: src/routes/login/+page.svelte
<script>
+++import { enhance } from '$app/forms';+++
/** @type {import('./$types').PageProps} */
let { form } = $props();
</script>
<form method="POST" +++use:enhance+++>
```
> [!NOTE] `use:enhance` can only be used with forms that have `method="POST"` and point to actions defined in a `+page.server.js` file. It will not work with `method="GET"`, which is the default for forms without a specified method. Attempting to use `use:enhance` on forms without `method="POST"` or posting to a `+server.js` endpoint will result in an error.
> [!NOTE] Yes, it's a little confusing that the `enhance` action and `<form action>` are both called 'action'. These docs are action-packed. Sorry.
Without an argument, `use:enhance` will emulate the browser-native behaviour, just without the full-page reloads. It will:
- update the `form` property, `page.form` and `page.status` on a successful or invalid response, but only if the action is on the same page you're submitting from. For example, if your form looks like `<form action="/somewhere/else" ..>`, the `form` prop and the `page.form` state will _not_ be updated. This is because in the native form submission case you would be redirected to the page the action is on. If you want to have them updated either way, use [`applyAction`](#Progressive-enhancement-Customising-use:enhance)
- reset the `<form>` element
- invalidate all data using `invalidateAll` on a successful response
- call `goto` on a redirect response
- render the nearest `+error` boundary if an error occurs
- [reset focus](accessibility#Focus-management) to the appropriate element
### Customising use:enhance
To customise the behaviour, you can provide a `SubmitFunction` that runs immediately before the form is submitted, and (optionally) returns a callback that runs with the `ActionResult`. Note that if you return a callback, the default behavior mentioned above is not triggered. To get it back, call `update`.
```svelte
<form
method="POST"
use:enhance={({ formElement, formData, action, cancel, submitter }) => {
// `formElement` is this `<form>` element
// `formData` is its `FormData` object that's about to be submitted
// `action` is the URL to which the form is posted
// calling `cancel()` will prevent the submission
// `submitter` is the `HTMLElement` that caused the form to be submitted
return async ({ result, update }) => {
// `result` is an `ActionResult` object
// `update` is a function which triggers the default logic that would be triggered if this callback wasn't set
};
}}
>
```
You can use these functions to show and hide loading UI, and so on.
If you return a callback, you may need to reproduce part of the default `use:enhance` behaviour, but without invalidating all data on a successful response. You can do so with `applyAction`:
```svelte
/// file: src/routes/login/+page.svelte
<script>
import { enhance, +++applyAction+++ } from '$app/forms';
/** @type {import('./$types').PageProps} */
let { form } = $props();
</script>
<form
method="POST"
use:enhance={({ formElement, formData, action, cancel }) => {
return async ({ result }) => {
// `result` is an `ActionResult` object
+++ if (result.type === 'redirect') {
goto(result.location);
} else {
await applyAction(result);
}+++
};
}}
>
```
The behaviour of `applyAction(result)` depends on `result.type`:
- `success`, `failure` — sets `page.status` to `result.status` and updates `form` and `page.form` to `result.data` (regardless of where you are submitting from, in contrast to `update` from `enhance`)
- `redirect` — calls `goto(result.location, { invalidateAll: true })`
- `error` — renders the nearest `+error` boundary with `result.error`
In all cases, [focus will be reset](accessibility#Focus-management).
### Custom event listener
We can also implement progressive enhancement ourselves, without `use:enhance`, with a normal event listener on the `<form>`:
```svelte
<!--- file: src/routes/login/+page.svelte --->
<script>
import { invalidateAll, goto } from '$app/navigation';
import { applyAction, deserialize } from '$app/forms';
/** @type {import('./$types').PageProps} */
let { form } = $props();
/** @param {SubmitEvent & { currentTarget: EventTarget & HTMLFormElement}} event */
async function handleSubmit(event) {
event.preventDefault();
const data = new FormData(event.currentTarget);
const response = await fetch(event.currentTarget.action, {
method: 'POST',
body: data
});
/** @type {import('@sveltejs/kit').ActionResult} */
const result = deserialize(await response.text());
if (result.type === 'success') {
// rerun all `load` functions, following the successful update
await invalidateAll();
}
applyAction(result);
}
</script>
<form method="POST" onsubmit={handleSubmit}>
<!-- content -->
</form>
```
Note that you need to `deserialize` the response before processing it further using the corresponding method from `$app/forms`. `JSON.parse()` isn't enough because form actions - like `load` functions - also support returning `Date` or `BigInt` objects.
If you have a `+server.js` alongside your `+page.server.js`, `fetch` requests will be routed there by default. To `POST` to an action in `+page.server.js` instead, use the custom `x-sveltekit-action` header:
```js
const response = await fetch(this.action, {
method: 'POST',
body: data,
+++ headers: {
'x-sveltekit-action': 'true'
}+++
});
```
## Alternatives
Form actions are the preferred way to send data to the server, since they can be progressively enhanced, but you can also use [`+server.js`](routing#server) files to expose (for example) a JSON API. Here's how such an interaction could look like:
```svelte
<!--- file: src/routes/send-message/+page.svelte --->
<script>
function rerun() {
fetch('/api/ci', {
method: 'POST'
});
}
</script>
<button onclick={rerun}>Rerun CI</button>
```
```js
// @errors: 2355 1360 2322
/// file: src/routes/api/ci/+server.js
/** @type {import('./$types').RequestHandler} */
export function POST() {
// do something
}
```
## GET vs POST
As we've seen, to invoke a form action you must use `method="POST"`.
Some forms don't need to `POST` data to the server — search inputs, for example. For these you can use `method="GET"` (or, equivalently, no `method` at all), and SvelteKit will treat them like `<a>` elements, using the client-side router instead of a full page navigation:
```html
<form action="/search">
<label>
Search
<input name="q">
</label>
</form>
```
Submitting this form will navigate to `/search?q=...` and invoke your load function but will not invoke an action. As with `<a>` elements, you can set the [`data-sveltekit-reload`](link-options#data-sveltekit-reload), [`data-sveltekit-replacestate`](link-options#data-sveltekit-replacestate), [`data-sveltekit-keepfocus`](link-options#data-sveltekit-keepfocus) and [`data-sveltekit-noscroll`](link-options#data-sveltekit-noscroll) attributes on the `<form>` to control the router's behaviour.
## Further reading
- [Tutorial: Forms](/tutorial/kit/the-form-element)
# Page options
By default, SvelteKit will render (or [prerender](glossary#Prerendering)) any component first on the server and send it to the client as HTML. It will then render the component again in the browser to make it interactive in a process called [**hydration**](glossary#Hydration). For this reason, you need to ensure that components can run in both places. SvelteKit will then initialize a [**router**](routing) that takes over subsequent navigations.
You can control each of these on a page-by-page basis by exporting options from [`+page.js`](routing#page-page.js) or [`+page.server.js`](routing#page-page.server.js), or for groups of pages using a shared [`+layout.js`](routing#layout-layout.js) or [`+layout.server.js`](routing#layout-layout.server.js). To define an option for the whole app, export it from the root layout. Child layouts and pages override values set in parent layouts, so — for example — you can enable prerendering for your entire app then disable it for pages that need to be dynamically rendered.
You can mix and match these options in different areas of your app. For example, you could prerender your marketing page for maximum speed, server-render your dynamic pages for SEO and accessibility and turn your admin section into an SPA by rendering it on the client only. This makes SvelteKit very versatile.
## prerender
It's likely that at least some routes of your app can be represented as a simple HTML file generated at build time. These routes can be [_prerendered_](glossary#Prerendering).
```js
/// file: +page.js/+page.server.js/+server.js
export const prerender = true;
```
Alternatively, you can set `export const prerender = true` in your root `+layout.js` or `+layout.server.js` and prerender everything except pages that are explicitly marked as _not_ prerenderable:
```js
/// file: +page.js/+page.server.js/+server.js
export const prerender = false;
```
Routes with `prerender = true` will be excluded from manifests used for dynamic SSR, making your server (or serverless/edge functions) smaller. In some cases you might want to prerender a route but also include it in the manifest (for example, with a route like `/blog/[slug]` where you want to prerender your most recent/popular content but server-render the long tail) — for these cases, there's a third option, 'auto':
```js
/// file: +page.js/+page.server.js/+server.js
export const prerender = 'auto';
```
> [!NOTE] If your entire app is suitable for prerendering, you can use [`adapter-static`](https://github.com/sveltejs/kit/tree/main/packages/adapter-static), which will output files suitable for use with any static webserver.
The prerenderer will start at the root of your app and generate files for any prerenderable pages or `+server.js` routes it finds. Each page is scanned for `<a>` elements that point to other pages that are candidates for prerendering — because of this, you generally don't need to specify which pages should be accessed. If you _do_ need to specify which pages should be accessed by the prerenderer, you can do so with [`config.kit.prerender.entries`](configuration#prerender), or by exporting an [`entries`](#entries) function from your dynamic route.
While prerendering, the value of `building` imported from [`$app/environment`]($app-environment) will be `true`.
### Prerendering server routes
Unlike the other page options, `prerender` also applies to `+server.js` files. These files are _not_ affected by layouts, but will inherit default values from the pages that fetch data from them, if any. For example if a `+page.js` contains this `load` function...
```js
/// file: +page.js
export const prerender = true;
/** @type {import('./$types').PageLoad} */
export async function load({ fetch }) {
const res = await fetch('/my-server-route.json');
return await res.json();
}
```
...then `src/routes/my-server-route.json/+server.js` will be treated as prerenderable if it doesn't contain its own `export const prerender = false`.
### When not to prerender
The basic rule is this: for a page to be prerenderable, any two users hitting it directly must get the same content from the server.
> [!NOTE] Not all pages are suitable for prerendering. Any content that is prerendered will be seen by all users. You can of course fetch personalized data in `onMount` in a prerendered page, but this may result in a poorer user experience since it will involve blank initial content or loading indicators.
Note that you can still prerender pages that load data based on the page's parameters, such as a `src/routes/blog/[slug]/+page.svelte` route.
Accessing [`url.searchParams`](load#Using-URL-data-url) during prerendering is forbidden. If you need to use it, ensure you are only doing so in the browser (for example in `onMount`).
Pages with [actions](form-actions) cannot be prerendered, because a server must be able to handle the action `POST` requests.
### Route conflicts
Because prerendering writes to the filesystem, it isn't possible to have two endpoints that would cause a directory and a file to have the same name. For example, `src/routes/foo/+server.js` and `src/routes/foo/bar/+server.js` would try to create `foo` and `foo/bar`, which is impossible.
For that reason among others, it's recommended that you always include a file extension — `src/routes/foo.json/+server.js` and `src/routes/foo/bar.json/+server.js` would result in `foo.json` and `foo/bar.json` files living harmoniously side-by-side.
For _pages_, we skirt around this problem by writing `foo/index.html` instead of `foo`.
### Troubleshooting
If you encounter an error like 'The following routes were marked as prerenderable, but were not prerendered' it's because the route in question (or a parent layout, if it's a page) has `export const prerender = true` but the page wasn't reached by the prerendering crawler and thus wasn't prerendered.
Since these routes cannot be dynamically server-rendered, this will cause errors when people try to access the route in question. There are a few ways to fix it:
* Ensure that SvelteKit can find the route by following links from [`config.kit.prerender.entries`](configuration#prerender) or the [`entries`](#entries) page option. Add links to dynamic routes (i.e. pages with `[parameters]` ) to this option if they are not found through crawling the other entry points, else they are not prerendered because SvelteKit doesn't know what value the parameters should have. Pages not marked as prerenderable will be ignored and their links to other pages will not be crawled, even if some of them would be prerenderable.
* Ensure that SvelteKit can find the route by discovering a link to it from one of your other prerendered pages that have server-side rendering enabled.
* Change `export const prerender = true` to `export const prerender = 'auto'`. Routes with `'auto'` can be dynamically server rendered
## entries
SvelteKit will discover pages to prerender automatically, by starting at _entry points_ and crawling them. By default, all your non-dynamic routes are considered entry points — for example, if you have these routes...
```bash
/ # non-dynamic
/blog # non-dynamic
/blog/[slug] # dynamic, because of `[slug]`
```
...SvelteKit will prerender `/` and `/blog`, and in the process discover links like `<a href="/blog/hello-world">` which give it new pages to prerender.
Most of the time, that's enough. In some situations, links to pages like `/blog/hello-world` might not exist (or might not exist on prerendered pages), in which case we need to tell SvelteKit about their existence.
This can be done with [`config.kit.prerender.entries`](configuration#prerender), or by exporting an `entries` function from a `+page.js`, a `+page.server.js` or a `+server.js` belonging to a dynamic route:
```js
/// file: src/routes/blog/[slug]/+page.server.js
/** @type {import('./$types').EntryGenerator} */
export function entries() {
return [
{ slug: 'hello-world' },
{ slug: 'another-blog-post' }
];
}
export const prerender = true;
```
`entries` can be an `async` function, allowing you to (for example) retrieve a list of posts from a CMS or database, in the example above.
## ssr
Normally, SvelteKit renders your page on the server first and sends that HTML to the client where it's [hydrated](glossary#Hydration). If you set `ssr` to `false`, it renders an empty 'shell' page instead. This is useful if your page is unable to be rendered on the server (because you use browser-only globals like `document` for example), but in most situations it's not recommended ([see appendix](glossary#SSR)).
```js
/// file: +page.js
export const ssr = false;
// If both `ssr` and `csr` are `false`, nothing will be rendered!
```
If you add `export const ssr = false` to your root `+layout.js`, your entire app will only be rendered on the client — which essentially means you turn your app into an SPA.
> [!NOTE] Even with `ssr` set to `false`, code that relies on browser APIs should be imported in your `+page.svelte` or `+layout.svelte` file instead. This is because page options can be overriden and need to be evaluated by importing your `+page.js` or `+layout.js` file on the server (if you have a runtime) or at build time (in case of prerendering).
## csr
Ordinarily, SvelteKit [hydrates](glossary#Hydration) your server-rendered HTML into an interactive client-side-rendered (CSR) page. Some pages don't require JavaScript at all — many blog posts and 'about' pages fall into this category. In these cases you can disable CSR:
```js
/// file: +page.js
export const csr = false;
// If both `csr` and `ssr` are `false`, nothing will be rendered!
```
Disabling CSR does not ship any JavaScript to the client. This means:
* The webpage should work with HTML and CSS only.
* `<script>` tags inside all Svelte components are removed.
* `<form>` elements cannot be [progressively enhanced](form-actions#Progressive-enhancement).
* Links are handled by the browser with a full-page navigation.
* Hot Module Replacement (HMR) will be disabled.
You can enable `csr` during development (for example to take advantage of HMR) like so:
```js
/// file: +page.js
import { dev } from '$app/environment';
export const csr = dev;
```
## trailingSlash
By default, SvelteKit will remove trailing slashes from URLs — if you visit `/about/`, it will respond with a redirect to `/about`. You can change this behaviour with the `trailingSlash` option, which can be one of `'never'` (the default), `'always'`, or `'ignore'`.
As with other page options, you can export this value from a `+layout.js` or a `+layout.server.js` and it will apply to all child pages. You can also export the configuration from `+server.js` files.
```js
/// file: src/routes/+layout.js
export const trailingSlash = 'always';
```
This option also affects [prerendering](#prerender). If `trailingSlash` is `always`, a route like `/about` will result in an `about/index.html` file, otherwise it will create `about.html`, mirroring static webserver conventions.
> [!NOTE] Ignoring trailing slashes is not recommended — the semantics of relative paths differ between the two cases (`./y` from `/x` is `/y`, but from `/x/` is `/x/y`), and `/x` and `/x/` are treated as separate URLs which is harmful to SEO.
## config
With the concept of [adapters](adapters), SvelteKit is able to run on a variety of platforms. Each of these might have specific configuration to further tweak the deployment — for example on Vercel you could choose to deploy some parts of your app on the edge and others on serverless environments.
`config` is an object with key-value pairs at the top level. Beyond that, the concrete shape is dependent on the adapter you're using. Every adapter should provide a `Config` interface to import for type safety. Consult the documentation of your adapter for more information.
```js
// @filename: ambient.d.ts
declare module 'some-adapter' {
export interface Config { runtime: string }
}
// @filename: index.js
// ---cut---
/// file: src/routes/+page.js
/** @type {import('some-adapter').Config} */
export const config = {
runtime: 'edge'
};
```
`config` objects are merged at the top level (but _not_ deeper levels). This means you don't need to repeat all the values in a `+page.js` if you want to only override some of the values in the upper `+layout.js`. For example this layout configuration...
```js
/// file: src/routes/+layout.js
export const config = {
runtime: 'edge',
regions: 'all',
foo: {
bar: true
}
}
```
...is overridden by this page configuration...
```js
/// file: src/routes/+page.js
export const config = {
regions: ['us1', 'us2'],
foo: {
baz: true
}
}
```
...which results in the config value `{ runtime: 'edge', regions: ['us1', 'us2'], foo: { baz: true } }` for that page.
## Further reading
- [Tutorial: Page options](/tutorial/kit/page-options)
# State management
If you're used to building client-only apps, state management in an app that spans server and client might seem intimidating. This section provides tips for avoiding some common gotchas.
## Avoid shared state on the server
Browsers are _stateful_ — state is stored in memory as the user interacts with the application. Servers, on the other hand, are _stateless_ — the content of the response is determined entirely by the content of the request.
Conceptually, that is. In reality, servers are often long-lived and shared by multiple users. For that reason it's important not to store data in shared variables. For example, consider this code:
```js
// @errors: 7034 7005
/// file: +page.server.js
let user;
/** @type {import('./$types').PageServerLoad} */
export function load() {
return { user };
}
/** @satisfies {import('./$types').Actions} */
export const actions = {
default: async ({ request }) => {
const data = await request.formData();
// NEVER DO THIS!
user = {
name: data.get('name'),
embarrassingSecret: data.get('secret')
};
}
}
```
The `user` variable is shared by everyone who connects to this server. If Alice submitted an embarrassing secret, and Bob visited the page after her, Bob would know Alice's secret. In addition, when Alice returns to the site later in the day, the server may have restarted, losing her data.
Instead, you should _authenticate_ the user using [`cookies`](load#Cookies) and persist the data to a database.
## No side-effects in load
For the same reason, your `load` functions should be _pure_ — no side-effects (except maybe the occasional `console.log(...)`). For example, you might be tempted to write to a store or global state inside a `load` function so that you can use the value in your components:
```js
/// file: +page.js
// @filename: ambient.d.ts
declare module '$lib/user' {
export const user: { set: (value: any) => void };
}
// @filename: index.js
// ---cut---
import { user } from '$lib/user';
/** @type {import('./$types').PageLoad} */
export async function load({ fetch }) {
const response = await fetch('/api/user');
// NEVER DO THIS!
user.set(await response.json());
}
```
As with the previous example, this puts one user's information in a place that is shared by _all_ users. Instead, just return the data...
```js
/// file: +page.js
/** @type {import('./$types').PageServerLoad} */
export async function load({ fetch }) {
const response = await fetch('/api/user');
+++ return {
user: await response.json()
};+++
}
```
...and pass it around to the components that need it, or use [`page.data`](load#page.data).
If you're not using SSR, then there's no risk of accidentally exposing one user's data to another. But you should still avoid side-effects in your `load` functions — your application will be much easier to reason about without them.
## Using state and stores with context
You might wonder how we're able to use `page.data` and other [app state]($app-state) (or [app stores]($app-stores)) if we can't use global state. The answer is that app state and app stores on the server use Svelte's [context API](/tutorial/svelte/context-api) — the state (or store) is attached to the component tree with `setContext`, and when you subscribe you retrieve it with `getContext`. We can do the same thing with our own state:
```svelte
<!--- file: src/routes/+layout.svelte --->
<script>
import { setContext } from 'svelte';
/** @type {import('./$types').LayoutProps} */
let { data } = $props();
// Pass a function referencing our state
// to the context for child components to access
setContext('user', () => data.user);
</script>
```
```svelte
<!--- file: src/routes/user/+page.svelte --->
<script>
import { getContext } from 'svelte';
// Retrieve user store from context
const user = getContext('user');
</script>
<p>Welcome {user().name}</p>
```
> [!NOTE] We're passing a function into `setContext` to keep reactivity across boundaries. Read more about it [here](/docs/svelte/$state#Passing-state-into-functions)
> [!LEGACY]
> You also use stores from `svelte/store` for this, but when using Svelte 5 it is recommended to make use of universal reactivity instead.
Updating the value of context-based state in deeper-level pages or components while the page is being rendered via SSR will not affect the value in the parent component because it has already been rendered by the time the state value is updated. In contrast, on the client (when CSR is enabled, which is the default) the value will be propagated and components, pages, and layouts higher in the hierarchy will react to the new value. Therefore, to avoid values 'flashing' during state updates during hydration, it is generally recommended to pass state down into components rather than up.
If you're not using SSR (and can guarantee that you won't need to use SSR in future) then you can safely keep state in a shared module, without using the context API.
## Component and page state is preserved
When you navigate around your application, SvelteKit reuses existing layout and page components. For example, if you have a route like this...
```svelte
<!--- file: src/routes/blog/[slug]/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
// THIS CODE IS BUGGY!
const wordCount = data.content.split(' ').length;
const estimatedReadingTime = wordCount / 250;
</script>
<header>
<h1>{data.title}</h1>
<p>Reading time: {Math.round(estimatedReadingTime)} minutes</p>
</header>
<div>{@html data.content}</div>
```
...then navigating from `/blog/my-short-post` to `/blog/my-long-post` won't cause the layout, page and any other components within to be destroyed and recreated. Instead the `data` prop (and by extension `data.title` and `data.content`) will update (as it would with any other Svelte component) and, because the code isn't rerunning, lifecycle methods like `onMount` and `onDestroy` won't rerun and `estimatedReadingTime` won't be recalculated.
Instead, we need to make the value [_reactive_](/tutorial/svelte/state):
```svelte
/// file: src/routes/blog/[slug]/+page.svelte
<script>
/** @type {import('./$types').PageProps} */
let { data } = $props();
+++ let wordCount = $derived(data.content.split(' ').length);
let estimatedReadingTime = $derived(wordCount / 250);+++
</script>
```
> [!NOTE] If your code in `onMount` and `onDestroy` has to run again after navigation you can use [afterNavigate]($app-navigation#afterNavigate) and [beforeNavigate]($app-navigation#beforeNavigate) respectively.
Reusing components like this means that things like sidebar scroll state are preserved, and you can easily animate between changing values. In the case that you do need to completely destroy and remount a component on navigation, you can use this pattern:
```svelte
<script>
import { page } from '$app/state';
</script>
{#key page.url.pathname}
<BlogPost title={data.title} content={data.title} />
{/key}
```
## Storing state in the URL
If you have state that should survive a reload and/or affect SSR, such as filters or sorting rules on a table, URL search parameters (like `?sort=price&order=ascending`) are a good place to put them. You can put them in `<a href="...">` or `<form action="...">` attributes, or set them programmatically via `goto('?key=value')`. They can be accessed inside `load` functions via the `url` parameter, and inside components via `page.url.searchParams`.
## Storing ephemeral state in snapshots
Some UI state, such as 'is the accordion open?', is disposable — if the user navigates away or refreshes the page, it doesn't matter if the state is lost. In some cases, you _do_ want the data to persist if the user navigates to a different page and comes back, but storing the state in the URL or in a database would be overkill. For this, SvelteKit provides [snapshots](snapshots), which let you associate component state with a history entry.
# Building your app
Building a SvelteKit app happens in two stages, which both happen when you run `vite build` (usually via `npm run build`).
Firstly, Vite creates an optimized production build of your server code, your browser code, and your service worker (if you have one). [Prerendering](page-options#prerender) is executed at this stage, if appropriate.
Secondly, an _adapter_ takes this production build and tunes it for your target environment — more on this on the following pages.
## During the build
SvelteKit will load your `+page/layout(.server).js` files (and all files they import) for analysis during the build. Any code that should _not_ be executed at this stage must check that `building` from [`$app/environment`]($app-environment) is `false`:
```js
+++import { building } from '$app/environment';+++
import { setupMyDatabase } from '$lib/server/database';
+++if (!building) {+++
setupMyDatabase();
+++}+++
export function load() {
// ...
}
```
## Preview your app
After building, you can view your production build locally with `vite preview` (via `npm run preview`). Note that this will run the app in Node, and so is not a perfect reproduction of your deployed app — adapter-specific adjustments like the [`platform` object](adapters#Platform-specific-context) do not apply to previews.
# Adapters
Before you can deploy your SvelteKit app, you need to _adapt_ it for your deployment target. Adapters are small plugins that take the built app as input and generate output for deployment.
Official adapters exist for a variety of platforms — these are documented on the following pages:
- [`@sveltejs/adapter-cloudflare`](adapter-cloudflare) for Cloudflare Pages
- [`@sveltejs/adapter-cloudflare-workers`](adapter-cloudflare-workers) for Cloudflare Workers
- [`@sveltejs/adapter-netlify`](adapter-netlify) for Netlify
- [`@sveltejs/adapter-node`](adapter-node) for Node servers
- [`@sveltejs/adapter-static`](adapter-static) for static site generation (SSG)
- [`@sveltejs/adapter-vercel`](adapter-vercel) for Vercel
Additional [community-provided adapters](https://sveltesociety.dev/packages?category=sveltekit-adapters) exist for other platforms.
## Using adapters
Your adapter is specified in `svelte.config.js`:
```js
/// file: svelte.config.js
// @filename: ambient.d.ts
declare module 'svelte-adapter-foo' {
const adapter: (opts: any) => import('@sveltejs/kit').Adapter;
export default adapter;
}
// @filename: index.js
// ---cut---
import adapter from 'svelte-adapter-foo';
/** @type {import('@sveltejs/kit').Config} */
const config = {
kit: {
adapter: adapter({
// adapter options go here
})
}
};
export default config;
```
## Platform-specific context
Some adapters may have access to additional information about the request. For example, Cloudflare Workers can access an `env` object containing KV namespaces etc. This can be passed to the `RequestEvent` used in [hooks](hooks) and [server routes](routing#server) as the `platform` property — consult each adapter's documentation to learn more.
# Zero-config deployments
When you create a new SvelteKit project with `npx sv create`, it installs [`adapter-auto`](https://github.com/sveltejs/kit/tree/main/packages/adapter-auto) by default. This adapter automatically installs and uses the correct adapter for supported environments when you deploy:
- [`@sveltejs/adapter-cloudflare`](adapter-cloudflare) for [Cloudflare Pages](https://developers.cloudflare.com/pages/)
- [`@sveltejs/adapter-netlify`](adapter-netlify) for [Netlify](https://netlify.com/)
- [`@sveltejs/adapter-vercel`](adapter-vercel) for [Vercel](https://vercel.com/)
- [`svelte-adapter-azure-swa`](https://github.com/geoffrich/svelte-adapter-azure-swa) for [Azure Static Web Apps](https://docs.microsoft.com/en-us/azure/static-web-apps/)
- [`svelte-kit-sst`](https://github.com/sst/sst/tree/master/packages/svelte-kit-sst) for [AWS via SST](https://sst.dev/docs/start/aws/svelte)
- [`@sveltejs/adapter-node`](adapter-node) for [Google Cloud Run](https://cloud.google.com/run)
It's recommended to install the appropriate adapter to your `devDependencies` once you've settled on a target environment, since this will add the adapter to your lockfile and slightly improve install times on CI.
## Environment-specific configuration
To add configuration options, such as `{ edge: true }` in [`adapter-vercel`](adapter-vercel) and [`adapter-netlify`](adapter-netlify), you must install the underlying adapter — `adapter-auto` does not take any options.
## Adding community adapters
You can add zero-config support for additional adapters by editing [adapters.js](https://github.com/sveltejs/kit/blob/main/packages/adapter-auto/adapters.js) and opening a pull request.
# Node servers
To generate a standalone Node server, use [`adapter-node`](https://github.com/sveltejs/kit/tree/main/packages/adapter-node).
## Usage
Install with `npm i -D @sveltejs/adapter-node`, then add the adapter to your `svelte.config.js`:
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-node';
export default {
kit: {
adapter: adapter()
}
};
```
## Deploying
First, build your app with `npm run build`. This will create the production server in the output directory specified in the adapter options, defaulting to `build`.
You will need the output directory, the project's `package.json`, and the production dependencies in `node_modules` to run the application. Production dependencies can be generated by copying the `package.json` and `package-lock.json` and then running `npm ci --omit dev` (you can skip this step if your app doesn't have any dependencies). You can then start your app with this command:
```bash
node build
```
Development dependencies will be bundled into your app using [Rollup](https://rollupjs.org). To control whether a given package is bundled or externalised, place it in `devDependencies` or `dependencies` respectively in your `package.json`.
### Compressing responses
You will typically want to compress responses coming from the server. If you are already deploying your server behind a reverse proxy for SSL or load balancing, it typically results in better performance to also handle compression at that layer since Node.js is single-threaded.
However, if you're building a [custom server](#Custom-server) and do want to add a compression middleware there, note that we would recommend using [`@polka/compression`](https://www.npmjs.com/package/@polka/compression) since SvelteKit streams responses and the more popular `compression` package does not support streaming and may cause errors when used.
## Environment variables
In `dev` and `preview`, SvelteKit will read environment variables from your `.env` file (or `.env.local`, or `.env.[mode]`, [as determined by Vite](https://vitejs.dev/guide/env-and-mode.html#env-files).)
In production, `.env` files are _not_ automatically loaded. To do so, install `dotenv` in your project...
```bash
npm install dotenv
```
...and invoke it before running the built app:
```bash
node +++-r dotenv/config+++ build
```
If you use Node.js v20.6+, you can use the [`--env-file`](https://nodejs.org/en/learn/command-line/how-to-read-environment-variables-from-nodejs) flag instead:
```bash
node +++--env-file=.env+++ build
```
### `PORT`, `HOST` and `SOCKET_PATH`
By default, the server will accept connections on `0.0.0.0` using port 3000. These can be customised with the `PORT` and `HOST` environment variables:
```
HOST=127.0.0.1 PORT=4000 node build
```
Alternatively, the server can be configured to accept connections on a specified socket path. When this is done using the `SOCKET_PATH` environment variable, the `HOST` and `PORT` environment variables will be disregarded.
```
SOCKET_PATH=/tmp/socket node build
```
### `ORIGIN`, `PROTOCOL_HEADER`, `HOST_HEADER`, and `PORT_HEADER`
HTTP doesn't give SvelteKit a reliable way to know the URL that is currently being requested. The simplest way to tell SvelteKit where the app is being served is to set the `ORIGIN` environment variable:
```
ORIGIN=https://my.site node build
# or e.g. for local previewing and testing
ORIGIN=http://localhost:3000 node build
```
With this, a request for the `/stuff` pathname will correctly resolve to `https://my.site/stuff`. Alternatively, you can specify headers that tell SvelteKit about the request protocol and host, from which it can construct the origin URL:
```
PROTOCOL_HEADER=x-forwarded-proto HOST_HEADER=x-forwarded-host node build
```
> [!NOTE] [`x-forwarded-proto`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Proto) and [`x-forwarded-host`](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/X-Forwarded-Host) are de facto standard headers that forward the original protocol and host if you're using a reverse proxy (think load balancers and CDNs). You should only set these variables if your server is behind a trusted reverse proxy; otherwise, it'd be possible for clients to spoof these headers.
>
> If you're hosting your proxy on a non-standard port and your reverse proxy supports `x-forwarded-port`, you can also set `PORT_HEADER=x-forwarded-port`.
If `adapter-node` can't correctly determine the URL of your deployment, you may experience this error when using [form actions](form-actions):
> [!NOTE] Cross-site POST form submissions are forbidden
### `ADDRESS_HEADER` and `XFF_DEPTH`
The [RequestEvent](@sveltejs-kit#RequestEvent) object passed to hooks and endpoints includes an `event.getClientAddress()` function that returns the client's IP address. By default this is the connecting `remoteAddress`. If your server is behind one or more proxies (such as a load balancer), this value will contain the innermost proxy's IP address rather than the client's, so we need to specify an `ADDRESS_HEADER` to read the address from:
```
ADDRESS_HEADER=True-Client-IP node build
```
> [!NOTE] Headers can easily be spoofed. As with `PROTOCOL_HEADER` and `HOST_HEADER`, you should [know what you're doing](https://adam-p.ca/blog/2022/03/x-forwarded-for/) before setting these.
If the `ADDRESS_HEADER` is `X-Forwarded-For`, the header value will contain a comma-separated list of IP addresses. The `XFF_DEPTH` environment variable should specify how many trusted proxies sit in front of your server. E.g. if there are three trusted proxies, proxy 3 will forward the addresses of the original connection and the first two proxies:
```
<client address>, <proxy 1 address>, <proxy 2 address>
```
Some guides will tell you to read the left-most address, but this leaves you [vulnerable to spoofing](https://adam-p.ca/blog/2022/03/x-forwarded-for/):
```
<spoofed address>, <client address>, <proxy 1 address>, <proxy 2 address>
```
We instead read from the _right_, accounting for the number of trusted proxies. In this case, we would use `XFF_DEPTH=3`.
> [!NOTE] If you need to read the left-most address instead (and don't care about spoofing) — for example, to offer a geolocation service, where it's more important for the IP address to be _real_ than _trusted_, you can do so by inspecting the `x-forwarded-for` header within your app.
### `BODY_SIZE_LIMIT`
The maximum request body size to accept in bytes including while streaming. The body size can also be specified with a unit suffix for kilobytes (`K`), megabytes (`M`), or gigabytes (`G`). For example, `512K` or `1M`. Defaults to 512kb. You can disable this option with a value of `Infinity` (0 in older versions of the adapter) and implement a custom check in [`handle`](hooks#Server-hooks-handle) if you need something more advanced.
### `SHUTDOWN_TIMEOUT`
The number of seconds to wait before forcefully closing any remaining connections after receiving a `SIGTERM` or `SIGINT` signal. Defaults to `30`. Internally the adapter calls [`closeAllConnections`](https://nodejs.org/api/http.html#servercloseallconnections). See [Graceful shutdown](#Graceful-shutdown) for more details.
### `IDLE_TIMEOUT`
When using systemd socket activation, `IDLE_TIMEOUT` specifies the number of seconds after which the app is automatically put to sleep when receiving no requests. If not set, the app runs continuously. See [Socket activation](#Socket-activation) for more details.
## Options
The adapter can be configured with various options:
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-node';
export default {
kit: {
adapter: adapter({
// default options are shown
out: 'build',
precompress: true,
envPrefix: ''
})
}
};
```
### out
The directory to build the server to. It defaults to `build` — i.e. `node build` would start the server locally after it has been created.
### precompress
Enables precompressing using gzip and brotli for assets and prerendered pages. It defaults to `true`.
### envPrefix
If you need to change the name of the environment variables used to configure the deployment (for example, to deconflict with environment variables you don't control), you can specify a prefix:
```js
envPrefix: 'MY_CUSTOM_';
```
```sh
MY_CUSTOM_HOST=127.0.0.1 \
MY_CUSTOM_PORT=4000 \
MY_CUSTOM_ORIGIN=https://my.site \
node build
```
## Graceful shutdown
By default `adapter-node` gracefully shuts down the HTTP server when a `SIGTERM` or `SIGINT` signal is received. It will:
1. reject new requests ([`server.close`](https://nodejs.org/api/http.html#serverclosecallback))
2. wait for requests that have already been made but not received a response yet to finish and close connections once they become idle ([`server.closeIdleConnections`](https://nodejs.org/api/http.html#servercloseidleconnections))
3. and finally, close any remaining connections that are still active after [`SHUTDOWN_TIMEOUT`](#Environment-variables-SHUTDOWN_TIMEOUT) seconds. ([`server.closeAllConnections`](https://nodejs.org/api/http.html#servercloseallconnections))
> [!NOTE] If you want to customize this behaviour you can use a [custom server](#Custom-server).
You can listen to the `sveltekit:shutdown` event which is emitted after the HTTP server has closed all connections. Unlike Node's `exit` event, the `sveltekit:shutdown` event supports asynchronous operations and is always emitted when all connections are closed even if the server has dangling work such as open database connections.
```js
// @errors: 2304
process.on('sveltekit:shutdown', async (reason) => {
await jobs.stop();
await db.close();
});
```
The parameter `reason` has one of the following values:
- `SIGINT` - shutdown was triggered by a `SIGINT` signal
- `SIGTERM` - shutdown was triggered by a `SIGTERM` signal
- `IDLE` - shutdown was triggered by [`IDLE_TIMEOUT`](#Environment-variables-IDLE_TIMEOUT)
## Socket activation
Most Linux operating systems today use a modern process manager called systemd to start the server and run and manage services. You can configure your server to allocate a socket and start and scale your app on demand. This is called [socket activation](http://0pointer.de/blog/projects/socket-activated-containers.html). In this case, the OS will pass two environment variables to your app — `LISTEN_PID` and `LISTEN_FDS`. The adapter will then listen on file descriptor 3 which refers to a systemd socket unit that you will have to create.
> [!NOTE] You can still use [`envPrefix`](#Options-envPrefix) with systemd socket activation. `LISTEN_PID` and `LISTEN_FDS` are always read without a prefix.
To take advantage of socket activation follow these steps.
1. Run your app as a [systemd service](https://www.freedesktop.org/software/systemd/man/latest/systemd.service.html). It can either run directly on the host system or inside a container (using Docker or a systemd portable service for example). If you additionally pass an [`IDLE_TIMEOUT`](#Environment-variables-IDLE_TIMEOUT) environment variable to your app it will gracefully shutdown if there are no requests for `IDLE_TIMEOUT` seconds. systemd will automatically start your app again when new requests are coming in.
```ini
/// file: /etc/systemd/system/myapp.service
[Service]
Environment=NODE_ENV=production IDLE_TIMEOUT=60
ExecStart=/usr/bin/node /usr/bin/myapp/build
```
2. Create an accompanying [socket unit](https://www.freedesktop.org/software/systemd/man/latest/systemd.socket.html). The adapter only accepts a single socket.
```ini
/// file: /etc/systemd/system/myapp.socket
[Socket]
ListenStream=3000
[Install]
WantedBy=sockets.target
```
3. Make sure systemd has recognised both units by running `sudo systemctl daemon-reload`. Then enable the socket on boot and start it immediately using `sudo systemctl enable --now myapp.socket`. The app will then automatically start once the first request is made to `localhost:3000`.
## Custom server
The adapter creates two files in your build directory — `index.js` and `handler.js`. Running `index.js` — e.g. `node build`, if you use the default build directory — will start a server on the configured port.
Alternatively, you can import the `handler.js` file, which exports a handler suitable for use with [Express](https://github.com/expressjs/express), [Connect](https://github.com/senchalabs/connect) or [Polka](https://github.com/lukeed/polka) (or even just the built-in [`http.createServer`](https://nodejs.org/dist/latest/docs/api/http.html#httpcreateserveroptions-requestlistener)) and set up your own server:
```js
// @errors: 2307 7006
/// file: my-server.js
import { handler } from './build/handler.js';
import express from 'express';
const app = express();
// add a route that lives separately from the SvelteKit app
app.get('/healthcheck', (req, res) => {
res.end('ok');
});
// let SvelteKit handle everything else, including serving prerendered pages and static assets
app.use(handler);
app.listen(3000, () => {
console.log('listening on port 3000');
});
```
# Static site generation
To use SvelteKit as a static site generator (SSG), use [`adapter-static`](https://github.com/sveltejs/kit/tree/main/packages/adapter-static).
This will prerender your entire site as a collection of static files. If you'd like to prerender only some pages and dynamically server-render others, you will need to use a different adapter together with [the `prerender` option](page-options#prerender).
## Usage
Install with `npm i -D @sveltejs/adapter-static`, then add the adapter to your `svelte.config.js`:
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-static';
export default {
kit: {
adapter: adapter({
// default options are shown. On some platforms
// these options are set automatically — see below
pages: 'build',
assets: 'build',
fallback: undefined,
precompress: false,
strict: true
})
}
};
```
...and add the [`prerender`](page-options#prerender) option to your root layout:
```js
/// file: src/routes/+layout.js
// This can be false if you're using a fallback (i.e. SPA mode)
export const prerender = true;
```
> [!NOTE] You must ensure SvelteKit's [`trailingSlash`](page-options#trailingSlash) option is set appropriately for your environment. If your host does not render `/a.html` upon receiving a request for `/a` then you will need to set `trailingSlash: 'always'` in your root layout to create `/a/index.html` instead.
## Zero-config support
Some platforms have zero-config support (more to come in future):
- [Vercel](https://vercel.com)
On these platforms, you should omit the adapter options so that `adapter-static` can provide the optimal configuration:
```js
// @errors: 2304
/// file: svelte.config.js
export default {
kit: {
adapter: adapter(---{...}---)
}
};
```
## Options
### pages
The directory to write prerendered pages to. It defaults to `build`.
### assets
The directory to write static assets (the contents of `static`, plus client-side JS and CSS generated by SvelteKit) to. Ordinarily this should be the same as `pages`, and it will default to whatever the value of `pages` is, but in rare circumstances you might need to output pages and assets to separate locations.
### fallback
Specify a fallback page for [SPA mode](single-page-apps), e.g. `index.html` or `200.html` or `404.html`.
### precompress
If `true`, precompresses files with brotli and gzip. This will generate `.br` and `.gz` files.
### strict
By default, `adapter-static` checks that either all pages and endpoints (if any) of your app were prerendered, or you have the `fallback` option set. This check exists to prevent you from accidentally publishing an app where some parts of it are not accessible, because they are not contained in the final output. If you know this is ok (for example when a certain page only exists conditionally), you can set `strict` to `false` to turn off this check.
## GitHub Pages
When building for [GitHub Pages](https://docs.github.com/en/pages/getting-started-with-github-pages/about-github-pages), if your repo name is not equivalent to `your-username.github.io`, make sure to update [`config.kit.paths.base`](configuration#paths) to match your repo name. This is because the site will be served from `https://your-username.github.io/your-repo-name` rather than from the root.
You'll also want to generate a fallback `404.html` page to replace the default 404 page shown by GitHub Pages.
A config for GitHub Pages might look like the following:
```js
// @errors: 2307 2322
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-static';
/** @type {import('@sveltejs/kit').Config} */
const config = {
kit: {
adapter: adapter({
fallback: '404.html'
}),
paths: {
base: process.argv.includes('dev') ? '' : process.env.BASE_PATH
}
}
};
export default config;
```
You can use GitHub actions to automatically deploy your site to GitHub Pages when you make a change. Here's an example workflow:
```yaml
### file: .github/workflows/deploy.yml
name: Deploy to GitHub Pages
on:
push:
branches: 'main'
jobs:
build_site:
runs-on: ubuntu-latest
steps:
- name: Checkout
uses: actions/checkout@v4
# If you're using pnpm, add this step then change the commands and cache key below to use `pnpm`
# - name: Install pnpm
# uses: pnpm/action-setup@v3
# with:
# version: 8
- name: Install Node.js
uses: actions/setup-node@v4
with:
node-version: 20
cache: npm
- name: Install dependencies
run: npm install
- name: build
env:
BASE_PATH: '/${{ github.event.repository.name }}'
run: |
npm run build
- name: Upload Artifacts
uses: actions/upload-pages-artifact@v3
with:
# this should match the `pages` option in your adapter-static options
path: 'build/'
deploy:
needs: build_site
runs-on: ubuntu-latest
permissions:
pages: write
id-token: write
environment:
name: github-pages
url: ${{ steps.deployment.outputs.page_url }}
steps:
- name: Deploy
id: deployment
uses: actions/deploy-pages@v4
```
If you're not using GitHub actions to deploy your site (for example, you're pushing the built site to its own repo), add an empty `.nojekyll` file in your `static` directory to prevent Jekyll from interfering.
# Single-page apps
You can turn any SvelteKit app, using any adapter, into a fully client-rendered single-page app (SPA) by disabling SSR at the root layout:
```js
/// file: src/routes/+layout.js
export const ssr = false;
```
> [!NOTE] In most situations this is not recommended: it harms SEO, tends to slow down perceived performance, and makes your app inaccessible to users if JavaScript fails or is disabled (which happens [more often than you probably think](https://kryogenix.org/code/browser/everyonehasjs.html)).
If you don't have any server-side logic (i.e. `+page.server.js`, `+layout.server.js` or `+server.js` files) you can use [`adapter-static`](adapter-static) to create your SPA by adding a _fallback page_.
## Usage
Install with `npm i -D @sveltejs/adapter-static`, then add the adapter to your `svelte.config.js` with the following options:
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-static';
export default {
kit: {
adapter: adapter({
fallback: '200.html' // may differ from host to host
})
}
};
```
The `fallback` page is an HTML page created by SvelteKit from your page template (e.g. `app.html`) that loads your app and navigates to the correct route. For example [Surge](https://surge.sh/help/adding-a-200-page-for-client-side-routing), a static web host, lets you add a `200.html` file that will handle any requests that don't correspond to static assets or prerendered pages.
On some hosts it may be `index.html` or something else entirely — consult your platform's documentation.
> [!NOTE] Note that the fallback page will always contain absolute asset paths (i.e. beginning with `/` rather than `.`) regardless of the value of [`paths.relative`](configuration#paths), since it is used to respond to requests for arbitrary paths.
## Apache
To run an SPA on [Apache](https://httpd.apache.org/), you should add a `static/.htaccess` file to route requests to the fallback page:
```
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^200\.html$ - [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /200.html [L]
</IfModule>
```
## Prerendering individual pages
If you want certain pages to be prerendered, you can re-enable `ssr` alongside `prerender` for just those parts of your app:
```js
/// file: src/routes/my-prerendered-page/+page.js
export const prerender = true;
export const ssr = true;
```
# Cloudflare Pages
To deploy to [Cloudflare Pages](https://developers.cloudflare.com/pages/), use [`adapter-cloudflare`](https://github.com/sveltejs/kit/tree/main/packages/adapter-cloudflare).
This adapter will be installed by default when you use [`adapter-auto`](adapter-auto). If you plan on staying with Cloudflare Pages, you can switch from [`adapter-auto`](adapter-auto) to using this adapter directly so that values specific to Cloudflare Workers are emulated during local development, type declarations are automatically applied, and the ability to set Cloudflare-specific options is provided.
## Comparisons
- `adapter-cloudflare` – supports all SvelteKit features; builds for [Cloudflare Pages](https://blog.cloudflare.com/cloudflare-pages-goes-full-stack/)
- `adapter-cloudflare-workers` – supports all SvelteKit features; builds for Cloudflare Workers
- `adapter-static` – only produces client-side static assets; compatible with Cloudflare Pages
## Usage
Install with `npm i -D @sveltejs/adapter-cloudflare`, then add the adapter to your `svelte.config.js`:
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-cloudflare';
export default {
kit: {
adapter: adapter({
// See below for an explanation of these options
routes: {
include: ['/*'],
exclude: ['<all>']
},
platformProxy: {
configPath: 'wrangler.toml',
environment: undefined,
experimentalJsonConfig: false,
persist: false
}
})
}
};
```
## Options
### routes
Allows you to customise the [`_routes.json`](https://developers.cloudflare.com/pages/platform/functions/routing/#create-a-_routesjson-file) file generated by `adapter-cloudflare`.
- `include` defines routes that will invoke a function, and defaults to `['/*']`
- `exclude` defines routes that will _not_ invoke a function — this is a faster and cheaper way to serve your app's static assets. This array can include the following special values:
- `<build>` contains your app's build artifacts (the files generated by Vite)
- `<files>` contains the contents of your `static` directory
- `<prerendered>` contains a list of prerendered pages
- `<all>` (the default) contains all of the above
You can have up to 100 `include` and `exclude` rules combined. Generally you can omit the `routes` options, but if (for example) your `<prerendered>` paths exceed that limit, you may find it helpful to manually create an `exclude` list that includes `'/articles/*'` instead of the auto-generated `['/articles/foo', '/articles/bar', '/articles/baz', ...]`.
### platformProxy
Preferences for the emulated `platform.env` local bindings. See the [getPlatformProxy](https://developers.cloudflare.com/workers/wrangler/api/#syntax) Wrangler API documentation for a full list of options.
## Deployment
Please follow the [Get Started Guide](https://developers.cloudflare.com/pages/get-started) for Cloudflare Pages to begin.
When configuring your project settings, you must use the following settings:
- **Framework preset** – SvelteKit
- **Build command** – `npm run build` or `vite build`
- **Build output directory** – `.svelte-kit/cloudflare`
## Runtime APIs
The [`env`](https://developers.cloudflare.com/workers/runtime-apis/fetch-event#parameters) object contains your project's [bindings](https://developers.cloudflare.com/pages/platform/functions/bindings/), which consist of KV/DO namespaces, etc. It is passed to SvelteKit via the `platform` property, along with [`context`](https://developers.cloudflare.com/workers/runtime-apis/handlers/fetch/#contextwaituntil), [`caches`](https://developers.cloudflare.com/workers/runtime-apis/cache/), and [`cf`](https://developers.cloudflare.com/workers/runtime-apis/request/#the-cf-property-requestinitcfproperties), meaning that you can access it in hooks and endpoints:
```js
// @errors: 7031
export async function POST({ request, platform }) {
const x = platform.env.YOUR_DURABLE_OBJECT_NAMESPACE.idFromName('x');
}
```
> [!NOTE] SvelteKit's built-in `$env` module should be preferred for environment variables.
To make these types available to your app, install `@cloudflare/workers-types` and reference them in your `src/app.d.ts`:
```ts
/// file: src/app.d.ts
+++import { KVNamespace, DurableObjectNamespace } from '@cloudflare/workers-types';+++
declare global {
namespace App {
interface Platform {
+++ env?: {
YOUR_KV_NAMESPACE: KVNamespace;
YOUR_DURABLE_OBJECT_NAMESPACE: DurableObjectNamespace;
};+++
}
}
}
export {};
```
### Testing Locally
Cloudflare Workers specific values in the `platform` property are emulated during dev and preview modes. Local [bindings](https://developers.cloudflare.com/workers/wrangler/configuration/#bindings) are created based on the configuration in your `wrangler.toml` file and are used to populate `platform.env` during development and preview. Use the adapter config [`platformProxy` option](#Options-platformProxy) to change your preferences for the bindings.
For testing the build, you should use [wrangler](https://developers.cloudflare.com/workers/cli-wrangler) **version 3**. Once you have built your site, run `wrangler pages dev .svelte-kit/cloudflare`.
## Notes
Functions contained in the `/functions` directory at the project's root will _not_ be included in the deployment, which is compiled to a [single `_worker.js` file](https://developers.cloudflare.com/pages/platform/functions/#advanced-mode). Functions should be implemented as [server endpoints](routing#server) in your SvelteKit app.
The `_headers` and `_redirects` files specific to Cloudflare Pages can be used for static asset responses (like images) by putting them into the `/static` folder.
However, they will have no effect on responses dynamically rendered by SvelteKit, which should return custom headers or redirect responses from [server endpoints](routing#server) or with the [`handle`](hooks#Server-hooks-handle) hook.
## Troubleshooting
### Further reading
You may wish to refer to [Cloudflare's documentation for deploying a SvelteKit site](https://developers.cloudflare.com/pages/framework-guides/deploy-a-svelte-kit-site).
### Accessing the file system
You can't use `fs` in Cloudflare Workers — you must [prerender](page-options#prerender) the routes in question.
# Cloudflare Workers
To deploy to [Cloudflare Workers](https://workers.cloudflare.com/), use [`adapter-cloudflare-workers`](https://github.com/sveltejs/kit/tree/main/packages/adapter-cloudflare-workers).
> [!NOTE] Unless you have a specific reason to use `adapter-cloudflare-workers`, it's recommended that you use `adapter-cloudflare` instead. Both adapters have equivalent functionality, but Cloudflare Pages offers features like GitHub integration with automatic builds and deploys, preview deployments, instant rollback and so on.
## Usage
Install with `npm i -D @sveltejs/adapter-cloudflare-workers`, then add the adapter to your `svelte.config.js`:
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-cloudflare-workers';
export default {
kit: {
adapter: adapter({
config: 'wrangler.toml',
platformProxy: {
configPath: 'wrangler.toml',
environment: undefined,
experimentalJsonConfig: false,
persist: false
}
})
}
};
```
## Options
### config
Path to your custom `wrangler.toml` or `wrangler.json` config file.
### platformProxy
Preferences for the emulated `platform.env` local bindings. See the [getPlatformProxy](https://developers.cloudflare.com/workers/wrangler/api/#syntax) Wrangler API documentation for a full list of options.
## Basic Configuration
This adapter expects to find a [wrangler.toml/wrangler.json](https://developers.cloudflare.com/workers/platform/sites/configuration) file in the project root. It should look something like this:
```toml
/// file: wrangler.toml
name = "<your-service-name>"
account_id = "<your-account-id>"
main = "./.cloudflare/worker.js"
site.bucket = "./.cloudflare/public"
build.command = "npm run build"
compatibility_date = "2021-11-12"
workers_dev = true
```
`<your-service-name>` can be anything. `<your-account-id>` can be found by logging into your [Cloudflare dashboard](https://dash.cloudflare.com) and grabbing it from the end of the URL:
```
https://dash.cloudflare.com/<your-account-id>
```
> [!NOTE] You should add the `.cloudflare` directory (or whichever directories you specified for `main` and `site.bucket`) to your `.gitignore`.
You will need to install [wrangler](https://developers.cloudflare.com/workers/wrangler/get-started/) and log in, if you haven't already:
```
npm i -g wrangler
wrangler login
```
Then, you can build your app and deploy it:
```sh
wrangler deploy
```
## Custom config
If you would like to use a config file other than `wrangler.toml` you can specify so using the [`config` option](#Options-config).
If you would like to enable [Node.js compatibility](https://developers.cloudflare.com/workers/runtime-apis/nodejs/#enable-nodejs-from-the-cloudflare-dashboard), you can add "nodejs_compat" flag to `wrangler.toml`:
```toml
/// file: wrangler.toml
compatibility_flags = [ "nodejs_compat" ]
```
## Runtime APIs
The [`env`](https://developers.cloudflare.com/workers/runtime-apis/fetch-event#parameters) object contains your project's [bindings](https://developers.cloudflare.com/pages/platform/functions/bindings/), which consist of KV/DO namespaces, etc. It is passed to SvelteKit via the `platform` property, along with [`context`](https://developers.cloudflare.com/workers/runtime-apis/handlers/fetch/#contextwaituntil), [`caches`](https://developers.cloudflare.com/workers/runtime-apis/cache/), and [`cf`](https://developers.cloudflare.com/workers/runtime-apis/request/#the-cf-property-requestinitcfproperties), meaning that you can access it in hooks and endpoints:
```js
// @errors: 7031
export async function POST({ request, platform }) {
const x = platform.env.YOUR_DURABLE_OBJECT_NAMESPACE.idFromName('x');
}
```
> [!NOTE] SvelteKit's built-in `$env` module should be preferred for environment variables.
To make these types available to your app, install `@cloudflare/workers-types` and reference them in your `src/app.d.ts`:
```ts
/// file: src/app.d.ts
+++import { KVNamespace, DurableObjectNamespace } from '@cloudflare/workers-types';+++
declare global {
namespace App {
interface Platform {
+++ env?: {
YOUR_KV_NAMESPACE: KVNamespace;
YOUR_DURABLE_OBJECT_NAMESPACE: DurableObjectNamespace;
};+++
}
}
}
export {};
```
### Testing Locally
Cloudflare Workers specific values in the `platform` property are emulated during dev and preview modes. Local [bindings](https://developers.cloudflare.com/workers/wrangler/configuration/#bindings) are created based on the configuration in your `wrangler.toml` file and are used to populate `platform.env` during development and preview. Use the adapter config [`platformProxy` option](#Options-platformProxy) to change your preferences for the bindings.
For testing the build, you should use [wrangler](https://developers.cloudflare.com/workers/cli-wrangler) **version 3**. Once you have built your site, run `wrangler dev`.
## Troubleshooting
### Worker size limits
When deploying to workers, the server generated by SvelteKit is bundled into a single file. Wrangler will fail to publish your worker if it exceeds [the size limits](https://developers.cloudflare.com/workers/platform/limits/#worker-size) after minification. You're unlikely to hit this limit usually, but some large libraries can cause this to happen. In that case, you can try to reduce the size of your worker by only importing such libraries on the client side. See [the FAQ](./faq#How-do-I-use-X-with-SvelteKit-How-do-I-use-a-client-side-only-library-that-depends-on-document-or-window) for more information.
### Accessing the file system
You can't use `fs` in Cloudflare Workers — you must [prerender](page-options#prerender) the routes in question.
# Netlify
To deploy to Netlify, use [`adapter-netlify`](https://github.com/sveltejs/kit/tree/main/packages/adapter-netlify).
This adapter will be installed by default when you use [`adapter-auto`](adapter-auto), but adding it to your project allows you to specify Netlify-specific options.
## Usage
Install with `npm i -D @sveltejs/adapter-netlify`, then add the adapter to your `svelte.config.js`:
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-netlify';
export default {
kit: {
// default options are shown
adapter: adapter({
// if true, will create a Netlify Edge Function rather
// than using standard Node-based functions
edge: false,
// if true, will split your app into multiple functions
// instead of creating a single one for the entire app.
// if `edge` is true, this option cannot be used
split: false
})
}
};
```
Then, make sure you have a [netlify.toml](https://docs.netlify.com/configure-builds/file-based-configuration) file in the project root. This will determine where to write static assets based on the `build.publish` settings, as per this sample configuration:
```toml
[build]
command = "npm run build"
publish = "build"
```
If the `netlify.toml` file or the `build.publish` value is missing, a default value of `"build"` will be used. Note that if you have set the publish directory in the Netlify UI to something else then you will need to set it in `netlify.toml` too, or use the default value of `"build"`.
### Node version
New projects will use the current Node LTS version by default. However, if you're upgrading a project you created a while ago it may be stuck on an older version. See [the Netlify docs](https://docs.netlify.com/configure-builds/manage-dependencies/#node-js-and-javascript) for details on manually specifying a current Node version.
## Netlify Edge Functions
SvelteKit supports [Netlify Edge Functions](https://docs.netlify.com/netlify-labs/experimental-features/edge-functions/). If you pass the option `edge: true` to the `adapter` function, server-side rendering will happen in a Deno-based edge function that's deployed close to the site visitor. If set to `false` (the default), the site will deploy to Node-based Netlify Functions.
```js
// @errors: 2307
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-netlify';
export default {
kit: {
adapter: adapter({
// will create a Netlify Edge Function using Deno-based
// rather than using standard Node-based functions
edge: true
})
}
};
```
## Netlify alternatives to SvelteKit functionality
You may build your app using functionality provided directly by SvelteKit without relying on any Netlify functionality. Using the SvelteKit versions of these features will allow them to be used in dev mode, tested with integration tests, and to work with other adapters should you ever decide to switch away from Netlify. However, in some scenarios you may find it beneficial to use the Netlify versions of these features. One example would be if you're migrating an app that's already hosted on Netlify to SvelteKit.
### Redirect rules
During compilation, redirect rules are automatically appended to your `_redirects` file. (If it doesn't exist yet, it will be created.) That means:
- `[[redirects]]` in `netlify.toml` will never match as `_redirects` has a [higher priority](https://docs.netlify.com/routing/redirects/#rule-processing-order). So always put your rules in the [`_redirects` file](https://docs.netlify.com/routing/redirects/#syntax-for-the-redirects-file).
- `_redirects` shouldn't have any custom "catch all" rules such as `/* /foobar/:splat`. Otherwise the automatically appended rule will never be applied as Netlify is only processing [the first matching rule](https://docs.netlify.com/routing/redirects/#rule-processing-order).
### Netlify Forms
1. Create your Netlify HTML form as described [here](https://docs.netlify.com/forms/setup/#html-forms), e.g. as `/routes/contact/+page.svelte`. (Don't forget to add the hidden `form-name` input element!)
2. Netlify's build bot parses your HTML files at deploy time, which means your form must be [prerendered](page-options#prerender) as HTML. You can either add `export const prerender = true` to your `contact.svelte` to prerender just that page or set the `kit.prerender.force: true` option to prerender all pages.
3. If your Netlify form has a [custom success message](https://docs.netlify.com/forms/setup/#success-messages) like `<form netlify ... action="/success">` then ensure the corresponding `/routes/success/+page.svelte` exists and is prerendered.
### Netlify Functions
With this adapter, SvelteKit endpoints are hosted as [Netlify Functions](https://docs.netlify.com/functions/overview/). Netlify function handlers have additional context, including [Netlify Identity](https://docs.netlify.com/visitor-access/identity/) information. You can access this context via the `event.platform.context` field inside your hooks and `+page.server` or `+layout.server` endpoints. These are [serverless functions](https://docs.netlify.com/functions/overview/) when the `edge` property is `false` in the adapter config or [edge functions](https://docs.netlify.com/edge-functions/overview/#app) when it is `true`.
```js
// @errors: 2705 7006
/// file: +page.server.js
export const load = async (event) => {
const context = event.platform.context;
console.log(context); // shows up in your functions log in the Netlify app
};
```
Additionally, you can add your own Netlify functions by creating a directory for them and adding the configuration to your `netlify.toml` file. For example:
```toml
[build]
command = "npm run build"
publish = "build"
[functions]
directory = "functions"
```
## Troubleshooting
### Accessing the file system
You can't use `fs` in edge deployments.
You _can_ use it in serverless deployments, but it won't work as expected, since files are not copied from your project into your deployment. Instead, use the [`read`]($app-server#read) function from `$app/server` to access your files. `read` does not work inside edge deployments (this may change in future).
Alternatively, you can [prerender](page-options#prerender) the routes in question.
# Vercel
To deploy to Vercel, use [`adapter-vercel`](https://github.com/sveltejs/kit/tree/main/packages/adapter-vercel).
This adapter will be installed by default when you use [`adapter-auto`](adapter-auto), but adding it to your project allows you to specify Vercel-specific options.
## Usage
Install with `npm i -D @sveltejs/adapter-vercel`, then add the adapter to your `svelte.config.js`:
```js
// @errors: 2307 2345
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-vercel';
export default {
kit: {
adapter: adapter({
// see below for options that can be set here
})
}
};
```
## Deployment configuration
To control how your routes are deployed to Vercel as functions, you can specify deployment configuration, either through the option shown above or with [`export const config`](page-options#config) inside `+server.js`, `+page(.server).js` and `+layout(.server).js` files.
For example you could deploy some parts of your app as [Edge Functions](https://vercel.com/docs/concepts/functions/edge-functions)...
```js
/// file: about/+page.js
/** @type {import('@sveltejs/adapter-vercel').Config} */
export const config = {
runtime: 'edge'
};
```
...and others as [Serverless Functions](https://vercel.com/docs/concepts/functions/serverless-functions) (note that by specifying `config` inside a layout, it applies to all child pages):
```js
/// file: admin/+layout.js
/** @type {import('@sveltejs/adapter-vercel').Config} */
export const config = {
runtime: 'nodejs22.x'
};
```
The following options apply to all functions:
- `runtime`: `'edge'`, `'nodejs18.x'`, `'nodejs20.x'` or `'nodejs22.x'`. By default, the adapter will select the `'nodejs<version>.x'` corresponding to the Node version your project is configured to use on the Vercel dashboard
- `regions`: an array of [edge network regions](https://vercel.com/docs/concepts/edge-network/regions) (defaulting to `["iad1"]` for serverless functions) or `'all'` if `runtime` is `edge` (its default). Note that multiple regions for serverless functions are only supported on Enterprise plans
- `split`: if `true`, causes a route to be deployed as an individual function. If `split` is set to `true` at the adapter level, all routes will be deployed as individual functions
Additionally, the following option applies to edge functions:
- `external`: an array of dependencies that esbuild should treat as external when bundling functions. This should only be used to exclude optional dependencies that will not run outside Node
And the following option apply to serverless functions:
- `memory`: the amount of memory available to the function. Defaults to `1024` Mb, and can be decreased to `128` Mb or [increased](https://vercel.com/docs/concepts/limits/overview#serverless-function-memory) in 64Mb increments up to `3008` Mb on Pro or Enterprise accounts
- `maxDuration`: [maximum execution duration](https://vercel.com/docs/functions/runtimes#max-duration) of the function. Defaults to `10` seconds for Hobby accounts, `15` for Pro and `900` for Enterprise
- `isr`: configuration Incremental Static Regeneration, described below
If your functions need to access data in a specific region, it's recommended that they be deployed in the same region (or close to it) for optimal performance.
## Image Optimization
You may set the `images` config to control how Vercel builds your images. See the [image configuration reference](https://vercel.com/docs/build-output-api/v3/configuration#images) for full details. As an example, you may set:
```js
/// file: svelte.config.js
import adapter from '@sveltejs/adapter-vercel';
export default {
kit: {
adapter: adapter({
images: {
sizes: [640, 828, 1200, 1920, 3840],
formats: ['image/avif', 'image/webp'],
minimumCacheTTL: 300,
domains: ['example-app.vercel.app'],
}
})
}
};
```
## Incremental Static Regeneration
Vercel supports [Incremental Static Regeneration](https://vercel.com/docs/incremental-static-regeneration) (ISR), which provides the performance and cost advantages of prerendered content with the flexibility of dynamically rendered content.
> Use ISR only on routes where every visitor should see the same content (much like when you prerender). If there's anything user-specific happening (like session cookies), they should happen on the client via JavaScript only to not leak sensitive information across visits
To add ISR to a route, include the `isr` property in your `config` object:
```js
// @errors: 2664
import { BYPASS_TOKEN } from '$env/static/private';
export const config = {
isr: {
expiration: 60,
bypassToken: BYPASS_TOKEN,
allowQuery: ['search']
}
};
```
> Using ISR on a route with `export const prerender = true` will have no effect, since the route is prerendered at build time
The `expiration` property is required; all others are optional. The properties are discussed in more detail below.
### expiration
The expiration time (in seconds) before the cached asset will be re-generated by invoking the Serverless Function. Setting the value to `false` means it will never expire. In that case, you likely want to define a bypass token to re-generate on demand.
### bypassToken
A random token that can be provided in the URL to bypass the cached version of the asset, by requesting the asset with a `__prerender_bypass=<token>` cookie.
Making a `GET` or `HEAD` request with `x-prerender-revalidate: <token>` will force the asset to be re-validated.
Note that the `BYPASS_TOKEN` string must be at least 32 characters long. You could generate one using the JavaScript console like so:
```js
crypto.randomUUID();
```
Set this string as an environment variable on Vercel by logging in and going to your project then Settings > Environment Variables. For "Key" put `BYPASS_TOKEN` and for "value" use the string generated above, then hit "Save".
To get this key known about for local development, you can use the [Vercel CLI](https://vercel.com/docs/cli/env) by running the `vercel env pull` command locally like so:
```sh
vercel env pull .env.development.local
```
### allowQuery
A list of valid query parameters that contribute to the cache key. Other parameters (such as utm tracking codes) will be ignored, ensuring that they do not result in content being re-generated unnecessarily. By default, query parameters are ignored.
> Pages that are [prerendered](page-options#prerender) will ignore ISR configuration.
## Environment variables
Vercel makes a set of [deployment-specific environment variables](https://vercel.com/docs/concepts/projects/environment-variables#system-environment-variables) available. Like other environment variables, these are accessible from `$env/static/private` and `$env/dynamic/private` (sometimes — more on that later), and inaccessible from their public counterparts. To access one of these variables from the client:
```js
// @errors: 2305
/// file: +layout.server.js
import { VERCEL_COMMIT_REF } from '$env/static/private';
/** @type {import('./$types').LayoutServerLoad} */
export function load() {
return {
deploymentGitBranch: VERCEL_COMMIT_REF
};
}
```
```svelte
<!--- file: +layout.svelte --->
<script>
/** @type {import('./$types').LayoutProps} */
let { data } = $props();
</script>
<p>This staging environment was deployed from {data.deploymentGitBranch}.</p>
```
Since all of these variables are unchanged between build time and run time when building on Vercel, we recommend using `$env/static/private` — which will statically replace the variables, enabling optimisations like dead code elimination — rather than `$env/dynamic/private`.
## Skew protection
When a new version of your app is deployed, assets belonging to the previous version may no longer be accessible. If a user is actively using your app when this happens, it can cause errors when they navigate — this is known as _version skew_. SvelteKit mitigates this by detecting errors resulting from version skew and causing a hard reload to get the latest version of the app, but this will cause any client-side state to be lost. (You can also proactively mitigate it by observing the [`updated`]($app-stores#updated) store value, which tells clients when a new version has been deployed.)
[Skew protection](https://vercel.com/docs/deployments/skew-protection) is a Vercel feature that routes client requests to their original deployment. When a user visits your app, a cookie is set with the deployment ID, and any subsequent requests will be routed to that deployment for as long as skew protection is active. When they reload the page, they will get the newest deployment. (The `updated` store is exempted from this behaviour, and so will continue to report new deployments.) To enable it, visit the Advanced section of your project settings on Vercel.
Cookie-based skew protection comes with one caveat: if a user has multiple versions of your app open in multiple tabs, requests from older versions will be routed to the newer one, meaning they will fall back to SvelteKit's built-in skew protection.
## Notes
### Vercel functions
If you have Vercel functions contained in the `api` directory at the project's root, any requests for `/api/*` will _not_ be handled by SvelteKit. You should implement these as [API routes](routing#server) in your SvelteKit app instead, unless you need to use a non-JavaScript language in which case you will need to ensure that you don't have any `/api/*` routes in your SvelteKit app.
### Node version
Projects created before a certain date may default to using an older Node version than what SvelteKit currently requires. You can [change the Node version in your project settings](https://vercel.com/docs/concepts/functions/serverless-functions/runtimes/node-js#node.js-version).
## Troubleshooting
### Accessing the file system
You can't use `fs` in edge functions.
You _can_ use it in serverless functions, but it won't work as expected, since files are not copied from your project into your deployment. Instead, use the [`read`]($app-server#read) function from `$app/server` to access your files. `read` does not work inside routes deployed as edge functions (this may change in future).
Alternatively, you can [prerender](page-options#prerender) the routes in question.
# Writing adapters
If an adapter for your preferred environment doesn't yet exist, you can build your own. We recommend [looking at the source for an adapter](https://github.com/sveltejs/kit/tree/main/packages) to a platform similar to yours and copying it as a starting point.
Adapter packages implement the following API, which creates an `Adapter`:
```js
// @errors: 2322
// @filename: ambient.d.ts
type AdapterSpecificOptions = any;
// @filename: index.js
// ---cut---
/** @param {AdapterSpecificOptions} options */
export default function (options) {
/** @type {import('@sveltejs/kit').Adapter} */
const adapter = {
name: 'adapter-package-name',
async adapt(builder) {
// adapter implementation
},
async emulate() {
return {
async platform({ config, prerender }) {
// the returned object becomes `event.platform` during dev, build and
// preview. Its shape is that of `App.Platform`
}
}
},
supports: {
read: ({ config, route }) => {
// Return `true` if the route with the given `config` can use `read`
// from `$app/server` in production, return `false` if it can't.
// Or throw a descriptive error describing how to configure the deployment
}
}
};
return adapter;
}
```
Of these, `name` and `adapt` are required. `emulate` and `supports` are optional.
Within the `adapt` method, there are a number of things that an adapter should do:
- Clear out the build directory
- Write SvelteKit output with `builder.writeClient`, `builder.writeServer`, and `builder.writePrerendered`
- Output code that:
- Imports `Server` from `${builder.getServerDirectory()}/index.js`
- Instantiates the app with a manifest generated with `builder.generateManifest({ relativePath })`
- Listens for requests from the platform, converts them to a standard [Request](https://developer.mozilla.org/en-US/docs/Web/API/Request) if necessary, calls the `server.respond(request, { getClientAddress })` function to generate a [Response](https://developer.mozilla.org/en-US/docs/Web/API/Response) and responds with it
- expose any platform-specific information to SvelteKit via the `platform` option passed to `server.respond`
- Globally shims `fetch` to work on the target platform, if necessary. SvelteKit provides a `@sveltejs/kit/node/polyfills` helper for platforms that can use `undici`
- Bundle the output to avoid needing to install dependencies on the target platform, if necessary
- Put the user's static files and the generated JS/CSS in the correct location for the target platform
Where possible, we recommend putting the adapter output under the `build/` directory with any intermediate output placed under `.svelte-kit/[adapter-name]`.
# Advanced routing
## Rest parameters
If the number of route segments is unknown, you can use rest syntax — for example you might implement GitHub's file viewer like so...
```bash
/[org]/[repo]/tree/[branch]/[...file]
```
...in which case a request for `/sveltejs/kit/tree/main/documentation/docs/04-advanced-routing.md` would result in the following parameters being available to the page:
```js
// @noErrors
{
org: 'sveltejs',
repo: 'kit',
branch: 'main',
file: 'documentation/docs/04-advanced-routing.md'
}
```
> [!NOTE] `src/routes/a/[...rest]/z/+page.svelte` will match `/a/z` (i.e. there's no parameter at all) as well as `/a/b/z` and `/a/b/c/z` and so on. Make sure you check that the value of the rest parameter is valid, for example using a [matcher](#Matching).
### 404 pages
Rest parameters also allow you to render custom 404s. Given these routes...
```tree
src/routes/
├ marx-brothers/
│ ├ chico/
│ ├ harpo/
│ ├ groucho/
│ └ +error.svelte
└ +error.svelte
```
...the `marx-brothers/+error.svelte` file will _not_ be rendered if you visit `/marx-brothers/karl`, because no route was matched. If you want to render the nested error page, you should create a route that matches any `/marx-brothers/*` request, and return a 404 from it:
```tree
src/routes/
├ marx-brothers/
+++| ├ [...path]/+++
│ ├ chico/
│ ├ harpo/
│ ├ groucho/
│ └ +error.svelte
└ +error.svelte
```
```js
/// file: src/routes/marx-brothers/[...path]/+page.js
import { error } from '@sveltejs/kit';
/** @type {import('./$types').PageLoad} */
export function load(event) {
error(404, 'Not Found');
}
```
> [!NOTE] If you don't handle 404 cases, they will appear in [`handleError`](hooks#Shared-hooks-handleError)
## Optional parameters
A route like `[lang]/home` contains a parameter named `lang` which is required. Sometimes it's beneficial to make these parameters optional, so that in this example both `home` and `en/home` point to the same page. You can do that by wrapping the parameter in another bracket pair: `[[lang]]/home`
Note that an optional route parameter cannot follow a rest parameter (`[...rest]/[[optional]]`), since parameters are matched 'greedily' and the optional parameter would always be unused.
## Matching
A route like `src/routes/fruits/[page]` would match `/fruits/apple`, but it would also match `/fruits/rocketship`. We don't want that. You can ensure that route parameters are well-formed by adding a _matcher_ — which takes the parameter string (`"apple"` or `"rocketship"`) and returns `true` if it is valid — to your [`params`](configuration#files) directory...
```js
/// file: src/params/fruit.js
/**
* @param {string} param
* @return {param is ('apple' | 'orange')}
* @satisfies {import('@sveltejs/kit').ParamMatcher}
*/
export function match(param) {
return param === 'apple' || param === 'orange';
}
```
...and augmenting your routes:
```
src/routes/fruits/[page+++=fruit+++]
```
If the pathname doesn't match, SvelteKit will try to match other routes (using the sort order specified below), before eventually returning a 404.
Each module in the `params` directory corresponds to a matcher, with the exception of `*.test.js` and `*.spec.js` files which may be used to unit test your matchers.
> [!NOTE] Matchers run both on the server and in the browser.
## Sorting
It's possible for multiple routes to match a given path. For example each of these routes would match `/foo-abc`:
```bash
src/routes/[...catchall]/+page.svelte
src/routes/[[a=x]]/+page.svelte
src/routes/[b]/+page.svelte
src/routes/foo-[c]/+page.svelte
src/routes/foo-abc/+page.svelte
```
SvelteKit needs to know which route is being requested. To do so, it sorts them according to the following rules...
- More specific routes are higher priority (e.g. a route with no parameters is more specific than a route with one dynamic parameter, and so on)
- Parameters with [matchers](#Matching) (`[name=type]`) are higher priority than those without (`[name]`)
- `[[optional]]` and `[...rest]` parameters are ignored unless they are the final part of the route, in which case they are treated with lowest priority. In other words `x/[[y]]/z` is treated equivalently to `x/z` for the purposes of sorting
- Ties are resolved alphabetically
...resulting in this ordering, meaning that `/foo-abc` will invoke `src/routes/foo-abc/+page.svelte`, and `/foo-def` will invoke `src/routes/foo-[c]/+page.svelte` rather than less specific routes:
```bash
src/routes/foo-abc/+page.svelte
src/routes/foo-[c]/+page.svelte
src/routes/[[a=x]]/+page.svelte
src/routes/[b]/+page.svelte
src/routes/[...catchall]/+page.svelte
```
## Encoding
Some characters can't be used on the filesystem — `/` on Linux and Mac, `\ / : * ? " < > |` on Windows. The `#` and `%` characters have special meaning in URLs, and the `[ ] ( )` characters have special meaning to SvelteKit, so these also can't be used directly as part of your route.
To use these characters in your routes, you can use hexadecimal escape sequences, which have the format `[x+nn]` where `nn` is a hexadecimal character code:
- `\` — `[x+5c]`
- `/` — `[x+2f]`
- `:` — `[x+3a]`
- `*` — `[x+2a]`
- `?` — `[x+3f]`
- `"` — `[x+22]`
- `<` — `[x+3c]`
- `>` — `[x+3e]`
- `|` — `[x+7c]`
- `#` — `[x+23]`
- `%` — `[x+25]`
- `[` — `[x+5b]`
- `]` — `[x+5d]`
- `(` — `[x+28]`
- `)` — `[x+29]`
For example, to create a `/smileys/:-)` route, you would create a `src/routes/smileys/[x+3a]-[x+29]/+page.svelte` file.
You can determine the hexadecimal code for a character with JavaScript:
```js
':'.charCodeAt(0).toString(16); // '3a', hence '[x+3a]'
```
You can also use Unicode escape sequences. Generally you won't need to as you can use the unencoded character directly, but if — for some reason — you can't have a filename with an emoji in it, for example, then you can use the escaped characters. In other words, these are equivalent:
```
src/routes/[u+d83e][u+dd2a]/+page.svelte
src/routes/🤪/+page.svelte
```
The format for a Unicode escape sequence is `[u+nnnn]` where `nnnn` is a valid value between `0000` and `10ffff`. (Unlike JavaScript string escaping, there's no need to use surrogate pairs to represent code points above `ffff`.) To learn more about Unicode encodings, consult [Programming with Unicode](https://unicodebook.readthedocs.io/unicode_encodings.html).
> [!NOTE] Since TypeScript [struggles](https://github.com/microsoft/TypeScript/issues/13399) with directories with a leading `.` character, you may find it useful to encode these characters when creating e.g. [`.well-known`](https://en.wikipedia.org/wiki/Well-known_URI) routes: `src/routes/[x+2e]well-known/...`
## Advanced layouts
By default, the _layout hierarchy_ mirrors the _route hierarchy_. In some cases, that might not be what you want.
### (group)
Perhaps you have some routes that are 'app' routes that should have one layout (e.g. `/dashboard` or `/item`), and others that are 'marketing' routes that should have a different layout (`/about` or `/testimonials`). We can group these routes with a directory whose name is wrapped in parentheses — unlike normal directories, `(app)` and `(marketing)` do not affect the URL pathname of the routes inside them:
```tree
src/routes/
+++│ (app)/+++
│ ├ dashboard/
│ ├ item/
│ └ +layout.svelte
+++│ (marketing)/+++
│ ├ about/
│ ├ testimonials/
│ └ +layout.svelte
├ admin/
└ +layout.svelte
```
You can also put a `+page` directly inside a `(group)`, for example if `/` should be an `(app)` or a `(marketing)` page.
### Breaking out of layouts
The root layout applies to every page of your app — if omitted, it defaults to `{@render children()}`. If you want some pages to have a different layout hierarchy than the rest, then you can put your entire app inside one or more groups _except_ the routes that should not inherit the common layouts.
In the example above, the `/admin` route does not inherit either the `(app)` or `(marketing)` layouts.
### +page@
Pages can break out of the current layout hierarchy on a route-by-route basis. Suppose we have an `/item/[id]/embed` route inside the `(app)` group from the previous example:
```tree
src/routes/
├ (app)/
│ ├ item/
│ │ ├ [id]/
│ │ │ ├ embed/
+++│ │ │ │ └ +page.svelte+++
│ │ │ └ +layout.svelte
│ │ └ +layout.svelte
│ └ +layout.svelte
└ +layout.svelte
```
Ordinarily, this would inherit the root layout, the `(app)` layout, the `item` layout and the `[id]` layout. We can reset to one of those layouts by appending `@` followed by the segment name — or, for the root layout, the empty string. In this example, we can choose from the following options:
- `+page@[id].svelte` - inherits from `src/routes/(app)/item/[id]/+layout.svelte`
- `+page@item.svelte` - inherits from `src/routes/(app)/item/+layout.svelte`
- `+page@(app).svelte` - inherits from `src/routes/(app)/+layout.svelte`
- `+page@.svelte` - inherits from `src/routes/+layout.svelte`
```tree
src/routes/
├ (app)/
│ ├ item/
│ │ ├ [id]/
│ │ │ ├ embed/
+++│ │ │ │ └ +page@(app).svelte+++
│ │ │ └ +layout.svelte
│ │ └ +layout.svelte
│ └ +layout.svelte
└ +layout.svelte
```
### +layout@
Like pages, layouts can _themselves_ break out of their parent layout hierarchy, using the same technique. For example, a `+layout@.svelte` component would reset the hierarchy for all its child routes.
```
src/routes/
├ (app)/
│ ├ item/
│ │ ├ [id]/
│ │ │ ├ embed/
│ │ │ │ └ +page.svelte // uses (app)/item/[id]/+layout.svelte
│ │ │ ├ +layout.svelte // inherits from (app)/item/+layout@.svelte
│ │ │ └ +page.svelte // uses (app)/item/+layout@.svelte
│ │ └ +layout@.svelte // inherits from root layout, skipping (app)/+layout.svelte
│ └ +layout.svelte
└ +layout.svelte
```
### When to use layout groups
Not all use cases are suited for layout grouping, nor should you feel compelled to use them. It might be that your use case would result in complex `(group)` nesting, or that you don't want to introduce a `(group)` for a single outlier. It's perfectly fine to use other means such as composition (reusable `load` functions or Svelte components) or if-statements to achieve what you want. The following example shows a layout that rewinds to the root layout and reuses components and functions that other layouts can also use:
```svelte
<!--- file: src/routes/nested/route/+layout@.svelte --->
<script>
import ReusableLayout from '$lib/ReusableLayout.svelte';
let { data, children } = $props();
</script>
<ReusableLayout {data}>
{@render children()}
</ReusableLayout>
```
```js
/// file: src/routes/nested/route/+layout.js
// @filename: ambient.d.ts
declare module "$lib/reusable-load-function" {
export function reusableLoad(event: import('@sveltejs/kit').LoadEvent): Promise<Record<string, any>>;
}
// @filename: index.js
// ---cut---
import { reusableLoad } from '$lib/reusable-load-function';
/** @type {import('./$types').PageLoad} */
export function load(event) {
// Add additional logic here, if needed
return reusableLoad(event);
}
```
## Further reading
- [Tutorial: Advanced Routing](/tutorial/kit/optional-params)
# Hooks
'Hooks' are app-wide functions you declare that SvelteKit will call in response to specific events, giving you fine-grained control over the framework's behaviour.
There are three hooks files, all optional:
- `src/hooks.server.js` — your app's server hooks
- `src/hooks.client.js` — your app's client hooks
- `src/hooks.js` — your app's hooks that run on both the client and server
Code in these modules will run when the application starts up, making them useful for initializing database clients and so on.
> [!NOTE] You can configure the location of these files with [`config.kit.files.hooks`](configuration#files).
## Server hooks
The following hooks can be added to `src/hooks.server.js`:
### handle
This function runs every time the SvelteKit server receives a [request](web-standards#Fetch-APIs-Request) — whether that happens while the app is running, or during [prerendering](page-options#prerender) — and determines the [response](web-standards#Fetch-APIs-Response). It receives an `event` object representing the request and a function called `resolve`, which renders the route and generates a `Response`. This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
```js
/// file: src/hooks.server.js
/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
if (event.url.pathname.startsWith('/custom')) {
return new Response('custom response');
}
const response = await resolve(event);
return response;
}
```
> [!NOTE] Requests for static assets — which includes pages that were already prerendered — are _not_ handled by SvelteKit.
If unimplemented, defaults to `({ event, resolve }) => resolve(event)`.
During prerendering, SvelteKit crawls your pages for links and renders each route it finds. Rendering the route invokes the `handle` function (and all other route dependencies, like `load`). If you need to exclude some code from running during this phase, check that the app is not [`building`]($app-environment#building) beforehand.
### locals
To add custom data to the request, which is passed to handlers in `+server.js` and server `load` functions, populate the `event.locals` object, as shown below.
```js
/// file: src/hooks.server.js
// @filename: ambient.d.ts
type User = {
name: string;
}
declare namespace App {
interface Locals {
user: User;
}
}
const getUserInformation: (cookie: string | void) => Promise<User>;
// @filename: index.js
// ---cut---
/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
event.locals.user = await getUserInformation(event.cookies.get('sessionid'));
const response = await resolve(event);
// Note that modifying response headers isn't always safe.
// Response objects can have immutable headers
// (e.g. Response.redirect() returned from an endpoint).
// Modifying immutable headers throws a TypeError.
// In that case, clone the response or avoid creating a
// response object with immutable headers.
response.headers.set('x-custom-header', 'potato');
return response;
}
```
You can define multiple `handle` functions and execute them with [the `sequence` helper function](@sveltejs-kit-hooks).
`resolve` also supports a second, optional parameter that gives you more control over how the response will be rendered. That parameter is an object that can have the following fields:
- `transformPageChunk(opts: { html: string, done: boolean }): MaybePromise<string | undefined>` — applies custom transforms to HTML. If `done` is true, it's the final chunk. Chunks are not guaranteed to be well-formed HTML (they could include an element's opening tag but not its closing tag, for example) but they will always be split at sensible boundaries such as `%sveltekit.head%` or layout/page components.
- `filterSerializedResponseHeaders(name: string, value: string): boolean` — determines which headers should be included in serialized responses when a `load` function loads a resource with `fetch`. By default, none will be included.
- `preload(input: { type: 'js' | 'css' | 'font' | 'asset', path: string }): boolean` — determines what files should be added to the `<head>` tag to preload it. The method is called with each file that was found at build time while constructing the code chunks — so if you for example have `import './styles.css` in your `+page.svelte`, `preload` will be called with the resolved path to that CSS file when visiting that page. Note that in dev mode `preload` is _not_ called, since it depends on analysis that happens at build time. Preloading can improve performance by downloading assets sooner, but it can also hurt if too much is downloaded unnecessarily. By default, `js` and `css` files will be preloaded. `asset` files are not preloaded at all currently, but we may add this later after evaluating feedback.
```js
/// file: src/hooks.server.js
/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
const response = await resolve(event, {
transformPageChunk: ({ html }) => html.replace('old', 'new'),
filterSerializedResponseHeaders: (name) => name.startsWith('x-'),
preload: ({ type, path }) => type === 'js' || path.includes('/important/')
});
return response;
}
```
Note that `resolve(...)` will never throw an error, it will always return a `Promise<Response>` with the appropriate status code. If an error is thrown elsewhere during `handle`, it is treated as fatal, and SvelteKit will respond with a JSON representation of the error or a fallback error page — which can be customised via `src/error.html` — depending on the `Accept` header. You can read more about error handling [here](errors).
### handleFetch
This function allows you to modify (or replace) a `fetch` request that happens inside a `load` or `action` function that runs on the server (or during pre-rendering).
For example, your `load` function might make a request to a public URL like `https://api.yourapp.com` when the user performs a client-side navigation to the respective page, but during SSR it might make sense to hit the API directly (bypassing whatever proxies and load balancers sit between it and the public internet).
```js
/// file: src/hooks.server.js
/** @type {import('@sveltejs/kit').HandleFetch} */
export async function handleFetch({ request, fetch }) {
if (request.url.startsWith('https://api.yourapp.com/')) {
// clone the original request, but change the URL
request = new Request(
request.url.replace('https://api.yourapp.com/', 'http://localhost:9999/'),
request
);
}
return fetch(request);
}
```
**Credentials**
For same-origin requests, SvelteKit's `fetch` implementation will forward `cookie` and `authorization` headers unless the `credentials` option is set to `"omit"`.
For cross-origin requests, `cookie` will be included if the request URL belongs to a subdomain of the app — for example if your app is on `my-domain.com`, and your API is on `api.my-domain.com`, cookies will be included in the request.
If your app and your API are on sibling subdomains — `www.my-domain.com` and `api.my-domain.com` for example — then a cookie belonging to a common parent domain like `my-domain.com` will _not_ be included, because SvelteKit has no way to know which domain the cookie belongs to. In these cases you will need to manually include the cookie using `handleFetch`:
```js
/// file: src/hooks.server.js
// @errors: 2345
/** @type {import('@sveltejs/kit').HandleFetch} */
export async function handleFetch({ event, request, fetch }) {
if (request.url.startsWith('https://api.my-domain.com/')) {
request.headers.set('cookie', event.request.headers.get('cookie'));
}
return fetch(request);
}
```
## Shared hooks
The following can be added to `src/hooks.server.js` _and_ `src/hooks.client.js`:
### handleError
If an [unexpected error](errors#Unexpected-errors) is thrown during loading or rendering, this function will be called with the `error`, `event`, `status` code and `message`. This allows for two things:
- you can log the error
- you can generate a custom representation of the error that is safe to show to users, omitting sensitive details like messages and stack traces. The returned value, which defaults to `{ message }`, becomes the value of `$page.error`.
For errors thrown from your code (or library code called by your code) the status will be 500 and the message will be "Internal Error". While `error.message` may contain sensitive information that should not be exposed to users, `message` is safe (albeit meaningless to the average user).
To add more information to the `$page.error` object in a type-safe way, you can customize the expected shape by declaring an `App.Error` interface (which must include `message: string`, to guarantee sensible fallback behavior). This allows you to — for example — append a tracking ID for users to quote in correspondence with your technical support staff:
```ts
/// file: src/app.d.ts
declare global {
namespace App {
interface Error {
message: string;
errorId: string;
}
}
}
export {};
```
```js
/// file: src/hooks.server.js
// @errors: 2322 2353
// @filename: ambient.d.ts
declare module '@sentry/sveltekit' {
export const init: (opts: any) => void;
export const captureException: (error: any, opts: any) => void;
}
// @filename: index.js
// ---cut---
import * as Sentry from '@sentry/sveltekit';
Sentry.init({/*...*/})
/** @type {import('@sveltejs/kit').HandleServerError} */
export async function handleError({ error, event, status, message }) {
const errorId = crypto.randomUUID();
// example integration with https://sentry.io/
Sentry.captureException(error, {
extra: { event, errorId, status }
});
return {
message: 'Whoops!',
errorId
};
}
```
```js
/// file: src/hooks.client.js
// @errors: 2322 2353
// @filename: ambient.d.ts
declare module '@sentry/sveltekit' {
export const init: (opts: any) => void;
export const captureException: (error: any, opts: any) => void;
}
// @filename: index.js
// ---cut---
import * as Sentry from '@sentry/sveltekit';
Sentry.init({/*...*/})
/** @type {import('@sveltejs/kit').HandleClientError} */
export async function handleError({ error, event, status, message }) {
const errorId = crypto.randomUUID();
// example integration with https://sentry.io/
Sentry.captureException(error, {
extra: { event, errorId, status }
});
return {
message: 'Whoops!',
errorId
};
}
```
> [!NOTE] In `src/hooks.client.js`, the type of `handleError` is `HandleClientError` instead of `HandleServerError`, and `event` is a `NavigationEvent` rather than a `RequestEvent`.
This function is not called for _expected_ errors (those thrown with the [`error`](@sveltejs-kit#error) function imported from `@sveltejs/kit`).
During development, if an error occurs because of a syntax error in your Svelte code, the passed in error has a `frame` property appended highlighting the location of the error.
> [!NOTE] Make sure that `handleError` _never_ throws an error
### init
This function runs once, when the server is created or the app starts in the browser, and is a useful place to do asynchronous work such as initializing a database connection.
> [!NOTE] If your environment supports top-level await, the `init` function is really no different from writing your initialisation logic at the top level of the module, but some environments — most notably, Safari — don't.
```js
/// file: src/hooks.server.js
import * as db from '$lib/server/database';
/** @type {import('@sveltejs/kit').ServerInit} */
export async function init() {
await db.connect();
}
```
> [!NOTE]
> In the browser, asynchronous work in `init` will delay hydration, so be mindful of what you put in there.
## Universal hooks
The following can be added to `src/hooks.js`. Universal hooks run on both server and client (not to be confused with shared hooks, which are environment-specific).
### reroute
This function runs before `handle` and allows you to change how URLs are translated into routes. The returned pathname (which defaults to `url.pathname`) is used to select the route and its parameters.
For example, you might have a `src/routes/[[lang]]/about/+page.svelte` page, which should be accessible as `/en/about` or `/de/ueber-uns` or `/fr/a-propos`. You could implement this with `reroute`:
```js
/// file: src/hooks.js
// @errors: 2345
// @errors: 2304
/** @type {Record<string, string>} */
const translated = {
'/en/about': '/en/about',
'/de/ueber-uns': '/de/about',
'/fr/a-propos': '/fr/about',
};
/** @type {import('@sveltejs/kit').Reroute} */
export function reroute({ url }) {
if (url.pathname in translated) {
return translated[url.pathname];
}
}
```
The `lang` parameter will be correctly derived from the returned pathname.
Using `reroute` will _not_ change the contents of the browser's address bar, or the value of `event.url`.
### transport
This is a collection of _transporters_, which allow you to pass custom types — returned from `load` and form actions — across the server/client boundary. Each transporter contains an `encode` function, which encodes values on the server (or returns `false` for anything that isn't an instance of the type) and a corresponding `decode` function:
```js
/// file: src/hooks.js
import { Vector } from '$lib/math';
/** @type {import('@sveltejs/kit').Transport} */
export const transport = {
Vector: {
encode: (value) => value instanceof Vector && [value.x, value.y],
decode: ([x, y]) => new Vector(x, y)
}
};
```
## Further reading
- [Tutorial: Hooks](/tutorial/kit/handle)
# Errors
Errors are an inevitable fact of software development. SvelteKit handles errors differently depending on where they occur, what kind of errors they are, and the nature of the incoming request.
## Error objects
SvelteKit distinguishes between expected and unexpected errors, both of which are represented as simple `{ message: string }` objects by default.
You can add additional properties, like a `code` or a tracking `id`, as shown in the examples below. (When using TypeScript this requires you to redefine the `Error` type as described in [type safety](errors#Type-safety)).
## Expected errors
An _expected_ error is one created with the [`error`](@sveltejs-kit#error) helper imported from `@sveltejs/kit`:
```js
/// file: src/routes/blog/[slug]/+page.server.js
// @filename: ambient.d.ts
declare module '$lib/server/database' {
export function getPost(slug: string): Promise<{ title: string, content: string } | undefined>
}
// @filename: index.js
// ---cut---
import { error } from '@sveltejs/kit';
import * as db from '$lib/server/database';
/** @type {import('./$types').PageServerLoad} */
export async function load({ params }) {
const post = await db.getPost(params.slug);
if (!post) {
error(404, {
message: 'Not found'
});
}
return { post };
}
```
This throws an exception that SvelteKit catches, causing it to set the response status code to 404 and render an [`+error.svelte`](routing#error) component, where `page.error` is the object provided as the second argument to `error(...)`.
```svelte
<!--- file: src/routes/+error.svelte --->
<script>
import { page } from '$app/state';
</script>
<h1>{page.error.message}</h1>
```
> [!LEGACY]
> `$app/state` was added in SvelteKit 2.12. If you're using an earlier version or are using Svelte 4, use `$app/stores` instead.
You can add extra properties to the error object if needed...
```js
// @filename: ambient.d.ts
declare global {
namespace App {
interface Error {
message: string;
code: string;
}
}
}
export {}
// @filename: index.js
import { error } from '@sveltejs/kit';
// ---cut---
error(404, {
message: 'Not found',
+++code: 'NOT_FOUND'+++
});
```
...otherwise, for convenience, you can pass a string as the second argument:
```js
import { error } from '@sveltejs/kit';
// ---cut---
---error(404, { message: 'Not found' });---
+++error(404, 'Not found');+++
```
> [!NOTE] [In SvelteKit 1.x](migrating-to-sveltekit-2#redirect-and-error-are-no-longer-thrown-by-you) you had to `throw` the `error` yourself
## Unexpected errors
An _unexpected_ error is any other exception that occurs while handling a request. Since these can contain sensitive information, unexpected error messages and stack traces are not exposed to users.
By default, unexpected errors are printed to the console (or, in production, your server logs), while the error that is exposed to the user has a generic shape:
```json
{ "message": "Internal Error" }
```
Unexpected errors will go through the [`handleError`](hooks#Shared-hooks-handleError) hook, where you can add your own error handling — for example, sending errors to a reporting service, or returning a custom error object which becomes `$page.error`.
## Responses
If an error occurs inside `handle` or inside a [`+server.js`](routing#server) request handler, SvelteKit will respond with either a fallback error page or a JSON representation of the error object, depending on the request's `Accept` headers.
You can customise the fallback error page by adding a `src/error.html` file:
```html
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8" />
<title>%sveltekit.error.message%</title>
</head>
<body>
<h1>My custom error page</h1>
<p>Status: %sveltekit.status%</p>
<p>Message: %sveltekit.error.message%</p>
</body>
</html>
```
SvelteKit will replace `%sveltekit.status%` and `%sveltekit.error.message%` with their corresponding values.
If the error instead occurs inside a `load` function while rendering a page, SvelteKit will render the [`+error.svelte`](routing#error) component nearest to where the error occurred. If the error occurs inside a `load` function in `+layout(.server).js`, the closest error boundary in the tree is an `+error.svelte` file _above_ that layout (not next to it).
The exception is when the error occurs inside the root `+layout.js` or `+layout.server.js`, since the root layout would ordinarily _contain_ the `+error.svelte` component. In this case, SvelteKit uses the fallback error page.
## Type safety
If you're using TypeScript and need to customize the shape of errors, you can do so by declaring an `App.Error` interface in your app (by convention, in `src/app.d.ts`, though it can live anywhere that TypeScript can 'see'):
```ts
/// file: src/app.d.ts
declare global {
namespace App {
interface Error {
+++ code: string;
id: string;+++
}
}
}
export {};
```
This interface always includes a `message: string` property.
## Further reading
- [Tutorial: Errors and redirects](/tutorial/kit/error-basics)
- [Tutorial: Hooks](/tutorial/kit/handle)
# Link options
In SvelteKit, `<a>` elements (rather than framework-specific `<Link>` components) are used to navigate between the routes of your app. If the user clicks on a link whose `href` is 'owned' by the app (as opposed to, say, a link to an external site) then SvelteKit will navigate to the new page by importing its code and then calling any `load` functions it needs to fetch data.
You can customise the behaviour of links with `data-sveltekit-*` attributes. These can be applied to the `<a>` itself, or to a parent element.
These options also apply to `<form>` elements with [`method="GET"`](form-actions#GET-vs-POST).
## data-sveltekit-preload-data
Before the browser registers that the user has clicked on a link, we can detect that they've hovered the mouse over it (on desktop) or that a `touchstart` or `mousedown` event was triggered. In both cases, we can make an educated guess that a `click` event is coming.
SvelteKit can use this information to get a head start on importing the code and fetching the page's data, which can give us an extra couple of hundred milliseconds — the difference between a user interface that feels laggy and one that feels snappy.
We can control this behaviour with the `data-sveltekit-preload-data` attribute, which can have one of two values:
- `"hover"` means that preloading will start if the mouse comes to a rest over a link. On mobile, preloading begins on `touchstart`
- `"tap"` means that preloading will start as soon as a `touchstart` or `mousedown` event is registered
The default project template has a `data-sveltekit-preload-data="hover"` attribute applied to the `<body>` element in `src/app.html`, meaning that every link is preloaded on hover by default:
```html
<body data-sveltekit-preload-data="hover">
<div style="display: contents">%sveltekit.body%</div>
</body>
```
Sometimes, calling `load` when the user hovers over a link might be undesirable, either because it's likely to result in false positives (a click needn't follow a hover) or because data is updating very quickly and a delay could mean staleness.
In these cases, you can specify the `"tap"` value, which causes SvelteKit to call `load` only when the user taps or clicks on a link:
```html
<a data-sveltekit-preload-data="tap" href="/stonks">
Get current stonk values
</a>
```
> [!NOTE] You can also programmatically invoke `preloadData` from `$app/navigation`.
Data will never be preloaded if the user has chosen reduced data usage, meaning [`navigator.connection.saveData`](https://developer.mozilla.org/en-US/docs/Web/API/NetworkInformation/saveData) is `true`.
## data-sveltekit-preload-code
Even in cases where you don't want to preload _data_ for a link, it can be beneficial to preload the _code_. The `data-sveltekit-preload-code` attribute works similarly to `data-sveltekit-preload-data`, except that it can take one of four values, in decreasing 'eagerness':
- `"eager"` means that links will be preloaded straight away
- `"viewport"` means that links will be preloaded once they enter the viewport
- `"hover"` - as above, except that only code is preloaded
- `"tap"` - as above, except that only code is preloaded
Note that `viewport` and `eager` only apply to links that are present in the DOM immediately following navigation — if a link is added later (in an `{#if ...}` block, for example) it will not be preloaded until triggered by `hover` or `tap`. This is to avoid performance pitfalls resulting from aggressively observing the DOM for changes.
> [!NOTE] Since preloading code is a prerequisite for preloading data, this attribute will only have an effect if it specifies a more eager value than any `data-sveltekit-preload-data` attribute that is present.
As with `data-sveltekit-preload-data`, this attribute will be ignored if the user has chosen reduced data usage.
## data-sveltekit-reload
Occasionally, we need to tell SvelteKit not to handle a link, but allow the browser to handle it. Adding a `data-sveltekit-reload` attribute to a link...
```html
<a data-sveltekit-reload href="/path">Path</a>
```
...will cause a full-page navigation when the link is clicked.
Links with a `rel="external"` attribute will receive the same treatment. In addition, they will be ignored during [prerendering](page-options#prerender).
## data-sveltekit-replacestate
Sometimes you don't want navigation to create a new entry in the browser's session history. Adding a `data-sveltekit-replacestate` attribute to a link...
```html
<a data-sveltekit-replacestate href="/path">Path</a>
```
...will replace the current `history` entry rather than creating a new one with `pushState` when the link is clicked.
## data-sveltekit-keepfocus
Sometimes you don't want [focus to be reset](accessibility#Focus-management) after navigation. For example, maybe you have a search form that submits as the user is typing, and you want to keep focus on the text input. Adding a `data-sveltekit-keepfocus` attribute to it...
```html
<form data-sveltekit-keepfocus>
<input type="text" name="query">
</form>
```
...will cause the currently focused element to retain focus after navigation. In general, avoid using this attribute on links, since the focused element would be the `<a>` tag (and not a previously focused element) and screen reader and other assistive technology users often expect focus to be moved after a navigation. You should also only use this attribute on elements that still exist after navigation. If the element no longer exists, the user's focus will be lost, making for a confusing experience for assistive technology users.
## data-sveltekit-noscroll
When navigating to internal links, SvelteKit mirrors the browser's default navigation behaviour: it will change the scroll position to 0,0 so that the user is at the very top left of the page (unless the link includes a `#hash`, in which case it will scroll to the element with a matching ID).
In certain cases, you may wish to disable this behaviour. Adding a `data-sveltekit-noscroll` attribute to a link...
```html
<a href="path" data-sveltekit-noscroll>Path</a>
```
...will prevent scrolling after the link is clicked.
## Disabling options
To disable any of these options inside an element where they have been enabled, use the `"false"` value:
```html
<div data-sveltekit-preload-data>
<!-- these links will be preloaded -->
<a href="/a">a</a>
<a href="/b">b</a>
<a href="/c">c</a>
<div data-sveltekit-preload-data="false">
<!-- these links will NOT be preloaded -->
<a href="/d">d</a>
<a href="/e">e</a>
<a href="/f">f</a>
</div>
</div>
```
To apply an attribute to an element conditionally, do this:
```svelte
<div data-sveltekit-preload-data={condition ? 'hover' : false}>
```
# Service workers
Service workers act as proxy servers that handle network requests inside your app. This makes it possible to make your app work offline, but even if you don't need offline support (or can't realistically implement it because of the type of app you're building), it's often worth using service workers to speed up navigation by precaching your built JS and CSS.
In SvelteKit, if you have a `src/service-worker.js` file (or `src/service-worker/index.js`) it will be bundled and automatically registered. You can change the [location of your service worker](configuration#files) if you need to.
You can [disable automatic registration](configuration#serviceWorker) if you need to register the service worker with your own logic or use another solution. The default registration looks something like this:
```js
if ('serviceWorker' in navigator) {
addEventListener('load', function () {
navigator.serviceWorker.register('./path/to/service-worker.js');
});
}
```
## Inside the service worker
Inside the service worker you have access to the [`$service-worker` module]($service-worker), which provides you with the paths to all static assets, build files and prerendered pages. You're also provided with an app version string, which you can use for creating a unique cache name, and the deployment's `base` path. If your Vite config specifies `define` (used for global variable replacements), this will be applied to service workers as well as your server/client builds.
The following example caches the built app and any files in `static` eagerly, and caches all other requests as they happen. This would make each page work offline once visited.
```js
// @errors: 2339
/// <reference types="@sveltejs/kit" />
import { build, files, version } from '$service-worker';
// Create a unique cache name for this deployment
const CACHE = `cache-${version}`;
const ASSETS = [
...build, // the app itself
...files // everything in `static`
];
self.addEventListener('install', (event) => {
// Create a new cache and add all files to it
async function addFilesToCache() {
const cache = await caches.open(CACHE);
await cache.addAll(ASSETS);
}
event.waitUntil(addFilesToCache());
});
self.addEventListener('activate', (event) => {
// Remove previous cached data from disk
async function deleteOldCaches() {
for (const key of await caches.keys()) {
if (key !== CACHE) await caches.delete(key);
}
}
event.waitUntil(deleteOldCaches());
});
self.addEventListener('fetch', (event) => {
// ignore POST requests etc
if (event.request.method !== 'GET') return;
async function respond() {
const url = new URL(event.request.url);
const cache = await caches.open(CACHE);
// `build`/`files` can always be served from the cache
if (ASSETS.includes(url.pathname)) {
const response = await cache.match(url.pathname);
if (response) {
return response;
}
}
// for everything else, try the network first, but
// fall back to the cache if we're offline
try {
const response = await fetch(event.request);
// if we're offline, fetch can return a value that is not a Response
// instead of throwing - and we can't pass this non-Response to respondWith
if (!(response instanceof Response)) {
throw new Error('invalid response from fetch');
}
if (response.status === 200) {
cache.put(event.request, response.clone());
}
return response;
} catch (err) {
const response = await cache.match(event.request);
if (response) {
return response;
}
// if there's no cache, then just error out
// as there is nothing we can do to respond to this request
throw err;
}
}
event.respondWith(respond());
});
```
> [!NOTE] Be careful when caching! In some cases, stale data might be worse than data that's unavailable while offline. Since browsers will empty caches if they get too full, you should also be careful about caching large assets like video files.
## During development
The service worker is bundled for production, but not during development. For that reason, only browsers that support [modules in service workers](https://web.dev/es-modules-in-sw) will be able to use them at dev time. If you are manually registering your service worker, you will need to pass the `{ type: 'module' }` option in development:
```js
import { dev } from '$app/environment';
navigator.serviceWorker.register('/service-worker.js', {
type: dev ? 'module' : 'classic'
});
```
> [!NOTE] `build` and `prerendered` are empty arrays during development
## Type safety
Setting up proper types for service workers requires some manual setup. Inside your `service-worker.js`, add the following to the top of your file:
```original-js
/// <reference types="@sveltejs/kit" />
/// <reference no-default-lib="true"/>
/// <reference lib="esnext" />
/// <reference lib="webworker" />
const sw = /** @type {ServiceWorkerGlobalScope} */ (/** @type {unknown} */ (self));
```
```generated-ts
/// <reference types="@sveltejs/kit" />
/// <reference no-default-lib="true"/>
/// <reference lib="esnext" />
/// <reference lib="webworker" />
const sw = self as unknown as ServiceWorkerGlobalScope;
```
This disables access to DOM typings like `HTMLElement` which are not available inside a service worker and instantiates the correct globals. The reassignment of `self` to `sw` allows you to type cast it in the process (there are a couple of ways to do this, but this is the easiest that requires no additional files). Use `sw` instead of `self` in the rest of the file. The reference to the SvelteKit types ensures that the `$service-worker` import has proper type definitions. If you import `$env/static/public` you either have to `// @ts-ignore` the import or add `/// <reference types="../.svelte-kit/ambient.d.ts" />` to the reference types.
## Other solutions
SvelteKit's service worker implementation is deliberately low-level. If you need a more full-fledged but also more opinionated solution, we recommend looking at solutions like [Vite PWA plugin](https://vite-pwa-org.netlify.app/frameworks/sveltekit.html), which uses [Workbox](https://web.dev/learn/pwa/workbox). For more general information on service workers, we recommend [the MDN web docs](https://developer.mozilla.org/en-US/docs/Web/API/Service_Worker_API/Using_Service_Workers).
# Server-only modules
Like a good friend, SvelteKit keeps your secrets. When writing your backend and frontend in the same repository, it can be easy to accidentally import sensitive data into your front-end code (environment variables containing API keys, for example). SvelteKit provides a way to prevent this entirely: server-only modules.
## Private environment variables
The [`$env/static/private`]($env-static-private) and [`$env/dynamic/private`]($env-dynamic-private) modules can only be imported into modules that only run on the server, such as [`hooks.server.js`](hooks#Server-hooks) or [`+page.server.js`](routing#page-page.server.js).
## Server-only utilities
The [`$app/server`]($app-server) module, which contains a [`read`]($app-server#read) function for reading assets from the filesystem, can likewise only be imported by code that runs on the server.
## Your modules
You can make your own modules server-only in two ways:
- adding `.server` to the filename, e.g. `secrets.server.js`
- placing them in `$lib/server`, e.g. `$lib/server/secrets.js`
## How it works
Any time you have public-facing code that imports server-only code (whether directly or indirectly)...
```js
// @errors: 7005
/// file: $lib/server/secrets.js
export const atlantisCoordinates = [/* redacted */];
```
```js
// @errors: 2307 7006 7005
/// file: src/routes/utils.js
export { atlantisCoordinates } from '$lib/server/secrets.js';
export const add = (a, b) => a + b;
```
```html
/// file: src/routes/+page.svelte
<script>
import { add } from './utils.js';
</script>
```
...SvelteKit will error:
```
Cannot import $lib/server/secrets.js into public-facing code:
- src/routes/+page.svelte
- src/routes/utils.js
- $lib/server/secrets.js
```
Even though the public-facing code — `src/routes/+page.svelte` — only uses the `add` export and not the secret `atlantisCoordinates` export, the secret code could end up in JavaScript that the browser downloads, and so the import chain is considered unsafe.
This feature also works with dynamic imports, even interpolated ones like ``await import(`./${foo}.js`)``, with one small caveat: during development, if there are two or more dynamic imports between the public-facing code and the server-only module, the illegal import will not be detected the first time the code is loaded.
> [!NOTE] Unit testing frameworks like Vitest do not distinguish between server-only and public-facing code. For this reason, illegal import detection is disabled when running tests, as determined by `process.env.TEST === 'true'`.
## Further reading
- [Tutorial: Environment variables](/tutorial/kit/env-static-private)
# Snapshots
Ephemeral DOM state — like scroll positions on sidebars, the content of `<input>` elements and so on — is discarded when you navigate from one page to another.
For example, if the user fills out a form but navigates away and then back before submitting, or if the user refreshes the page, the values they filled in will be lost. In cases where it's valuable to preserve that input, you can take a _snapshot_ of DOM state, which can then be restored if the user navigates back.
To do this, export a `snapshot` object with `capture` and `restore` methods from a `+page.svelte` or `+layout.svelte`:
```svelte
<!--- file: +page.svelte --->
<script>
let comment = $state('');
/** @type {import('./$types').Snapshot<string>} */
export const snapshot = {
capture: () => comment,
restore: (value) => comment = value
};
</script>
<form method="POST">
<label for="comment">Comment</label>
<textarea id="comment" bind:value={comment} />
<button>Post comment</button>
</form>
```
When you navigate away from this page, the `capture` function is called immediately before the page updates, and the returned value is associated with the current entry in the browser's history stack. If you navigate back, the `restore` function is called with the stored value as soon as the page is updated.
The data must be serializable as JSON so that it can be persisted to `sessionStorage`. This allows the state to be restored when the page is reloaded, or when the user navigates back from a different site.
> [!NOTE] Avoid returning very large objects from `capture` — once captured, objects will be retained in memory for the duration of the session, and in extreme cases may be too large to persist to `sessionStorage`.
# Shallow routing
As you navigate around a SvelteKit app, you create _history entries_. Clicking the back and forward buttons traverses through this list of entries, re-running any `load` functions and replacing page components as necessary.
Sometimes, it's useful to create history entries _without_ navigating. For example, you might want to show a modal dialog that the user can dismiss by navigating back. This is particularly valuable on mobile devices, where swipe gestures are often more natural than interacting directly with the UI. In these cases, a modal that is _not_ associated with a history entry can be a source of frustration, as a user may swipe backwards in an attempt to dismiss it and find themselves on the wrong page.
SvelteKit makes this possible with the [`pushState`]($app-navigation#pushState) and [`replaceState`]($app-navigation#replaceState) functions, which allow you to associate state with a history entry without navigating. For example, to implement a history-driven modal:
```svelte
<!--- file: +page.svelte --->
<script>
import { pushState } from '$app/navigation';
import { page } from '$app/state';
import Modal from './Modal.svelte';
function showModal() {
pushState('', {
showModal: true
});
}
</script>
{#if page.state.showModal}
<Modal close={() => history.back()} />
{/if}
```
The modal can be dismissed by navigating back (unsetting `page.state.showModal`) or by interacting with it in a way that causes the `close` callback to run, which will navigate back programmatically.
## API
The first argument to `pushState` is the URL, relative to the current URL. To stay on the current URL, use `''`.
The second argument is the new page state, which can be accessed via the [page object]($app-state#page) as `page.state`. You can make page state type-safe by declaring an [`App.PageState`](types#PageState) interface (usually in `src/app.d.ts`).
To set page state without creating a new history entry, use `replaceState` instead of `pushState`.
> [!LEGACY]
> `page.state` from `$app/state` was added in SvelteKit 2.12. If you're using an earlier version or are using Svelte 4, use `$page.state` from `$app/stores` instead.
## Loading data for a route
When shallow routing, you may want to render another `+page.svelte` inside the current page. For example, clicking on a photo thumbnail could pop up the detail view without navigating to the photo page.
For this to work, you need to load the data that the `+page.svelte` expects. A convenient way to do this is to use [`preloadData`]($app-navigation#preloadData) inside the `click` handler of an `<a>` element. If the element (or a parent) uses [`data-sveltekit-preload-data`](link-options#data-sveltekit-preload-data), the data will have already been requested, and `preloadData` will reuse that request.
```svelte
<!--- file: src/routes/photos/+page.svelte --->
<script>
import { preloadData, pushState, goto } from '$app/navigation';
import { page } from '$app/state';
import Modal from './Modal.svelte';
import PhotoPage from './[id]/+page.svelte';
let { data } = $props();
</script>
{#each data.thumbnails as thumbnail}
<a
href="/photos/{thumbnail.id}"
onclick={async (e) => {
if (innerWidth < 640 // bail if the screen is too small
|| e.shiftKey // or the link is opened in a new window
|| e.metaKey || e.ctrlKey // or a new tab (mac: metaKey, win/linux: ctrlKey)
// should also consider clicking with a mouse scroll wheel
) return;
// prevent navigation
e.preventDefault();
const { href } = e.currentTarget;
// run `load` functions (or rather, get the result of the `load` functions
// that are already running because of `data-sveltekit-preload-data`)
const result = await preloadData(href);
if (result.type === 'loaded' && result.status === 200) {
pushState(href, { selected: result.data });
} else {
// something bad happened! try navigating
goto(href);
}
}}
>
<img alt={thumbnail.alt} src={thumbnail.src} />
</a>
{/each}
{#if page.state.selected}
<Modal onclose={() => history.back()}>
<!-- pass page data to the +page.svelte component,
just like SvelteKit would on navigation -->
<PhotoPage data={page.state.selected} />
</Modal>
{/if}
```
## Caveats
During server-side rendering, `page.state` is always an empty object. The same is true for the first page the user lands on — if the user reloads the page (or returns from another document), state will _not_ be applied until they navigate.
Shallow routing is a feature that requires JavaScript to work. Be mindful when using it and try to think of sensible fallback behavior in case JavaScript isn't available.
# Packaging
You can use SvelteKit to build apps as well as component libraries, using the `@sveltejs/package` package (`npx sv create` has an option to set this up for you).
When you're creating an app, the contents of `src/routes` is the public-facing stuff; [`src/lib`]($lib) contains your app's internal library.
A component library has the exact same structure as a SvelteKit app, except that `src/lib` is the public-facing bit, and your root `package.json` is used to publish the package. `src/routes` might be a documentation or demo site that accompanies the library, or it might just be a sandbox you use during development.
Running the `svelte-package` command from `@sveltejs/package` will take the contents of `src/lib` and generate a `dist` directory (which can be [configured](#Options)) containing the following:
- All the files in `src/lib`. Svelte components will be preprocessed, TypeScript files will be transpiled to JavaScript.
- Type definitions (`d.ts` files) which are generated for Svelte, JavaScript and TypeScript files. You need to install `typescript >= 4.0.0` for this. Type definitions are placed next to their implementation, hand-written `d.ts` files are copied over as is. You can [disable generation](#Options), but we strongly recommend against it — people using your library might use TypeScript, for which they require these type definition files.
> [!NOTE] `@sveltejs/package` version 1 generated a `package.json`. This is no longer the case and it will now use the `package.json` from your project and validate that it is correct instead. If you're still on version 1, see [this PR](https://github.com/sveltejs/kit/pull/8922) for migration instructions.
## Anatomy of a package.json
Since you're now building a library for public use, the contents of your `package.json` will become more important. Through it, you configure the entry points of your package, which files are published to npm, and which dependencies your library has. Let's go through the most important fields one by one.
### name
This is the name of your package. It will be available for others to install using that name, and visible on `https://npmjs.com/package/<name>`.
```json
{
"name": "your-library"
}
```
Read more about it [here](https://docs.npmjs.com/cli/v9/configuring-npm/package-json#name).
### license
Every package should have a license field so people know how they are allowed to use it. A very popular license which is also very permissive in terms of distribution and reuse without warranty is `MIT`.
```json
{
"license": "MIT"
}
```
Read more about it [here](https://docs.npmjs.com/cli/v9/configuring-npm/package-json#license). Note that you should also include a `LICENSE` file in your package.
### files
This tells npm which files it will pack up and upload to npm. It should contain your output folder (`dist` by default). Your `package.json` and `README` and `LICENSE` will always be included, so you don't need to specify them.
```json
{
"files": ["dist"]
}
```
To exclude unnecessary files (such as unit tests, or modules that are only imported from `src/routes` etc) you can add them to an `.npmignore` file. This will result in smaller packages that are faster to install.
Read more about it [here](https://docs.npmjs.com/cli/v9/configuring-npm/package-json#files).
### exports
The `"exports"` field contains the package's entry points. If you set up a new library project through `npx sv create`, it's set to a single export, the package root:
```json
{
"exports": {
".": {
"types": "./dist/index.d.ts",
"svelte": "./dist/index.js"
}
}
}
```
This tells bundlers and tooling that your package only has one entry point, the root, and everything should be imported through that, like this:
```js
// @errors: 2307
import { Something } from 'your-library';
```
The `types` and `svelte` keys are [export conditions](https://nodejs.org/api/packages.html#conditional-exports). They tell tooling what file to import when they look up the `your-library` import:
- TypeScript sees the `types` condition and looks up the type definition file. If you don't publish type definitions, omit this condition.
- Svelte-aware tooling sees the `svelte` condition and knows this is a Svelte component library. If you publish a library that does not export any Svelte components and that could also work in non-Svelte projects (for example a Svelte store library), you can replace this condition with `default`.
> [!NOTE] Previous versions of `@sveltejs/package` also added a `package.json` export. This is no longer part of the template because all tooling can now deal with a `package.json` not being explicitly exported.
You can adjust `exports` to your liking and provide more entry points. For example, if instead of a `src/lib/index.js` file that re-exported components you wanted to expose a `src/lib/Foo.svelte` component directly, you could create the following export map...
```json
{
"exports": {
"./Foo.svelte": {
"types": "./dist/Foo.svelte.d.ts",
"svelte": "./dist/Foo.svelte"
}
}
}
```
...and a consumer of your library could import the component like so:
```js
// @filename: ambient.d.ts
declare module 'your-library/Foo.svelte';
// @filename: index.js
// ---cut---
import Foo from 'your-library/Foo.svelte';
```
> [!NOTE] Beware that doing this will need additional care if you provide type definitions. Read more about the caveat [here](#TypeScript)
In general, each key of the exports map is the path the user will have to use to import something from your package, and the value is the path to the file that will be imported or a map of export conditions which in turn contains these file paths.
Read more about `exports` [here](https://nodejs.org/docs/latest-v18.x/api/packages.html#package-entry-points).
### svelte
This is a legacy field that enabled tooling to recognise Svelte component libraries. It's no longer necessary when using the `svelte` [export condition](#Anatomy-of-a-package.json-exports), but for backwards compatibility with outdated tooling that doesn't yet know about export conditions it's good to keep it around. It should point towards your root entry point.
```json
{
"svelte": "./dist/index.js"
}
```
### sideEffects
The `sideEffects` field in `package.json` is used by bundlers to determine if a module may contain code that has side effects. A module is considered to have side effects if it makes changes that are observable from other scripts outside the module when it's imported. For example, side effects include modifying global variables or the prototype of built-in JavaScript objects. Because a side effect could potentially affect the behavior of other parts of the application, these files/modules will be included in the final bundle regardless of whether their exports are used in the application. It is a best practice to avoid side effects in your code.
Setting the `sideEffects` field in `package.json` can help the bundler to be more aggressive in eliminating unused exports from the final bundle, a process known as tree-shaking. This results in smaller and more efficient bundles. Different bundlers handle `sideEffects` in various manners. While not necessary for Vite, we recommend that libraries state that all CSS files have side effects so that your library will be [compatible with webpack](https://webpack.js.org/guides/tree-shaking/#mark-the-file-as-side-effect-free). This is the configuration that comes with newly created projects:
```json
/// file: package.json
{
"sideEffects": ["**/*.css"]
}
```
> If the scripts in your library have side effects, ensure that you update the `sideEffects` field. All scripts are marked as side effect free by default in newly created projects. If a file with side effects is incorrectly marked as having no side effects, it can result in broken functionality.
If your package has files with side effects, you can specify them in an array:
```json
/// file: package.json
{
"sideEffects": [
"**/*.css",
"./dist/sideEffectfulFile.js"
]
}
```
This will treat only the specified files as having side effects.
## TypeScript
You should ship type definitions for your library even if you don't use TypeScript yourself so that people who do get proper intellisense when using your library. `@sveltejs/package` makes the process of generating types mostly opaque to you. By default, when packaging your library, type definitions are auto-generated for JavaScript, TypeScript and Svelte files. All you need to ensure is that the `types` condition in the [exports](#Anatomy-of-a-package.json-exports) map points to the correct files. When initialising a library project through `npx sv create`, this is automatically setup for the root export.
If you have something else than a root export however — for example providing a `your-library/foo` import — you need to take additional care for providing type definitions. Unfortunately, TypeScript by default will _not_ resolve the `types` condition for an export like `{ "./foo": { "types": "./dist/foo.d.ts", ... }}`. Instead, it will search for a `foo.d.ts` relative to the root of your library (i.e. `your-library/foo.d.ts` instead of `your-library/dist/foo.d.ts`). To fix this, you have two options:
The first option is to require people using your library to set the `moduleResolution` option in their `tsconfig.json` (or `jsconfig.json`) to `bundler` (available since TypeScript 5, the best and recommended option in the future), `node16` or `nodenext`. This opts TypeScript into actually looking at the exports map and resolving the types correctly.
The second option is to (ab)use the `typesVersions` feature from TypeScript to wire up the types. This is a field inside `package.json` TypeScript uses to check for different type definitions depending on the TypeScript version, and also contains a path mapping feature for that. We leverage that path mapping feature to get what we want. For the mentioned `foo` export above, the corresponding `typesVersions` looks like this:
```json
{
"exports": {
"./foo": {
"types": "./dist/foo.d.ts",
"svelte": "./dist/foo.js"
}
},
"typesVersions": {
">4.0": {
"foo": ["./dist/foo.d.ts"]
}
}
}
```
`>4.0` tells TypeScript to check the inner map if the used TypeScript version is greater than 4 (which should in practice always be true). The inner map tells TypeScript that the typings for `your-library/foo` are found within `./dist/foo.d.ts`, which essentially replicates the `exports` condition. You also have `*` as a wildcard at your disposal to make many type definitions at once available without repeating yourself. Note that if you opt into `typesVersions` you have to declare all type imports through it, including the root import (which is defined as `"index.d.ts": [..]`).
You can read more about that feature [here](https://www.typescriptlang.org/docs/handbook/declaration-files/publishing.html#version-selection-with-typesversions).
## Best practices
You should avoid using SvelteKit-specific modules like `$app/environment` in your packages unless you intend for them to only be consumable by other SvelteKit projects. E.g. rather than using `import { browser } from '$app/environment'` you could use `import { BROWSER } from 'esm-env'` ([see esm-env docs](https://github.com/benmccann/esm-env)). You may also wish to pass in things like the current URL or a navigation action as a prop rather than relying directly on `$app/state`, `$app/navigation`, etc. Writing your app in this more generic fashion will also make it easier to setup tools for testing, UI demos and so on.
Ensure that you add [aliases](configuration#alias) via `svelte.config.js` (not `vite.config.js` or `tsconfig.json`), so that they are processed by `svelte-package`.
You should think carefully about whether or not the changes you make to your package are a bug fix, a new feature, or a breaking change, and update the package version accordingly. Note that if you remove any paths from `exports` or any `export` conditions inside them from your existing library, that should be regarded as a breaking change.
```json
{
"exports": {
".": {
"types": "./dist/index.d.ts",
// changing `svelte` to `default` is a breaking change:
--- "svelte": "./dist/index.js"---
+++ "default": "./dist/index.js"+++
},
// removing this is a breaking change:
--- "./foo": {
"types": "./dist/foo.d.ts",
"svelte": "./dist/foo.js",
"default": "./dist/foo.js"
},---
// adding this is ok:
+++ "./bar": {
"types": "./dist/bar.d.ts",
"svelte": "./dist/bar.js",
"default": "./dist/bar.js"
}+++
}
}
```
## Source maps
You can create so-called declaration maps (`d.ts.map` files) by setting `"declarationMap": true` in your `tsconfig.json`. This will allow editors such as VS Code to go to the original `.ts` or `.svelte` file when using features like _Go to Definition_. This means you also need to publish your source files alongside your dist folder in a way that the relative path inside the declaration files leads to a file on disk. Assuming that you have all your library code inside `src/lib` as suggested by Svelte's CLI, this is as simple as adding `src/lib` to `files` in your `package.json`:
```json
{
"files": [
"dist",
"!dist/**/*.test.*",
"!dist/**/*.spec.*",
+++"src/lib",
"!src/lib/**/*.test.*",
"!src/lib/**/*.spec.*"+++
]
}
```
## Options
`svelte-package` accepts the following options:
- `-w`/`--watch` — watch files in `src/lib` for changes and rebuild the package
- `-i`/`--input` — the input directory which contains all the files of the package. Defaults to `src/lib`
- `-o`/`--output` — the output directory where the processed files are written to. Your `package.json`'s `exports` should point to files inside there, and the `files` array should include that folder. Defaults to `dist`
- `-t`/`--types` — whether or not to create type definitions (`d.ts` files). We strongly recommend doing this as it fosters ecosystem library quality. Defaults to `true`
- `--tsconfig` - the path to a tsconfig or jsconfig. When not provided, searches for the next upper tsconfig/jsconfig in the workspace path.
## Publishing
To publish the generated package:
```sh
npm publish
```
## Caveats
All relative file imports need to be fully specified, adhering to Node's ESM algorithm. This means that for a file like `src/lib/something/index.js`, you must include the filename with the extension:
```js
// @errors: 2307
import { something } from './something+++/index.js+++';
```
If you are using TypeScript, you need to import `.ts` files the same way, but using a `.js` file ending, _not_ a `.ts` file ending. (This is a TypeScript design decision outside our control.) Setting `"moduleResolution": "NodeNext"` in your `tsconfig.json` or `jsconfig.json` will help you with this.
All files except Svelte files (preprocessed) and TypeScript files (transpiled to JavaScript) are copied across as-is.
# Auth
Auth refers to authentication and authorization, which are common needs when building a web application. Authentication means verifying that the user is who they say they are based on their provided credentials. Authorization means determining which actions they are allowed to take.
## Sessions vs tokens
After the user has provided their credentials such as a username and password, we want to allow them to use the application without needing to provide their credentials again for future requests. Users are commonly authenticated on subsequent requests with either a session identifier or signed token such as a JSON Web Token (JWT).
Session IDs are most commonly stored in a database. They can be immediately revoked, but require a database query to be made on each request.
In contrast, JWT generally are not checked against a datastore, which means they cannot be immediately revoked. The advantage of this method is improved latency and reduced load on your datastore.
## Integration points
Auth [cookies](@sveltejs-kit#Cookies) can be checked inside [server hooks](hooks#Server-hooks). If a user is found matching the provided credentials, the user information can be stored in [`locals`](hooks#Server-hooks-handle).
## Guides
[Lucia](https://lucia-auth.com/) is a good reference for session-based web app auth. It contains example code snippets and projects for implementing session-based auth within SvelteKit and other JS projects. You can add code which follows the Lucia guide to your project with `npx sv create` when creating a new project or `npx sv add lucia` for an existing project.
An auth system is tightly coupled to a web framework because most of the code lies in validating user input, handling errors, and directing users to the appropriate next page. As a result, many of the generic JS auth libraries include one or more web frameworks within them. For this reason, many users will find it preferrable to follow a SvelteKit-specific guide such as the examples found in [Lucia](https://lucia-auth.com/) rather than having multiple web frameworks inside their project.
# Performance
Out of the box, SvelteKit does a lot of work to make your applications as performant as possible:
- Code-splitting, so that only the code you need for the current page is loaded
- Asset preloading, so that 'waterfalls' (of files requesting other files) are prevented
- File hashing, so that your assets can be cached forever
- Request coalescing, so that data fetched from separate server `load` functions is grouped into a single HTTP request
- Parallel loading, so that separate universal `load` functions fetch data simultaneously
- Data inlining, so that requests made with `fetch` during server rendering can be replayed in the browser without issuing a new request
- Conservative invalidation, so that `load` functions are only re-run when necessary
- Prerendering (configurable on a per-route basis, if necessary) so that pages without dynamic data can be served instantaneously
- Link preloading, so that data and code requirements for a client-side navigation are eagerly anticipated
Nevertheless, we can't (yet) eliminate all sources of slowness. To eke out maximum performance, you should be mindful of the following tips.
## Diagnosing issues
Google's [PageSpeed Insights](https://pagespeed.web.dev/) and (for more advanced analysis) [WebPageTest](https://www.webpagetest.org/) are excellent ways to understand the performance characteristics of a site that is already deployed to the internet.
Your browser also includes useful developer tools for analysing your site, whether deployed or running locally:
* Chrome - [Lighthouse](https://developer.chrome.com/docs/lighthouse/overview#devtools), [Network](https://developer.chrome.com/docs/devtools/network), and [Performance](https://developer.chrome.com/docs/devtools/performance) devtools
* Edge - [Lighthouse](https://learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/lighthouse/lighthouse-tool), [Network](https://learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/network/), and [Performance](https://learn.microsoft.com/en-us/microsoft-edge/devtools-guide-chromium/evaluate-performance/) devtools
* Firefox - [Network](https://firefox-source-docs.mozilla.org/devtools-user/network_monitor/) and [Performance](https://hacks.mozilla.org/2022/03/performance-tool-in-firefox-devtools-reloaded/) devtools
* Safari - [enhancing the performance of your webpage](https://developer.apple.com/library/archive/documentation/NetworkingInternetWeb/Conceptual/Web_Inspector_Tutorial/EnhancingyourWebpagesPerformance/EnhancingyourWebpagesPerformance.html)
Note that your site running locally in `dev` mode will exhibit different behaviour than your production app, so you should do performance testing in [preview](building-your-app#Preview-your-app) mode after building.
### Instrumenting
If you see in the network tab of your browser that an API call is taking a long time and you'd like to understand why, you may consider instrumenting your backend with a tool like [OpenTelemetry](https://opentelemetry.io/) or [Server-Timing headers](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Server-Timing).
## Optimizing assets
### Images
Reducing the size of image files is often one of the most impactful changes you can make to a site's performance. Svelte provides the `@sveltejs/enhanced-img` package, detailed on the [images](images) page, for making this easier. Additionally, Lighthouse is useful for identifying the worst offenders.
### Videos
Video files can be very large, so extra care should be taken to ensure that they're optimized:
- Compress videos with tools such as [Handbrake](https://handbrake.fr/). Consider converting the videos to web-friendly formats such as `.webm` or `.mp4`.
- You can [lazy-load videos](https://web.dev/articles/lazy-loading-video) located below the fold with `preload="none"` (though note that this will slow down playback when the user _does_ initiate it).
- Strip the audio track out of muted videos using a tool like [FFmpeg](https://ffmpeg.org/).
### Fonts
SvelteKit automatically preloads critical `.js` and `.css` files when the user visits a page, but it does _not_ preload fonts by default, since this may cause unnecessary files (such as font weights that are referenced by your CSS but not actually used on the current page) to be downloaded. Having said that, preloading fonts correctly can make a big difference to how fast your site feels. In your [`handle`](hooks#Server-hooks-handle) hook, you can call `resolve` with a `preload` filter that includes your fonts.
You can reduce the size of font files by [subsetting](https://web.dev/learn/performance/optimize-web-fonts#subset_your_web_fonts) your fonts.
## Reducing code size
### Svelte version
We recommend running the latest version of Svelte. Svelte 5 is smaller and faster than Svelte 4, which is smaller and faster than Svelte 3.
### Packages
[`rollup-plugin-visualizer`](https://www.npmjs.com/package/rollup-plugin-visualizer) can be helpful for identifying which packages are contributing the most to the size of your site. You may also find opportunities to remove code by manually inspecting the build output (use `build: { minify: false }` in your [Vite config](https://vitejs.dev/config/build-options.html#build-minify) to make the output readable, but remember to undo that before deploying your app), or via the network tab of your browser's devtools.
### External scripts
Try to minimize the number of third-party scripts running in the browser. For example, instead of using JavaScript-based analytics consider using server-side implementations, such as those offered by many platforms with SvelteKit adapters including [Cloudflare](https://www.cloudflare.com/web-analytics/), [Netlify](https://docs.netlify.com/monitor-sites/site-analytics/), and [Vercel](https://vercel.com/docs/analytics).
To run third party scripts in a web worker (which avoids blocking the main thread), use [Partytown's SvelteKit integration](https://partytown.builder.io/sveltekit).
### Selective loading
Code imported with static `import` declarations will be automatically bundled with the rest of your page. If there is a piece of code you need only when some condition is met, use the dynamic `import(...)` form to selectively lazy-load the component.
## Navigation
### Preloading
You can speed up client-side navigations by eagerly preloading the necessary code and data, using [link options](link-options). This is configured by default on the `<body>` element when you create a new SvelteKit app.
### Non-essential data
For slow-loading data that isn't needed immediately, the object returned from your `load` function can contain promises rather than the data itself. For server `load` functions, this will cause the data to [stream](load#Streaming-with-promises) in after the navigation (or initial page load).
### Preventing waterfalls
One of the biggest performance killers is what is referred to as a _waterfall_, which is a series of requests that is made sequentially. This can happen on the server or in the browser.
- Asset waterfalls can occur in the browser when your HTML requests JS which requests CSS which requests a background image and web font. SvelteKit will largely solve this class of problems for you by adding [`modulepreload`](https://developer.mozilla.org/en-US/docs/Web/HTML/Attributes/rel/modulepreload) tags or headers, but you should view [the network tab in your devtools](#Diagnosing-issues) to check whether additional resources need to be preloaded. Pay special attention to this if you use web [fonts](#Optimizing-assets-Fonts) since they need to be handled manually.
- If a universal `load` function makes an API call to fetch the current user, then uses the details from that response to fetch a list of saved items, and then uses _that_ response to fetch the details for each item, the browser will end up making multiple sequential requests. This is deadly for performance, especially for users that are physically located far from your backend. Avoid this issue by using [server `load` functions](load#Universal-vs-server) where possible.
- Server `load` functions are also not immune to waterfalls (though they are much less costly since they rarely involve roundtrips with high latency). For example if you query a database to get the current user and then use that data to make a second query for a list of saved items, it will typically be more performant to issue a single query with a database join.
## Hosting
Your frontend should be located in the same data center as your backend to minimize latency. For sites with no central backend, many SvelteKit adapters support deploying to the _edge_, which means handling each user's requests from a nearby server. This can reduce load times significantly. Some adapters even support [configuring deployment on a per-route basis](page-options#config). You should also consider serving images from a CDN (which are typically edge networks) — the hosts for many SvelteKit adapters will do this automatically.
Ensure your host uses HTTP/2 or newer. Vite's code splitting creates numerous small files for improved cacheability, which results in excellent performance, but this does assume that your files can be loaded in parallel with HTTP/2.
## Further reading
For the most part, building a performant SvelteKit app is the same as building any performant web app. You should be able to apply information from general performance resources such as [Core Web Vitals](https://web.dev/explore/learn-core-web-vitals) to any web experience you build.
# Images
Images can have a big impact on your app's performance. For best results, you should optimize them by doing the following:
- generate optimal formats like `.avif` and `.webp`
- create different sizes for different screens
- ensure that assets can be cached effectively
Doing this manually is tedious. There are a variety of techniques you can use, depending on your needs and preferences.
## Vite's built-in handling
[Vite will automatically process imported assets](https://vitejs.dev/guide/assets.html) for improved performance. This includes assets referenced via the CSS `url()` function. Hashes will be added to the filenames so that they can be cached, and assets smaller than `assetsInlineLimit` will be inlined. Vite's asset handling is most often used for images, but is also useful for video, audio, etc.
```svelte
<script>
import logo from '$lib/assets/logo.png';
</script>
<img alt="The project logo" src={logo} />
```
## @sveltejs/enhanced-img
`@sveltejs/enhanced-img` is a plugin offered on top of Vite's built-in asset handling. It provides plug and play image processing that serves smaller file formats like `avif` or `webp`, automatically sets the intrinsic `width` and `height` of the image to avoid layout shift, creates images of multiple sizes for various devices, and strips EXIF data for privacy. It will work in any Vite-based project including, but not limited to, SvelteKit projects.
> [!NOTE] As a build plugin, `@sveltejs/enhanced-img` can only optimize files located on your machine during the build process. If you have an image located elsewhere (such as a path served from your database, CMS, or backend), please read about [loading images dynamically from a CDN](#Loading-images-dynamically-from-a-CDN).
>
> **WARNING**: The `@sveltejs/enhanced-img` package is experimental. It uses pre-1.0 versioning and may introduce breaking changes with every minor version release.
### Setup
Install:
```bash
npm install --save-dev @sveltejs/enhanced-img
```
Adjust `vite.config.js`:
```js
import { sveltekit } from '@sveltejs/kit/vite';
+++import { enhancedImages } from '@sveltejs/enhanced-img';+++
import { defineConfig } from 'vite';
export default defineConfig({
plugins: [
+++enhancedImages(),+++
sveltekit()
]
});
```
Building will take longer on the first build due to the computational expense of transforming images. However, the build output will be cached in `./node_modules/.cache/imagetools` so that subsequent builds will be fast.
### Basic usage
Use in your `.svelte` components by using `<enhanced:img>` rather than `<img>` and referencing the image file with a [Vite asset import](https://vitejs.dev/guide/assets.html#static-asset-handling) path:
```svelte
<enhanced:img src="./path/to/your/image.jpg" alt="An alt text" />
```
At build time, your `<enhanced:img>` tag will be replaced with an `<img>` wrapped by a `<picture>` providing multiple image types and sizes. It's only possible to downscale images without losing quality, which means that you should provide the highest resolution image that you need — smaller versions will be generated for the various device types that may request an image.
You should provide your image at 2x resolution for HiDPI displays (a.k.a. retina displays). `<enhanced:img>` will automatically take care of serving smaller versions to smaller devices.
If you wish to add styles to your `<enhanced:img>`, you should add a `class` and target that.
### Dynamically choosing an image
You can also manually import an image asset and pass it to an `<enhanced:img>`. This is useful when you have a collection of static images and would like to dynamically choose one or [iterate over them](https://github.com/sveltejs/kit/blob/0ab1733e394b6310895a1d3bf0f126ce34531170/sites/kit.svelte.dev/src/routes/home/Showcase.svelte). In this case you will need to update both the `import` statement and `<img>` element as shown below to indicate you'd like process them.
```svelte
<script>
import MyImage from './path/to/your/image.jpg?enhanced';
</script>
<enhanced:img src={MyImage} alt="some alt text" />
```
You can also use [Vite's `import.meta.glob`](https://vitejs.dev/guide/features.html#glob-import). Note that you will have to specify `enhanced` via a [custom query](https://vitejs.dev/guide/features.html#custom-queries):
```svelte
<script>
const imageModules = import.meta.glob(
'/path/to/assets/*.{avif,gif,heif,jpeg,jpg,png,tiff,webp,svg}',
{
eager: true,
query: {
enhanced: true
}
}
)
</script>
{#each Object.entries(imageModules) as [_path, module]}
<enhanced:img src={module.default} alt="some alt text" />
{/each}
```
### Intrinsic Dimensions
`width` and `height` are optional as they can be inferred from the source image and will be automatically added when the `<enhanced:img>` tag is preprocessed. With these attributes, the browser can reserve the correct amount of space, preventing [layout shift](https://web.dev/articles/cls). If you'd like to use a different `width` and `height` you can style the image with CSS. Because the preprocessor adds a `width` and `height` for you, if you'd like one of the dimensions to be automatically calculated then you will need to specify that:
```svelte
<style>
.hero-image img {
width: var(--size);
height: auto;
}
</style>
```
### `srcset` and `sizes`
If you have a large image, such as a hero image taking the width of the design, you should specify `sizes` so that smaller versions are requested on smaller devices. E.g. if you have a 1280px image you may want to specify something like:
```svelte
<enhanced:img src="./image.png" sizes="min(1280px, 100vw)"/>
```
If `sizes` is specified, `<enhanced:img>` will generate small images for smaller devices and populate the `srcset` attribute.
The smallest picture generated automatically will have a width of 540px. If you'd like smaller images or would otherwise like to specify custom widths, you can do that with the `w` query parameter:
```svelte
<enhanced:img
src="./image.png?w=1280;640;400"
sizes="(min-width:1920px) 1280px, (min-width:1080px) 640px, (min-width:768px) 400px"
/>
```
If `sizes` is not provided, then a HiDPI/Retina image and a standard resolution image will be generated. The image you provide should be 2x the resolution you wish to display so that the browser can display that image on devices with a high [device pixel ratio](https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio).
### Per-image transforms
By default, enhanced images will be transformed to more efficient formats. However, you may wish to apply other transforms such as a blur, quality, flatten, or rotate operation. You can run per-image transforms by appending a query string:
```svelte
<enhanced:img src="./path/to/your/image.jpg?blur=15" alt="An alt text" />
```
[See the imagetools repo for the full list of directives](https://github.com/JonasKruckenberg/imagetools/blob/main/docs/directives.md).
## Loading images dynamically from a CDN
In some cases, the images may not be accessible at build time — e.g. they may live inside a content management system or elsewhere.
Using a content delivery network (CDN) can allow you to optimize these images dynamically, and provides more flexibility with regards to sizes, but it may involve some setup overhead and usage costs. Depending on caching strategy, the browser may not be able to use a cached copy of the asset until a [304 response](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status/304) is received from the CDN. Building HTML to target CDNs allows using an `<img>` tag since the CDN can serve the appropriate format based on the `User-Agent` header, whereas build-time optimizations must produce `<picture>` tags with multiple sources. Finally, some CDNs may generate images lazily, which could have a negative performance impact for sites with low traffic and frequently changing images.
CDNs can generally be used without any need for a library. However, there are a number of libraries with Svelte support that make it easier. [`@unpic/svelte`](https://unpic.pics/img/svelte/) is a CDN-agnostic library with support for a large number of providers. You may also find that specific CDNs like [Cloudinary](https://svelte.cloudinary.dev/) have Svelte support. Finally, some content management systems (CMS) which support Svelte (such as [Contentful](https://www.contentful.com/sveltekit-starter-guide/), [Storyblok](https://github.com/storyblok/storyblok-svelte), and [Contentstack](https://www.contentstack.com/docs/developers/sample-apps/build-a-starter-website-with-sveltekit-and-contentstack)) have built-in support for image handling.
## Best practices
- For each image type, use the appropriate solution from those discussed above. You can mix and match all three solutions in one project. For example, you may use Vite's built-in handling to provide images for `<meta>` tags, display images on your homepage with `@sveltejs/enhanced-img`, and display user-submitted content with a dynamic approach.
- Consider serving all images via CDN regardless of the image optimization types you use. CDNs reduce latency by distributing copies of static assets globally.
- Your original images should have a good quality/resolution and should have 2x the width it will be displayed at to serve HiDPI devices. Image processing can size images down to save bandwidth when serving smaller screens, but it would be a waste of bandwidth to invent pixels to size images up.
- For images which are much larger than the width of a mobile device (roughly 400px), such as a hero image taking the width of the page design, specify `sizes` so that smaller images can be served on smaller devices.
- For important images, such as the [largest contentful paint (LCP)](https://web.dev/articles/lcp) image, set `fetchpriority="high"` and avoid `loading="lazy"` to prioritize loading as early as possible.
- Give the image a container or styling so that it is constrained and does not jump around while the page is loading affecting your [cumulative layout shift (CLS)](https://web.dev/articles/cls). `width` and `height` help the browser to reserve space while the image is still loading, so `@sveltejs/enhanced-img` will add a `width` and `height` for you.
- Always provide a good `alt` text. The Svelte compiler will warn you if you don't do this.
- Do not use `em` or `rem` in `sizes` and change the default size of these measures. When used in `sizes` or `@media` queries, `em` and `rem` are both defined to mean the user's default `font-size`. For a `sizes` declaration like `sizes="(min-width: 768px) min(100vw, 108rem), 64rem"`, the actual `em` or `rem` that controls how the image is laid out on the page can be different if changed by CSS. For example, do not do something like `html { font-size: 62.5%; }` as the slot reserved by the browser preloader will now end up being larger than the actual slot of the CSS object model once it has been created.
# Accessibility
SvelteKit strives to provide an accessible platform for your app by default. Svelte's [compile-time accessibility checks](../svelte/compiler-warnings) will also apply to any SvelteKit application you build.
Here's how SvelteKit's built-in accessibility features work and what you need to do to help these features to work as well as possible. Keep in mind that while SvelteKit provides an accessible foundation, you are still responsible for making sure your application code is accessible. If you're new to accessibility, see the ["further reading"](accessibility#Further-reading) section of this guide for additional resources.
We recognize that accessibility can be hard to get right. If you want to suggest improvements to how SvelteKit handles accessibility, please [open a GitHub issue](https://github.com/sveltejs/kit/issues).
## Route announcements
In traditional server-rendered applications, every navigation (e.g. clicking on an `<a>` tag) triggers a full page reload. When this happens, screen readers and other assistive technology will read out the new page's title so that users understand that the page has changed.
Since navigation between pages in SvelteKit happens without reloading the page (known as [client-side routing](glossary#Routing)), SvelteKit injects a [live region](https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/ARIA_Live_Regions) onto the page that will read out the new page name after each navigation. This determines the page name to announce by inspecting the `<title>` element.
Because of this behavior, every page in your app should have a unique, descriptive title. In SvelteKit, you can do this by placing a `<svelte:head>` element on each page:
```svelte
<!--- file: src/routes/+page.svelte --->
<svelte:head>
<title>Todo List</title>
</svelte:head>
```
This will allow screen readers and other assistive technology to identify the new page after a navigation occurs. Providing a descriptive title is also important for [SEO](seo#Manual-setup-title-and-meta).
## Focus management
In traditional server-rendered applications, every navigation will reset focus to the top of the page. This ensures that people browsing the web with a keyboard or screen reader will start interacting with the page from the beginning.
To simulate this behavior during client-side routing, SvelteKit focuses the `<body>` element after each navigation and [enhanced form submission](form-actions#Progressive-enhancement). There is one exception - if an element with the [`autofocus`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/autofocus) attribute is present, SvelteKit will focus that element instead. Make sure to [consider the implications for assistive technology](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/autofocus#accessibility_considerations) when using that attribute.
If you want to customize SvelteKit's focus management, you can use the `afterNavigate` hook:
```js
/// <reference types="@sveltejs/kit" />
// ---cut---
import { afterNavigate } from '$app/navigation';
afterNavigate(() => {
/** @type {HTMLElement | null} */
const to_focus = document.querySelector('.focus-me');
to_focus?.focus();
});
```
You can also programmatically navigate to a different page using the [`goto`]($app-navigation#goto) function. By default, this will have the same client-side routing behavior as clicking on a link. However, `goto` also accepts a `keepFocus` option that will preserve the currently-focused element instead of resetting focus. If you enable this option, make sure the currently-focused element still exists on the page after navigation. If the element no longer exists, the user's focus will be lost, making for a confusing experience for assistive technology users.
## The "lang" attribute
By default, SvelteKit's page template sets the default language of the document to English. If your content is not in English, you should update the `<html>` element in `src/app.html` to have the correct [`lang`](https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/lang#accessibility) attribute. This will ensure that any assistive technology reading the document uses the correct pronunciation. For example, if your content is in German, you should update `app.html` to the following:
```html
/// file: src/app.html
<html lang="de">
```
If your content is available in multiple languages, you should set the `lang` attribute based on the language of the current page. You can do this with SvelteKit's [handle hook](hooks#Server-hooks-handle):
```html
/// file: src/app.html
<html lang="%lang%">
```
```js
/// file: src/hooks.server.js
/**
* @param {import('@sveltejs/kit').RequestEvent} event
*/
function get_lang(event) {
return 'en';
}
// ---cut---
/** @type {import('@sveltejs/kit').Handle} */
export function handle({ event, resolve }) {
return resolve(event, {
transformPageChunk: ({ html }) => html.replace('%lang%', get_lang(event))
});
}
```
## Further reading
For the most part, building an accessible SvelteKit app is the same as building an accessible web app. You should be able to apply information from the following general accessibility resources to any web experience you build:
- [MDN Web Docs: Accessibility](https://developer.mozilla.org/en-US/docs/Learn/Accessibility)
- [The A11y Project](https://www.a11yproject.com/)
- [How to Meet WCAG (Quick Reference)](https://www.w3.org/WAI/WCAG21/quickref/)
# SEO
The most important aspect of SEO is to create high-quality content that is widely linked to from around the web. However, there are a few technical considerations for building sites that rank well.
## Out of the box
### SSR
While search engines have got better in recent years at indexing content that was rendered with client-side JavaScript, server-side rendered content is indexed more frequently and reliably. SvelteKit employs SSR by default, and while you can disable it in [`handle`](hooks#Server-hooks-handle), you should leave it on unless you have a good reason not to.
> [!NOTE] SvelteKit's rendering is highly configurable and you can implement [dynamic rendering](https://developers.google.com/search/docs/advanced/javascript/dynamic-rendering) if necessary. It's not generally recommended, since SSR has other benefits beyond SEO.
### Performance
Signals such as [Core Web Vitals](https://web.dev/vitals/#core-web-vitals) impact search engine ranking. Because Svelte and SvelteKit introduce minimal overhead, it's easier to build high performance sites. You can test your site's performance using Google's [PageSpeed Insights](https://pagespeed.web.dev/) or [Lighthouse](https://developers.google.com/web/tools/lighthouse). Read [the performance page](performance) for more details.
### Normalized URLs
SvelteKit redirects pathnames with trailing slashes to ones without (or vice versa depending on your [configuration](page-options#trailingSlash)), as duplicate URLs are bad for SEO.
## Manual setup
### <title> and <meta>
Every page should have well-written and unique `<title>` and `<meta name="description">` elements inside a [`<svelte:head>`](../svelte/svelte-head). Guidance on how to write descriptive titles and descriptions, along with other suggestions on making content understandable by search engines, can be found on Google's [Lighthouse SEO audits](https://web.dev/lighthouse-seo/) documentation.
> [!NOTE] A common pattern is to return SEO-related `data` from page [`load`](load) functions, then use it (as [`page.data`]($app-state)) in a `<svelte:head>` in your root [layout](routing#layout).
### Sitemaps
[Sitemaps](https://developers.google.com/search/docs/advanced/sitemaps/build-sitemap) help search engines prioritize pages within your site, particularly when you have a large amount of content. You can create a sitemap dynamically using an endpoint:
```js
/// file: src/routes/sitemap.xml/+server.js
export async function GET() {
return new Response(
`
<?xml version="1.0" encoding="UTF-8" ?>
<urlset
xmlns="https://www.sitemaps.org/schemas/sitemap/0.9"
xmlns:xhtml="https://www.w3.org/1999/xhtml"
xmlns:mobile="https://www.google.com/schemas/sitemap-mobile/1.0"
xmlns:news="https://www.google.com/schemas/sitemap-news/0.9"
xmlns:image="https://www.google.com/schemas/sitemap-image/1.1"
xmlns:video="https://www.google.com/schemas/sitemap-video/1.1"
>
<!-- <url> elements go here -->
</urlset>`.trim(),
{
headers: {
'Content-Type': 'application/xml'
}
}
);
}
```
### AMP
An unfortunate reality of modern web development is that it is sometimes necessary to create an [Accelerated Mobile Pages (AMP)](https://amp.dev/) version of your site. In SvelteKit this can be done by setting the [`inlineStyleThreshold`](configuration#inlineStyleThreshold) option...
```js
/// file: svelte.config.js
/** @type {import('@sveltejs/kit').Config} */
const config = {
kit: {
// since <link rel="stylesheet"> isn't
// allowed, inline all styles
inlineStyleThreshold: Infinity
}
};
export default config;
```
...disabling `csr` in your root `+layout.js`/`+layout.server.js`...
```js
/// file: src/routes/+layout.server.js
export const csr = false;
```
...adding `amp` to your `app.html`
```html
<html amp>
...
```
...and transforming the HTML using `transformPageChunk` along with `transform` imported from `@sveltejs/amp`:
```js
/// file: src/hooks.server.js
import * as amp from '@sveltejs/amp';
/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
let buffer = '';
return await resolve(event, {
transformPageChunk: ({ html, done }) => {
buffer += html;
if (done) return amp.transform(buffer);
}
});
}
```
To prevent shipping any unused CSS as a result of transforming the page to amp, we can use [`dropcss`](https://www.npmjs.com/package/dropcss):
```js
// @filename: ambient.d.ts
declare module 'dropcss';
// @filename: index.js
// ---cut---
/// file: src/hooks.server.js
// @errors: 2307
import * as amp from '@sveltejs/amp';
import dropcss from 'dropcss';
/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
let buffer = '';
return await resolve(event, {
transformPageChunk: ({ html, done }) => {
buffer += html;
if (done) {
let css = '';
const markup = amp
.transform(buffer)
.replace('⚡', 'amp') // dropcss can't handle this character
.replace(/<style amp-custom([^>]*?)>([^]+?)<\/style>/, (match, attributes, contents) => {
css = contents;
return `<style amp-custom${attributes}></style>`;
});
css = dropcss({ css, html: markup }).css;
return markup.replace('</style>', `${css}</style>`);
}
}
});
}
```
> [!NOTE] It's a good idea to use the `handle` hook to validate the transformed HTML using `amphtml-validator`, but only if you're prerendering pages since it's very slow.
# Frequently asked questions
## Other resources
Please see [the Svelte FAQ](../svelte/faq) and [`vite-plugin-svelte` FAQ](https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md) as well for the answers to questions deriving from those libraries.
## What can I make with SvelteKit?
SvelteKit can be used to create most kinds of applications. Out of the box, SvelteKit supports many features including:
- Dynamic page content with [load](load) functions and [API routes](routing#server).
- SEO-friendly dynamic content with [server-side rendering (SSR)](glossary#SSR).
- User-friendly progressively-enhanced interactive pages with SSR and [Form Actions](form-actions).
- Static pages with [prerendering](page-options#prerender).
SvelteKit can also be deployed to a wide spectrum of hosted architectures via [adapters](adapters). In cases where SSR is used (or server-side logic is added without prerendering), those functions will be adapted to the target backend. Some examples include:
- Self-hosted dynamic web applications with a [Node.js backend](adapter-node).
- Serverless web applications with backend loaders and APIs deployed as remote functions. See [zero-config deployments](adapter-auto) for popular deployment options.
- [Static pre-rendered sites](adapter-static) such as a blog or multi-page site hosted on a CDN or static host. Statically-generated sites are shipped without a backend.
- [Single-page Applications (SPAs)](single-page-apps) with client-side routing and rendering for API-driven dynamic content. SPAs are shipped without a backend and are not server-rendered. This option is commonly chosen when bundling SvelteKit with an app written in PHP, .Net, Java, C, Golang, Rust, etc.
- A mix of the above; some routes can be static, and some routes can use backend functions to fetch dynamic information. This can be configured with [page options](page-options) that includes the option to opt out of SSR.
In order to support SSR, a JS backend — such as Node.js or Deno-based server, serverless function, or edge function — is required.
It is also possible to write custom adapters or leverage community adapters to deploy SvelteKit to more platforms such as specialized server environments, browser extensions, or native applications. See [integrations](./integrations) for more examples and integrations.
## How do I include details from package.json in my application?
You cannot directly require JSON files, since SvelteKit expects [`svelte.config.js`](./configuration) to be an ES module. If you'd like to include your application's version number or other information from `package.json` in your application, you can load JSON like so:
```js
/// file: svelte.config.js
// @filename: index.js
/// <reference types="@types/node" />
import { URL } from 'node:url';
// ---cut---
import { readFileSync } from 'node:fs';
import { fileURLToPath } from 'node:url';
const path = fileURLToPath(new URL('package.json', import.meta.url));
const pkg = JSON.parse(readFileSync(path, 'utf8'));
```
## How do I fix the error I'm getting trying to include a package?
Most issues related to including a library are due to incorrect packaging. You can check if a library's packaging is compatible with Node.js by entering it into [the publint website](https://publint.dev/).
Here are a few things to keep in mind when checking if a library is packaged correctly:
- `exports` takes precedence over the other entry point fields such as `main` and `module`. Adding an `exports` field may not be backwards-compatible as it prevents deep imports.
- ESM files should end with `.mjs` unless `"type": "module"` is set in which any case CommonJS files should end with `.cjs`.
- `main` should be defined if `exports` is not. It should be either a CommonJS or ESM file and adhere to the previous bullet. If a `module` field is defined, it should refer to an ESM file.
- Svelte components should be distributed as uncompiled `.svelte` files with any JS in the package written as ESM only. Custom script and style languages, like TypeScript and SCSS, should be preprocessed as vanilla JS and CSS respectively. We recommend using [`svelte-package`](./packaging) for packaging Svelte libraries, which will do this for you.
Libraries work best in the browser with Vite when they distribute an ESM version, especially if they are dependencies of a Svelte component library. You may wish to suggest to library authors that they provide an ESM version. However, CommonJS (CJS) dependencies should work as well since, by default, [`vite-plugin-svelte` will ask Vite to pre-bundle them](https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md#what-is-going-on-with-vite-and-pre-bundling-dependencies) using `esbuild` to convert them to ESM.
If you are still encountering issues we recommend searching both [the Vite issue tracker](https://github.com/vitejs/vite/issues) and the issue tracker of the library in question. Sometimes issues can be worked around by fiddling with the [`optimizeDeps`](https://vitejs.dev/config/#dep-optimization-options) or [`ssr`](https://vitejs.dev/config/#ssr-options) config values though we recommend this as only a short-term workaround in favor of fixing the library in question.
## How do I use the view transitions API with SvelteKit?
While SvelteKit does not have any specific integration with [view transitions](https://developer.chrome.com/docs/web-platform/view-transitions/), you can call `document.startViewTransition` in [`onNavigate`]($app-navigation#onNavigate) to trigger a view transition on every client-side navigation.
```js
// @errors: 2339 2810
import { onNavigate } from '$app/navigation';
onNavigate((navigation) => {
if (!document.startViewTransition) return;
return new Promise((resolve) => {
document.startViewTransition(async () => {
resolve();
await navigation.complete;
});
});
});
```
For more, see ["Unlocking view transitions"](/blog/view-transitions) on the Svelte blog.
## How do I use X with SvelteKit?
Make sure you've read the [documentation section on integrations](./integrations). If you're still having trouble, solutions to common issues are listed below.
### How do I setup a database?
Put the code to query your database in a [server route](./routing#server) - don't query the database in .svelte files. You can create a `db.js` or similar that sets up a connection immediately and makes the client accessible throughout the app as a singleton. You can execute any one-time setup code in `hooks.server.js` and import your database helpers into any endpoint that needs them.
### How do I use a client-side only library that depends on `document` or `window`?
If you need access to the `document` or `window` variables or otherwise need code to run only on the client-side you can wrap it in a `browser` check:
```js
/// <reference types="@sveltejs/kit" />
// ---cut---
import { browser } from '$app/environment';
if (browser) {
// client-only code here
}
```
You can also run code in `onMount` if you'd like to run it after the component has been first rendered to the DOM:
```js
// @filename: ambient.d.ts
// @lib: ES2015
declare module 'some-browser-only-library';
// @filename: index.js
// ---cut---
import { onMount } from 'svelte';
onMount(async () => {
const { method } = await import('some-browser-only-library');
method('hello world');
});
```
If the library you'd like to use is side-effect free you can also statically import it and it will be tree-shaken out in the server-side build where `onMount` will be automatically replaced with a no-op:
```js
// @filename: ambient.d.ts
// @lib: ES2015
declare module 'some-browser-only-library';
// @filename: index.js
// ---cut---
import { onMount } from 'svelte';
import { method } from 'some-browser-only-library';
onMount(() => {
method('hello world');
});
```
Finally, you may also consider using an `{#await}` block:
```svelte
<!--- file: index.svelte --->
<script>
import { browser } from '$app/environment';
const ComponentConstructor = browser ?
import('some-browser-only-library').then((module) => module.Component) :
new Promise(() => {});
</script>
{#await ComponentConstructor}
<p>Loading...</p>
{:then component}
<svelte:component this={component} />
{:catch error}
<p>Something went wrong: {error.message}</p>
{/await}
```
### How do I use a different backend API server?
You can use [`event.fetch`](./load#Making-fetch-requests) to request data from an external API server, but be aware that you would need to deal with [CORS](https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS), which will result in complications such as generally requiring requests to be preflighted resulting in higher latency. Requests to a separate subdomain may also increase latency due to an additional DNS lookup, TLS setup, etc. If you wish to use this method, you may find [`handleFetch`](./hooks#Server-hooks-handleFetch) helpful.
Another approach is to set up a proxy to bypass CORS headaches. In production, you would rewrite a path like `/api` to the API server; for local development, use Vite's [`server.proxy`](https://vitejs.dev/config/server-options.html#server-proxy) option.
How to setup rewrites in production will depend on your deployment platform. If rewrites aren't an option, you could alternatively add an [API route](./routing#server):
```js
/// file: src/routes/api/[...path]/+server.js
/** @type {import('./$types').RequestHandler} */
export function GET({ params, url }) {
return fetch(`https://my-api-server.com/${params.path + url.search}`);
}
```
(Note that you may also need to proxy `POST`/`PATCH` etc requests, and forward `request.headers`, depending on your needs.)
### How do I use middleware?
`adapter-node` builds a middleware that you can use with your own server for production mode. In dev, you can add middleware to Vite by using a Vite plugin. For example:
```js
// @errors: 2322
// @filename: ambient.d.ts
declare module '@sveltejs/kit/vite'; // TODO this feels unnecessary, why can't it 'see' the declarations?
// @filename: index.js
// ---cut---
import { sveltekit } from '@sveltejs/kit/vite';
/** @type {import('vite').Plugin} */
const myPlugin = {
name: 'log-request-middleware',
configureServer(server) {
server.middlewares.use((req, res, next) => {
console.log(`Got request ${req.url}`);
next();
});
}
};
/** @type {import('vite').UserConfig} */
const config = {
plugins: [myPlugin, sveltekit()]
};
export default config;
```
See [Vite's `configureServer` docs](https://vitejs.dev/guide/api-plugin.html#configureserver) for more details including how to control ordering.
### Does it work with Yarn 2?
Sort of. The Plug'n'Play feature, aka 'pnp', is broken (it deviates from the Node module resolution algorithm, and [doesn't yet work with native JavaScript modules](https://github.com/yarnpkg/berry/issues/638) which SvelteKit — along with an [increasing number of packages](https://blog.sindresorhus.com/get-ready-for-esm-aa53530b3f77) — uses). You can use `nodeLinker: 'node-modules'` in your [`.yarnrc.yml`](https://yarnpkg.com/configuration/yarnrc#nodeLinker) file to disable pnp, but it's probably easier to just use npm or [pnpm](https://pnpm.io/), which is similarly fast and efficient but without the compatibility headaches.
### How do I use with Yarn 3?
Currently ESM Support within the latest Yarn (version 3) is considered [experimental](https://github.com/yarnpkg/berry/pull/2161).
The below seems to work although your results may vary.
First create a new application:
```sh
yarn create svelte myapp
cd myapp
```
And enable Yarn Berry:
```sh
yarn set version berry
yarn install
```
#### Yarn 3 global cache
One of the more interesting features of Yarn Berry is the ability to have a single global cache for packages, instead of having multiple copies for each project on the disk. However, setting `enableGlobalCache` to true causes building to fail, so it is recommended to add the following to the `.yarnrc.yml` file:
```yaml
nodeLinker: node-modules
```
This will cause packages to be downloaded into a local node_modules directory but avoids the above problem and is your best bet for using version 3 of Yarn at this point in time.
# Integrations
## `vitePreprocess`
Including [`vitePreprocess`](https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/preprocess.md) in your project will allow you to use the various flavors of CSS that Vite supports: PostCSS, SCSS, Less, Stylus, and SugarSS. If you set your project up with TypeScript it will be included by default:
```js
// svelte.config.js
import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
export default {
preprocess: [vitePreprocess()]
};
```
You will also need to use a preprocessor if you're using TypeScript with Svelte 4. TypeScript is supported natively in Svelte 5 if you're using only the type syntax. To use more complex TypeScript syntax in Svelte 5, you will need still need a preprocessor and can use `vitePreprocess({ script: true })`.
## Adders
Run `npx sv add` to setup many different complex integrations with a single command including:
- prettier (formatting)
- eslint (linting)
- vitest (unit testing)
- playwright (e2e testing)
- lucia (auth)
- tailwind (CSS)
- drizzle (DB)
- paraglide (i18n)
- mdsvex (markdown)
- storybook (frontend workshop)
## Directory
See [sveltesociety.dev](https://sveltesociety.dev/) for a full listing of [packages](https://sveltesociety.dev/packages) and [templates](https://sveltesociety.dev/templates) available for use with Svelte and SvelteKit.
## Additional integrations
### `svelte-preprocess`
`svelte-preprocess` has some additional functionality not found in `vitePreprocess` such as support for Pug, Babel, and global styles. However, `vitePreprocess` may be faster and require less configuration, so it is used by default. Note that CoffeeScript is [not supported](https://github.com/sveltejs/kit/issues/2920#issuecomment-996469815) by SvelteKit.
You will need to install `svelte-preprocess` with `npm install --save-dev svelte-preprocess` and [add it to your `svelte.config.js`](https://github.com/sveltejs/svelte-preprocess/blob/main/docs/usage.md#with-svelte-config). After that, you will often need to [install the corresponding library](https://github.com/sveltejs/svelte-preprocess/blob/main/docs/getting-started.md) such as `npm install -D sass` or `npm install -D less`.
## Vite plugins
Since SvelteKit projects are built with Vite, you can use Vite plugins to enhance your project. See a list of available plugins at [`vitejs/awesome-vite`](https://github.com/vitejs/awesome-vite?tab=readme-ov-file#plugins).
## Integration FAQs
The SvelteKit FAQ has a [how to do X with SvelteKit](./faq#How-do-I-use-X-with-SvelteKit), which may be helpful if you still have questions.
# Breakpoint Debugging
In addition to the [`@debug`](../svelte/@debug) tag, you can also debug Svelte and SvelteKit projects using breakpoints within various tools and development environments. This includes both frontend and backend code.
The following guides assume your JavaScript runtime environment is Node.js.
## Visual Studio Code
With the built-in debug terminal, you can set up breakpoints in source files within VSCode.
1. Open the command palette: `CMD/Ctrl` + `Shift` + `P`.
2. Find and launch "Debug: JavaScript Debug Terminal".
3. Start your project using the debug terminal. For example: `npm run dev`.
4. Set some breakpoints in your client or server-side source code.
5. Trigger the breakpoint.
### Launch via debug pane
You may alternatively set up a `.vscode/launch.json` in your project. To set one up automatically:
1. Go to the "Run and Debug" pane.
2. In the "Run" select menu, choose "Node.js...".
3. Select the "run script" that corresponds to your project, such as "Run script: dev".
4. Press the "Start debugging" play button, or hit `F5` to begin breakpoint debugging.
Here's an example `launch.json`:
```json
{
"version": "0.2.0",
"configurations": [
{
"command": "npm run dev",
"name": "Run development server",
"request": "launch",
"type": "node-terminal"
}
]
}
```
Further reading: <https://code.visualstudio.com/docs/editor/debugging>.
## Other Editors
If you use a different editor, these community guides might be useful for you:
- [WebStorm Svelte: Debug Your Application](https://www.jetbrains.com/help/webstorm/svelte.html#ws_svelte_debug)
- [Debugging JavaScript Frameworks in Neovim](https://theosteiner.de/debugging-javascript-frameworks-in-neovim)
## Google Chrome and Microsoft Edge Developer Tools
It's possible to debug Node.js applications using a browser-based debugger.
> [!NOTE] Note this only works with debugging client-side SvelteKit source maps.
1. Run the `--inspect` flag when starting the Vite server with Node.js. For instance: `NODE_OPTIONS="--inspect" npm run dev`
2. Open your site in a new tab. Typically at `localhost:5173`.
3. Open your browser's dev tools, and click on the "Open dedicated DevTools for Node.js" icon near the top-left. It should display the Node.js logo.
4. Set up breakpoints and debug your application.
You may alternatively open the debugger devtools by navigating to `chrome://inspect` in Google Chrome, or `edge://inspect` in Microsoft Edge.
## References
- [Debugging Node.js](https://nodejs.org/en/learn/getting-started/debugging)
# Migrating to SvelteKit v2
Upgrading from SvelteKit version 1 to version 2 should be mostly seamless. There are a few breaking changes to note, which are listed here. You can use `npx sv migrate sveltekit-2` to migrate some of these changes automatically.
We highly recommend upgrading to the most recent 1.x version before upgrading to 2.0, so that you can take advantage of targeted deprecation warnings. We also recommend [updating to Svelte 4](../svelte/v4-migration-guide) first: Later versions of SvelteKit 1.x support it, and SvelteKit 2.0 requires it.
## `redirect` and `error` are no longer thrown by you
Previously, you had to `throw` the values returned from `error(...)` and `redirect(...)` yourself. In SvelteKit 2 this is no longer the case — calling the functions is sufficient.
```js
import { error } from '@sveltejs/kit'
// ...
---throw error(500, 'something went wrong');---
+++error(500, 'something went wrong');+++
```
`svelte-migrate` will do these changes automatically for you.
If the error or redirect is thrown inside a `try {...}` block (hint: don't do this!), you can distinguish them from unexpected errors using [`isHttpError`](@sveltejs-kit#isHttpError) and [`isRedirect`](@sveltejs-kit#isRedirect) imported from `@sveltejs/kit`.
## path is required when setting cookies
When receiving a `Set-Cookie` header that doesn't specify a `path`, browsers will [set the cookie path](https://www.rfc-editor.org/rfc/rfc6265#section-5.1.4) to the parent of the resource in question. This behaviour isn't particularly helpful or intuitive, and frequently results in bugs because the developer expected the cookie to apply to the domain as a whole.
As of SvelteKit 2.0, you need to set a `path` when calling `cookies.set(...)`, `cookies.delete(...)` or `cookies.serialize(...)` so that there's no ambiguity. Most of the time, you probably want to use `path: '/'`, but you can set it to whatever you like, including relative paths — `''` means 'the current path', `'.'` means 'the current directory'.
```js
/** @type {import('./$types').PageServerLoad} */
export function load({ cookies }) {
cookies.set(name, value, +++{ path: '/' }+++);
return { response }
}
```
`svelte-migrate` will add comments highlighting the locations that need to be adjusted.
## Top-level promises are no longer awaited
In SvelteKit version 1, if the top-level properties of the object returned from a `load` function were promises, they were automatically awaited. With the introduction of [streaming](/blog/streaming-snapshots-sveltekit) this behavior became a bit awkward as it forces you to nest your streamed data one level deep.
As of version 2, SvelteKit no longer differentiates between top-level and non-top-level promises. To get back the blocking behavior, use `await` (with `Promise.all` to prevent waterfalls, where appropriate):
```js
// @filename: ambient.d.ts
declare const url: string;
// @filename: index.js
// ---cut---
// If you have a single promise
/** @type {import('./$types').PageServerLoad} */
export +++async+++ function load({ fetch }) {
const response = +++await+++ fetch(url).then(r => r.json());
return { response }
}
```
```js
// @filename: ambient.d.ts
declare const url1: string;
declare const url2: string;
// @filename: index.js
// ---cut---
// If you have multiple promises
/** @type {import('./$types').PageServerLoad} */
export +++async+++ function load({ fetch }) {
--- const a = fetch(url1).then(r => r.json());---
--- const b = fetch(url2).then(r => r.json());---
+++ const [a, b] = await Promise.all([
fetch(url1).then(r => r.json()),
fetch(url2).then(r => r.json()),
]);+++
return { a, b };
}
```
## goto(...) changes
`goto(...)` no longer accepts external URLs. To navigate to an external URL, use `window.location.href = url`. The `state` object now determines `$page.state` and must adhere to the `App.PageState` interface, if declared. See [shallow routing](shallow-routing) for more details.
## paths are now relative by default
In SvelteKit 1, `%sveltekit.assets%` in your `app.html` was replaced with a relative path by default (i.e. `.` or `..` or `../..` etc, depending on the path being rendered) during server-side rendering unless the [`paths.relative`](configuration#paths) config option was explicitly set to `false`. The same was true for `base` and `assets` imported from `$app/paths`, but only if the `paths.relative` option was explicitly set to `true`.
This inconsistency is fixed in version 2. Paths are either always relative or always absolute, depending on the value of [`paths.relative`](configuration#paths). It defaults to `true` as this results in more portable apps: if the `base` is something other than the app expected (as is the case when viewed on the [Internet Archive](https://archive.org/), for example) or unknown at build time (as is the case when deploying to [IPFS](https://ipfs.tech/) and so on), fewer things are likely to break.
## Server fetches are not trackable anymore
Previously it was possible to track URLs from `fetch`es on the server in order to rerun load functions. This poses a possible security risk (private URLs leaking), and as such it was behind the `dangerZone.trackServerFetches` setting, which is now removed.
## `preloadCode` arguments must be prefixed with `base`
SvelteKit exposes two functions, [`preloadCode`]($app-navigation#preloadCode) and [`preloadData`]($app-navigation#preloadData), for programmatically loading the code and data associated with a particular path. In version 1, there was a subtle inconsistency — the path passed to `preloadCode` did not need to be prefixed with the `base` path (if set), while the path passed to `preloadData` did.
This is fixed in SvelteKit 2 — in both cases, the path should be prefixed with `base` if it is set.
Additionally, `preloadCode` now takes a single argument rather than _n_ arguments.
## `resolvePath` has been removed
SvelteKit 1 included a function called `resolvePath` which allows you to resolve a route ID (like `/blog/[slug]`) and a set of parameters (like `{ slug: 'hello' }`) to a pathname. Unfortunately the return value didn't include the `base` path, limiting its usefulness in cases where `base` was set.
As such, SvelteKit 2 replaces `resolvePath` with a (slightly better named) function called `resolveRoute`, which is imported from `$app/paths` and which takes `base` into account.
```js
---import { resolvePath } from '@sveltejs/kit';
import { base } from '$app/paths';---
+++import { resolveRoute } from '$app/paths';+++
---const path = base + resolvePath('/blog/[slug]', { slug });---
+++const path = resolveRoute('/blog/[slug]', { slug });+++
```
`svelte-migrate` will do the method replacement for you, though if you later prepend the result with `base`, you need to remove that yourself.
## Improved error handling
Errors are handled inconsistently in SvelteKit 1. Some errors trigger the `handleError` hook but there is no good way to discern their status (for example, the only way to tell a 404 from a 500 is by seeing if `event.route.id` is `null`), while others (such as 405 errors for `POST` requests to pages without actions) don't trigger `handleError` at all, but should. In the latter case, the resulting `$page.error` will deviate from the [`App.Error`](types#Error) type, if it is specified.
SvelteKit 2 cleans this up by calling `handleError` hooks with two new properties: `status` and `message`. For errors thrown from your code (or library code called by your code) the status will be `500` and the message will be `Internal Error`. While `error.message` may contain sensitive information that should not be exposed to users, `message` is safe.
## Dynamic environment variables cannot be used during prerendering
The `$env/dynamic/public` and `$env/dynamic/private` modules provide access to _run time_ environment variables, as opposed to the _build time_ environment variables exposed by `$env/static/public` and `$env/static/private`.
During prerendering in SvelteKit 1, they are one and the same. As such, prerendered pages that make use of 'dynamic' environment variables are really 'baking in' build time values, which is incorrect. Worse, `$env/dynamic/public` is populated in the browser with these stale values if the user happens to land on a prerendered page before navigating to dynamically-rendered pages.
Because of this, dynamic environment variables can no longer be read during prerendering in SvelteKit 2 — you should use the `static` modules instead. If the user lands on a prerendered page, SvelteKit will request up-to-date values for `$env/dynamic/public` from the server (by default from a module called `/_app/env.js`) instead of reading them from the server-rendered HTML.
## `form` and `data` have been removed from `use:enhance` callbacks
If you provide a callback to [`use:enhance`](form-actions#Progressive-enhancement-use:enhance), it will be called with an object containing various useful properties.
In SvelteKit 1, those properties included `form` and `data`. These were deprecated some time ago in favour of `formElement` and `formData`, and have been removed altogether in SvelteKit 2.
## Forms containing file inputs must use `multipart/form-data`
If a form contains an `<input type="file">` but does not have an `enctype="multipart/form-data"` attribute, non-JS submissions will omit the file. SvelteKit 2 will throw an error if it encounters a form like this during a `use:enhance` submission to ensure that your forms work correctly when JavaScript is not present.
## Generated `tsconfig.json` is more strict
Previously, the generated `tsconfig.json` was trying its best to still produce a somewhat valid config when your `tsconfig.json` included `paths` or `baseUrl`. In SvelteKit 2, the validation is more strict and will warn when you use either `paths` or `baseUrl` in your `tsconfig.json`. These settings are used to generate path aliases and you should use [the `alias` config](configuration#alias) option in your `svelte.config.js` instead, to also create a corresponding alias for the bundler.
## `getRequest` no longer throws errors
The `@sveltejs/kit/node` module exports helper functions for use in Node environments, including `getRequest` which turns a Node [`ClientRequest`](https://nodejs.org/api/http.html#class-httpclientrequest) into a standard [`Request`](https://developer.mozilla.org/en-US/docs/Web/API/Request) object.
In SvelteKit 1, `getRequest` could throw if the `Content-Length` header exceeded the specified size limit. In SvelteKit 2, the error will not be thrown until later, when the request body (if any) is being read. This enables better diagnostics and simpler code.
## `vitePreprocess` is no longer exported from `@sveltejs/kit/vite`
Since `@sveltejs/vite-plugin-svelte` is now a peer dependency, SvelteKit 2 no longer re-exports `vitePreprocess`. You should import it directly from `@sveltejs/vite-plugin-svelte`.
## Updated dependency requirements
SvelteKit 2 requires Node `18.13` or higher, and the following minimum dependency versions:
- `svelte@4`
- `vite@5`
- `typescript@5`
- `@sveltejs/vite-plugin-svelte@3` (this is now required as a `peerDependency` of SvelteKit — previously it was directly depended upon)
- `@sveltejs/adapter-cloudflare@3` (if you're using these adapters)
- `@sveltejs/adapter-cloudflare-workers@2`
- `@sveltejs/adapter-netlify@3`
- `@sveltejs/adapter-node@2`
- `@sveltejs/adapter-static@3`
- `@sveltejs/adapter-vercel@4`
`svelte-migrate` will update your `package.json` for you.
As part of the TypeScript upgrade, the generated `tsconfig.json` (the one your `tsconfig.json` extends from) now uses `"moduleResolution": "bundler"` (which is recommended by the TypeScript team, as it properly resolves types from packages with an `exports` map in package.json) and `verbatimModuleSyntax` (which replaces the existing `importsNotUsedAsValues ` and `preserveValueImports` flags — if you have those in your `tsconfig.json`, remove them. `svelte-migrate` will do this for you).
## SvelteKit 2.12: $app/stores deprecated
SvelteKit 2.12 introduced `$app/state` based on the [Svelte 5 runes API](/docs/svelte/what-are-runes). `$app/state` provides everything that `$app/stores` provides but with more flexibility as to where and how you use it. Most importantly, the `page` object is now fine-grained, e.g. updates to `page.state` will not invalidate `page.data` and vice-versa.
As a consequence, `$app/stores` is deprecated and subject to be removed in SvelteKit 3. We recommend [upgrading to Svelte 5](/docs/svelte/v5-migration-guide), if you haven't already, and then migrate away from `$app/stores`. Most of the replacements should be pretty simple: Replace the `$app/stores` import with `$app/state` and remove the `$` prefixes from the usage sites.
```svelte
<script>
---import { page } from '$app/stores';---
+++import { page } from '$app/state';+++
</script>
---{$page.data}---
+++{page.data}+++
```
Use `npx sv migrate app-state` to auto-migrate most of your `$app/stores` usages inside `.svelte` components.
# Migrating from Sapper
SvelteKit is the successor to Sapper and shares many elements of its design.
If you have an existing Sapper app that you plan to migrate to SvelteKit, there are a number of changes you will need to make. You may find it helpful to view [some examples](additional-resources#Examples) while migrating.
## package.json
### type: "module"
Add `"type": "module"` to your `package.json`. You can do this step separately from the rest as part of an incremental migration if you are using Sapper 0.29.3
or newer.
### dependencies
Remove `polka` or `express`, if you're using one of those, and any middleware such as `sirv` or `compression`.
### devDependencies
Remove `sapper` from your `devDependencies` and replace it with `@sveltejs/kit` and whichever [adapter](adapters) you plan to use (see [next section](migrating#Project-files-Configuration)).
### scripts
Any scripts that reference `sapper` should be updated:
- `sapper build` should become `vite build` using the Node [adapter](adapters)
- `sapper export` should become `vite build` using the static [adapter](adapters)
- `sapper dev` should become `vite dev`
- `node __sapper__/build` should become `node build`
## Project files
The bulk of your app, in `src/routes`, can be left where it is, but several project files will need to be moved or updated.
### Configuration
Your `webpack.config.js` or `rollup.config.js` should be replaced with a `svelte.config.js`, as documented [here](configuration). Svelte preprocessor options should be moved to `config.preprocess`.
You will need to add an [adapter](adapters). `sapper build` is roughly equivalent to [adapter-node](https://github.com/sveltejs/kit/tree/main/packages/adapter-node) while `sapper export` is roughly equivalent to [adapter-static](https://github.com/sveltejs/kit/tree/main/packages/adapter-static), though you might prefer to use an adapter designed for the platform you're deploying to.
If you were using plugins for filetypes that are not automatically handled by [Vite](https://vitejs.dev), you will need to find Vite equivalents and add them to the [Vite config](project-structure#Project-files-vite.config.js).
### src/client.js
This file has no equivalent in SvelteKit. Any custom logic (beyond `sapper.start(...)`) should be expressed in your `+layout.svelte` file, inside an `onMount` callback.
### src/server.js
When using `adapter-node` the equivalent is a [custom server](adapter-node#Custom-server). Otherwise, this file has no direct equivalent, since SvelteKit apps can run in serverless environments.
### src/service-worker.js
Most imports from `@sapper/service-worker` have equivalents in [`$service-worker`]($service-worker):
- `files` is unchanged
- `routes` has been removed
- `shell` is now `build`
- `timestamp` is now `version`
### src/template.html
The `src/template.html` file should be renamed `src/app.html`.
Remove `%sapper.base%`, `%sapper.scripts%` and `%sapper.styles%`. Replace `%sapper.head%` with `%sveltekit.head%` and `%sapper.html%` with `%sveltekit.body%`. The `<div id="sapper">` is no longer necessary.
### src/node_modules
A common pattern in Sapper apps is to put your internal library in a directory inside `src/node_modules`. This doesn't work with Vite, so we use [`src/lib`]($lib) instead.
## Pages and layouts
### Renamed files
Routes now are made up of the folder name exclusively to remove ambiguity, the folder names leading up to a `+page.svelte` correspond to the route. See [the routing docs](routing) for an overview. The following shows a old/new comparison:
| Old | New |
| ------------------------- | ------------------------- |
| routes/about/index.svelte | routes/about/+page.svelte |
| routes/about.svelte | routes/about/+page.svelte |
Your custom error page component should be renamed from `_error.svelte` to `+error.svelte`. Any `_layout.svelte` files should likewise be renamed `+layout.svelte`. [Any other files are ignored](routing#Other-files).
### Imports
The `goto`, `prefetch` and `prefetchRoutes` imports from `@sapper/app` should be replaced with `goto`, `preloadData` and `preloadCode` imports respectively from [`$app/navigation`]($app-navigation).
The `stores` import from `@sapper/app` should be replaced — see the [Stores](migrating#Pages-and-layouts-Stores) section below.
Any files you previously imported from directories in `src/node_modules` will need to be replaced with [`$lib`]($lib) imports.
### Preload
As before, pages and layouts can export a function that allows data to be loaded before rendering takes place.
This function has been renamed from `preload` to [`load`](load), it now lives in a `+page.js` (or `+layout.js`) next to its `+page.svelte` (or `+layout.svelte`), and its API has changed. Instead of two arguments — `page` and `session` — there is a single `event` argument.
There is no more `this` object, and consequently no `this.fetch`, `this.error` or `this.redirect`. Instead, you can get [`fetch`](load#Making-fetch-requests) from the input methods, and both [`error`](load#Errors) and [`redirect`](load#Redirects) are now thrown.
### Stores
In Sapper, you would get references to provided stores like so:
```js
// @filename: ambient.d.ts
declare module '@sapper/app';
// @filename: index.js
// ---cut---
import { stores } from '@sapper/app';
const { preloading, page, session } = stores();
```
The `page` store still exists; `preloading` has been replaced with a `navigating` store that contains `from` and `to` properties. `page` now has `url` and `params` properties, but no `path` or `query`.
You access them differently in SvelteKit. `stores` is now `getStores`, but in most cases it is unnecessary since you can import `navigating`, and `page` directly from [`$app/stores`]($app-stores). If you're on Svelte 5 and SvelteKit 2.12 or higher, consider using [`$app/state`]($app-state) instead.
### Routing
Regex routes are no longer supported. Instead, use [advanced route matching](advanced-routing#Matching).
### Segments
Previously, layout components received a `segment` prop indicating the child segment. This has been removed; you should use the more flexible `$page.url.pathname` (or `page.url.pathname`) value to derive the segment you're interested in.
### URLs
In Sapper, all relative URLs were resolved against the base URL — usually `/`, unless the `basepath` option was used — rather than against the current page.
This caused problems and is no longer the case in SvelteKit. Instead, relative URLs are resolved against the current page (or the destination page, for `fetch` URLs in `load` functions) instead. In most cases, it's easier to use root-relative (i.e. starts with `/`) URLs, since their meaning is not context-dependent.
### <a> attributes
- `sapper:prefetch` is now `data-sveltekit-preload-data`
- `sapper:noscroll` is now `data-sveltekit-noscroll`
## Endpoints
In Sapper, [server routes](routing#server) received the `req` and `res` objects exposed by Node's `http` module (or the augmented versions provided by frameworks like Polka and Express).
SvelteKit is designed to be agnostic as to where the app is running — it could be running on a Node server, but could equally be running on a serverless platform or in a Cloudflare Worker. For that reason, you no longer interact directly with `req` and `res`. Your endpoints will need to be updated to match the new signature.
To support this environment-agnostic behavior, `fetch` is now available in the global context, so you don't need to import `node-fetch`, `cross-fetch`, or similar server-side fetch implementations in order to use it.
## Integrations
See [integrations](./integrations) for detailed information about integrations.
### HTML minifier
Sapper includes `html-minifier` by default. SvelteKit does not include this, but you can add it as a prod dependency and then use it through a [hook](hooks#Server-hooks-handle):
```js
// @filename: ambient.d.ts
/// <reference types="@sveltejs/kit" />
declare module 'html-minifier';
// @filename: index.js
// ---cut---
import { minify } from 'html-minifier';
import { building } from '$app/environment';
const minification_options = {
collapseBooleanAttributes: true,
collapseWhitespace: true,
conservativeCollapse: true,
decodeEntities: true,
html5: true,
ignoreCustomComments: [/^#/],
minifyCSS: true,
minifyJS: false,
removeAttributeQuotes: true,
removeComments: false, // some hydration code needs comments, so leave them in
removeOptionalTags: true,
removeRedundantAttributes: true,
removeScriptTypeAttributes: true,
removeStyleLinkTypeAttributes: true,
sortAttributes: true,
sortClassName: true
};
/** @type {import('@sveltejs/kit').Handle} */
export async function handle({ event, resolve }) {
let page = '';
return resolve(event, {
transformPageChunk: ({ html, done }) => {
page += html;
if (done) {
return building ? minify(page, minification_options) : page;
}
}
});
}
```
Note that `prerendering` is `false` when using `vite preview` to test the production build of the site, so to verify the results of minifying, you'll need to inspect the built HTML files directly.
# Additional resources
## FAQs
Please see the [SvelteKit FAQ](faq) for solutions to common issues and helpful tips and tricks.
The [Svelte FAQ](../svelte/faq) and [`vite-plugin-svelte` FAQ](https://github.com/sveltejs/vite-plugin-svelte/blob/main/docs/faq.md) may also be helpful for questions deriving from those libraries.
## Examples
We've written and published a few different SvelteKit sites as examples:
- [`sveltejs/realworld`](https://github.com/sveltejs/realworld) contains an example blog site
- [A HackerNews clone](https://github.com/sveltejs/sites/tree/master/sites/hn.svelte.dev)
- [`svelte.dev`](https://github.com/sveltejs/svelte.dev)
SvelteKit users have also published plenty of examples on GitHub, under the [#sveltekit](https://github.com/topics/sveltekit) and [#sveltekit-template](https://github.com/topics/sveltekit-template) topics, as well as on [the Svelte Society site](https://sveltesociety.dev/templates?category=sveltekit). Note that these have not been vetted by the maintainers and may not be up to date.
## Support
You can ask for help on [Discord](/chat) and [StackOverflow](https://stackoverflow.com/questions/tagged/sveltekit). Please first search for information related to your issue in the FAQ, Google or another search engine, issue tracker, and Discord chat history in order to be respectful of others' time. There are many more people asking questions than answering them, so this will help in allowing the community to grow in a scalable fashion.
# Glossary
The core of SvelteKit provides a highly configurable rendering engine. This section describes some of the terms used when discussing rendering. A reference for setting these options is provided in the documentation above.
## CSR
Client-side rendering (CSR) is the generation of the page contents in the web browser using JavaScript.
In SvelteKit, client-side rendering will be used by default, but you can turn off JavaScript with [the `csr = false` page option](page-options#csr).
## Hydration
Svelte components store some state and update the DOM when the state is updated. When fetching data during SSR, by default SvelteKit will store this data and transmit it to the client along with the server-rendered HTML. The components can then be initialized on the client with that data without having to call the same API endpoints again. Svelte will then check that the DOM is in the expected state and attach event listeners in a process called hydration. Once the components are fully hydrated, they can react to changes to their properties just like any newly created Svelte component.
In SvelteKit, pages will be hydrated by default, but you can turn off JavaScript with [the `csr = false` page option](page-options#csr).
## Prerendering
Prerendering means computing the contents of a page at build time and saving the HTML for display. This approach has the same benefits as traditional server-rendered pages, but avoids recomputing the page for each visitor and so scales nearly for free as the number of visitors increases. The tradeoff is that the build process is more expensive and prerendered content can only be updated by building and deploying a new version of the application.
Not all pages can be prerendered. The basic rule is this: for content to be prerenderable, any two users hitting it directly must get the same content from the server, and the page must not contain [actions](form-actions). Note that you can still prerender content that is loaded based on the page's parameters as long as all users will be seeing the same prerendered content.
Pre-rendered pages are not limited to static content. You can build personalized pages if user-specific data is fetched and rendered client-side. This is subject to the caveat that you will experience the downsides of not doing SSR for that content as discussed above.
In SvelteKit, you can control prerendering with [the `prerender` page option](page-options#prerender) and [`prerender` config](configuration#prerender) in `svelte.config.js`.
## Routing
By default, when you navigate to a new page (by clicking on a link or using the browser's forward or back buttons), SvelteKit will intercept the attempted navigation and handle it instead of allowing the browser to send a request to the server for the destination page. SvelteKit will then update the displayed contents on the client by rendering the component for the new page, which in turn can make calls to the necessary API endpoints. This process of updating the page on the client in response to attempted navigation is called client-side routing.
In SvelteKit, client-side routing will be used by default, but you can skip it with [`data-sveltekit-reload`](link-options#data-sveltekit-reload).
## SPA
A single-page app (SPA) is an application in which all requests to the server load a single HTML file which then does client-side rendering of the requested contents based on the requested URL. All navigation is handled on the client-side in a process called client-side routing with per-page contents being updated and common layout elements remaining largely unchanged. SPAs do not provide SSR, which has the shortcoming described above. However, some applications are not greatly impacted by these shortcomings such as a complex business application behind a login where SEO would not be important and it is known that users will be accessing the application from a consistent computing environment.
In SvelteKit, you can [build an SPA with `adapter-static`](single-page-apps).
## SSG
Static Site Generation (SSG) is a term that refers to a site where every page is prerendered. SvelteKit was not built to do only static site generation like some tools and so may not scale as well to efficiently render a very large number of pages as tools built specifically for that purpose. However, in contrast to most purpose-built SSGs, SvelteKit does nicely allow for mixing and matching different rendering types on different pages. One benefit of fully prerendering a site is that you do not need to maintain or pay for servers to perform SSR. Once generated, the site can be served from CDNs, leading to great "time to first byte" performance. This delivery model is often referred to as JAMstack.
In SvelteKit, you can do static site generation by using [`adapter-static`](adapter-static) or by configuring every page to be prerendered using [the `prerender` page option](page-options#prerender) or [`prerender` config](configuration#prerender) in `svelte.config.js`.
## SSR
Server-side rendering (SSR) is the generation of the page contents on the server. SSR is generally preferred for SEO. While some search engines can index content that is dynamically generated on the client-side it may take longer even in these cases. It also tends to improve perceived performance and makes your app accessible to users if JavaScript fails or is disabled (which happens [more often than you probably think](https://kryogenix.org/code/browser/everyonehasjs.html)).
In SvelteKit, pages are server-side rendered by default. You can disable SSR with [the `ssr` page option](page-options#ssr).
# @sveltejs/kit
```js
// @noErrors
import {
Server,
VERSION,
error,
fail,
isActionFailure,
isHttpError,
isRedirect,
json,
redirect,
text
} from '@sveltejs/kit';
```
## Server
<div class="ts-block">
```dts
class Server {/*…*/}
```
<div class="ts-block-property">
```dts
constructor(manifest: SSRManifest);
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
init(options: ServerInitOptions): Promise<void>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
respond(request: Request, options: RequestOptions): Promise<Response>;
```
<div class="ts-block-property-details"></div>
</div></div>
## VERSION
<div class="ts-block">
```dts
const VERSION: string;
```
</div>
## error
Throws an error with a HTTP status code and an optional message.
When called during request handling, this will cause SvelteKit to
return an error response without invoking `handleError`.
Make sure you're not catching the thrown error, which would prevent SvelteKit from handling it.
<div class="ts-block">
```dts
function error(status: number, body: App.Error): never;
```
</div>
<div class="ts-block">
```dts
function error(
status: number,
body?: {
message: string;
} extends App.Error
? App.Error | string | undefined
: never
): never;
```
</div>
## fail
Create an `ActionFailure` object.
<div class="ts-block">
```dts
function fail(status: number): ActionFailure<undefined>;
```
</div>
<div class="ts-block">
```dts
function fail<
T extends Record<string, unknown> | undefined = undefined
>(status: number, data: T): ActionFailure<T>;
```
</div>
## isActionFailure
Checks whether this is an action failure thrown by `fail`.
<div class="ts-block">
```dts
function isActionFailure(e: unknown): e is ActionFailure;
```
</div>
## isHttpError
Checks whether this is an error thrown by `error`.
<div class="ts-block">
```dts
function isHttpError<T extends number>(
e: unknown,
status?: T | undefined
): e is HttpError_1 & {
status: T extends undefined ? never : T;
};
```
</div>
## isRedirect
Checks whether this is a redirect thrown by `redirect`.
<div class="ts-block">
```dts
function isRedirect(e: unknown): e is Redirect_1;
```
</div>
## json
Create a JSON `Response` object from the supplied data.
<div class="ts-block">
```dts
function json(
data: any,
init?: ResponseInit | undefined
): Response;
```
</div>
## redirect
Redirect a request. When called during request handling, SvelteKit will return a redirect response.
Make sure you're not catching the thrown redirect, which would prevent SvelteKit from handling it.
Most common status codes:
* `303 See Other`: redirect as a GET request (often used after a form POST request)
* `307 Temporary Redirect`: redirect will keep the request method
* `308 Permanent Redirect`: redirect will keep the request method, SEO will be transferred to the new page
[See all redirect status codes](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#redirection_messages)
<div class="ts-block">
```dts
function redirect(
status:
| 300
| 301
| 302
| 303
| 304
| 305
| 306
| 307
| 308
| ({} & number),
location: string | URL
): never;
```
</div>
## text
Create a `Response` object from the supplied body.
<div class="ts-block">
```dts
function text(
body: string,
init?: ResponseInit | undefined
): Response;
```
</div>
## Action
Shape of a form action method that is part of `export const actions = {..}` in `+page.server.js`.
See [form actions](/docs/kit/form-actions) for more information.
<div class="ts-block">
```dts
type Action<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
OutputData extends Record<string, any> | void = Record<
string,
any
> | void,
RouteId extends string | null = string | null
> = (
event: RequestEvent<Params, RouteId>
) => MaybePromise<OutputData>;
```
</div>
## ActionFailure
<div class="ts-block">
```dts
interface ActionFailure<
T extends Record<string, unknown> | undefined = undefined
> {/*…*/}
```
<div class="ts-block-property">
```dts
status: number;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
data: T;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
[uniqueSymbol]: true;
```
<div class="ts-block-property-details"></div>
</div></div>
## ActionResult
When calling a form action via fetch, the response will be one of these shapes.
```svelte
<form method="post" use:enhance={() => {
return ({ result }) => {
// result is of type ActionResult
};
}}
```
<div class="ts-block">
```dts
type ActionResult<
Success extends
| Record<string, unknown>
| undefined = Record<string, any>,
Failure extends
| Record<string, unknown>
| undefined = Record<string, any>
> =
| { type: 'success'; status: number; data?: Success }
| { type: 'failure'; status: number; data?: Failure }
| { type: 'redirect'; status: number; location: string }
| { type: 'error'; status?: number; error: any };
```
</div>
## Actions
Shape of the `export const actions = {..}` object in `+page.server.js`.
See [form actions](/docs/kit/form-actions) for more information.
<div class="ts-block">
```dts
type Actions<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
OutputData extends Record<string, any> | void = Record<
string,
any
> | void,
RouteId extends string | null = string | null
> = Record<string, Action<Params, OutputData, RouteId>>;
```
</div>
## Adapter
[Adapters](/docs/kit/adapters) are responsible for taking the production build and turning it into something that can be deployed to a platform of your choosing.
<div class="ts-block">
```dts
interface Adapter {/*…*/}
```
<div class="ts-block-property">
```dts
name: string;
```
<div class="ts-block-property-details">
The name of the adapter, using for logging. Will typically correspond to the package name.
</div>
</div>
<div class="ts-block-property">
```dts
adapt: (builder: Builder) => MaybePromise<void>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `builder` An object provided by SvelteKit that contains methods for adapting the app
</div>
This function is called after SvelteKit has built your app.
</div>
</div>
<div class="ts-block-property">
```dts
supports?: {/*…*/}
```
<div class="ts-block-property-details">
Checks called during dev and build to determine whether specific features will work in production with this adapter
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
read?: (details: { config: any; route: { id: string } }) => boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `config` The merged route config
</div>
Test support for `read` from `$app/server`
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```dts
emulate?: () => MaybePromise<Emulator>;
```
<div class="ts-block-property-details">
Creates an `Emulator`, which allows the adapter to influence the environment
during dev, build and prerendering
</div>
</div></div>
## AfterNavigate
The argument passed to [`afterNavigate`](/docs/kit/$app-navigation#afterNavigate) callbacks.
<div class="ts-block">
```dts
interface AfterNavigate extends Omit<Navigation, 'type'> {/*…*/}
```
<div class="ts-block-property">
```dts
type: Exclude<NavigationType, 'leave'>;
```
<div class="ts-block-property-details">
The type of navigation:
- `enter`: The app has hydrated
- `form`: The user submitted a `<form>`
- `link`: Navigation was triggered by a link click
- `goto`: Navigation was triggered by a `goto(...)` call or a redirect
- `popstate`: Navigation was triggered by back/forward navigation
</div>
</div>
<div class="ts-block-property">
```dts
willUnload: false;
```
<div class="ts-block-property-details">
Since `afterNavigate` callbacks are called after a navigation completes, they will never be called with a navigation that unloads the page.
</div>
</div></div>
## AwaitedActions
<div class="ts-block">
```dts
type AwaitedActions<
T extends Record<string, (...args: any) => any>
> = OptionalUnion<
{
[Key in keyof T]: UnpackValidationError<
Awaited<ReturnType<T[Key]>>
>;
}[keyof T]
>;
```
</div>
## BeforeNavigate
The argument passed to [`beforeNavigate`](/docs/kit/$app-navigation#beforeNavigate) callbacks.
<div class="ts-block">
```dts
interface BeforeNavigate extends Navigation {/*…*/}
```
<div class="ts-block-property">
```dts
cancel: () => void;
```
<div class="ts-block-property-details">
Call this to prevent the navigation from starting.
</div>
</div></div>
## Builder
This object is passed to the `adapt` function of adapters.
It contains various methods and properties that are useful for adapting the app.
<div class="ts-block">
```dts
interface Builder {/*…*/}
```
<div class="ts-block-property">
```dts
log: Logger;
```
<div class="ts-block-property-details">
Print messages to the console. `log.info` and `log.minor` are silent unless Vite's `logLevel` is `info`.
</div>
</div>
<div class="ts-block-property">
```dts
rimraf: (dir: string) => void;
```
<div class="ts-block-property-details">
Remove `dir` and all its contents.
</div>
</div>
<div class="ts-block-property">
```dts
mkdirp: (dir: string) => void;
```
<div class="ts-block-property-details">
Create `dir` and any required parent directories.
</div>
</div>
<div class="ts-block-property">
```dts
config: ValidatedConfig;
```
<div class="ts-block-property-details">
The fully resolved `svelte.config.js`.
</div>
</div>
<div class="ts-block-property">
```dts
prerendered: Prerendered;
```
<div class="ts-block-property-details">
Information about prerendered pages and assets, if any.
</div>
</div>
<div class="ts-block-property">
```dts
routes: RouteDefinition[];
```
<div class="ts-block-property-details">
An array of all routes (including prerendered)
</div>
</div>
<div class="ts-block-property">
```dts
createEntries: (fn: (route: RouteDefinition) => AdapterEntry) => Promise<void>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `fn` A function that groups a set of routes into an entry point
- <span class="tag deprecated">deprecated</span> Use `builder.routes` instead
</div>
Create separate functions that map to one or more routes of your app.
</div>
</div>
<div class="ts-block-property">
```dts
findServerAssets: (routes: RouteDefinition[]) => string[];
```
<div class="ts-block-property-details">
Find all the assets imported by server files belonging to `routes`
</div>
</div>
<div class="ts-block-property">
```dts
generateFallback: (dest: string) => Promise<void>;
```
<div class="ts-block-property-details">
Generate a fallback page for a static webserver to use when no route is matched. Useful for single-page apps.
</div>
</div>
<div class="ts-block-property">
```dts
generateEnvModule: () => void;
```
<div class="ts-block-property-details">
Generate a module exposing build-time environment variables as `$env/dynamic/public`.
</div>
</div>
<div class="ts-block-property">
```dts
generateManifest: (opts: { relativePath: string; routes?: RouteDefinition[] }) => string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `opts` a relative path to the base directory of the app and optionally in which format (esm or cjs) the manifest should be generated
</div>
Generate a server-side manifest to initialise the SvelteKit [server](/docs/kit/@sveltejs-kit#Server) with.
</div>
</div>
<div class="ts-block-property">
```dts
getBuildDirectory: (name: string) => string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `name` path to the file, relative to the build directory
</div>
Resolve a path to the `name` directory inside `outDir`, e.g. `/path/to/.svelte-kit/my-adapter`.
</div>
</div>
<div class="ts-block-property">
```dts
getClientDirectory: () => string;
```
<div class="ts-block-property-details">
Get the fully resolved path to the directory containing client-side assets, including the contents of your `static` directory.
</div>
</div>
<div class="ts-block-property">
```dts
getServerDirectory: () => string;
```
<div class="ts-block-property-details">
Get the fully resolved path to the directory containing server-side code.
</div>
</div>
<div class="ts-block-property">
```dts
getAppPath: () => string;
```
<div class="ts-block-property-details">
Get the application path including any configured `base` path, e.g. `my-base-path/_app`.
</div>
</div>
<div class="ts-block-property">
```dts
writeClient: (dest: string) => string[];
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `dest` the destination folder
- <span class="tag">returns</span> an array of files written to `dest`
</div>
Write client assets to `dest`.
</div>
</div>
<div class="ts-block-property">
```dts
writePrerendered: (dest: string) => string[];
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `dest` the destination folder
- <span class="tag">returns</span> an array of files written to `dest`
</div>
Write prerendered files to `dest`.
</div>
</div>
<div class="ts-block-property">
```dts
writeServer: (dest: string) => string[];
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `dest` the destination folder
- <span class="tag">returns</span> an array of files written to `dest`
</div>
Write server-side code to `dest`.
</div>
</div>
<div class="ts-block-property">
```dts
copy: (
from: string,
to: string,
opts?: {
filter?(basename: string): boolean;
replace?: Record<string, string>;
}
) => string[];
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `from` the source file or directory
- `to` the destination file or directory
- `opts.filter` a function to determine whether a file or directory should be copied
- `opts.replace` a map of strings to replace
- <span class="tag">returns</span> an array of files that were copied
</div>
Copy a file or directory.
</div>
</div>
<div class="ts-block-property">
```dts
compress: (directory: string) => Promise<void>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `directory` The directory containing the files to be compressed
</div>
Compress files in `directory` with gzip and brotli, where appropriate. Generates `.gz` and `.br` files alongside the originals.
</div>
</div></div>
## ClientInit
<blockquote class="since note">
Available since 2.10.0
</blockquote>
The [`init`](/docs/kit/hooks#Shared-hooks-init) will be invoked once the app starts in the browser
<div class="ts-block">
```dts
type ClientInit = () => MaybePromise<void>;
```
</div>
## Config
See the [configuration reference](/docs/kit/configuration) for details.
## Cookies
<div class="ts-block">
```dts
interface Cookies {/*…*/}
```
<div class="ts-block-property">
```dts
get: (name: string, opts?: import('cookie').CookieParseOptions) => string | undefined;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `name` the name of the cookie
- `opts` the options, passed directly to `cookie.parse`. See documentation [here](https://github.com/jshttp/cookie#cookieparsestr-options)
</div>
Gets a cookie that was previously set with `cookies.set`, or from the request headers.
</div>
</div>
<div class="ts-block-property">
```dts
getAll: (opts?: import('cookie').CookieParseOptions) => Array<{ name: string; value: string }>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `opts` the options, passed directly to `cookie.parse`. See documentation [here](https://github.com/jshttp/cookie#cookieparsestr-options)
</div>
Gets all cookies that were previously set with `cookies.set`, or from the request headers.
</div>
</div>
<div class="ts-block-property">
```dts
set: (
name: string,
value: string,
opts: import('cookie').CookieSerializeOptions & { path: string }
) => void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `name` the name of the cookie
- `value` the cookie value
- `opts` the options, passed directly to `cookie.serialize`. See documentation [here](https://github.com/jshttp/cookie#cookieserializename-value-options)
</div>
Sets a cookie. This will add a `set-cookie` header to the response, but also make the cookie available via `cookies.get` or `cookies.getAll` during the current request.
The `httpOnly` and `secure` options are `true` by default (except on http://localhost, where `secure` is `false`), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The `sameSite` option defaults to `lax`.
You must specify a `path` for the cookie. In most cases you should explicitly set `path: '/'` to make the cookie available throughout your app. You can use relative paths, or set `path: ''` to make the cookie only available on the current path and its children
</div>
</div>
<div class="ts-block-property">
```dts
delete: (name: string, opts: import('cookie').CookieSerializeOptions & { path: string }) => void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `name` the name of the cookie
- `opts` the options, passed directly to `cookie.serialize`. The `path` must match the path of the cookie you want to delete. See documentation [here](https://github.com/jshttp/cookie#cookieserializename-value-options)
</div>
Deletes a cookie by setting its value to an empty string and setting the expiry date in the past.
You must specify a `path` for the cookie. In most cases you should explicitly set `path: '/'` to make the cookie available throughout your app. You can use relative paths, or set `path: ''` to make the cookie only available on the current path and its children
</div>
</div>
<div class="ts-block-property">
```dts
serialize: (
name: string,
value: string,
opts: import('cookie').CookieSerializeOptions & { path: string }
) => string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `name` the name of the cookie
- `value` the cookie value
- `opts` the options, passed directly to `cookie.serialize`. See documentation [here](https://github.com/jshttp/cookie#cookieserializename-value-options)
</div>
Serialize a cookie name-value pair into a `Set-Cookie` header string, but don't apply it to the response.
The `httpOnly` and `secure` options are `true` by default (except on http://localhost, where `secure` is `false`), and must be explicitly disabled if you want cookies to be readable by client-side JavaScript and/or transmitted over HTTP. The `sameSite` option defaults to `lax`.
You must specify a `path` for the cookie. In most cases you should explicitly set `path: '/'` to make the cookie available throughout your app. You can use relative paths, or set `path: ''` to make the cookie only available on the current path and its children
</div>
</div></div>
## Emulator
A collection of functions that influence the environment during dev, build and prerendering
<div class="ts-block">
```dts
interface Emulator {/*…*/}
```
<div class="ts-block-property">
```dts
platform?(details: { config: any; prerender: PrerenderOption }): MaybePromise<App.Platform>;
```
<div class="ts-block-property-details">
A function that is called with the current route `config` and `prerender` option
and returns an `App.Platform` object
</div>
</div></div>
## Handle
The [`handle`](/docs/kit/hooks#Server-hooks-handle) hook runs every time the SvelteKit server receives a [request](/docs/kit/web-standards#Fetch-APIs-Request) and
determines the [response](/docs/kit/web-standards#Fetch-APIs-Response).
It receives an `event` object representing the request and a function called `resolve`, which renders the route and generates a `Response`.
This allows you to modify response headers or bodies, or bypass SvelteKit entirely (for implementing routes programmatically, for example).
<div class="ts-block">
```dts
type Handle = (input: {
event: RequestEvent;
resolve: (
event: RequestEvent,
opts?: ResolveOptions
) => MaybePromise<Response>;
}) => MaybePromise<Response>;
```
</div>
## HandleClientError
The client-side [`handleError`](/docs/kit/hooks#Shared-hooks-handleError) hook runs when an unexpected error is thrown while navigating.
If an unexpected error is thrown during loading or the following render, this function will be called with the error and the event.
Make sure that this function _never_ throws an error.
<div class="ts-block">
```dts
type HandleClientError = (input: {
error: unknown;
event: NavigationEvent;
status: number;
message: string;
}) => MaybePromise<void | App.Error>;
```
</div>
## HandleFetch
The [`handleFetch`](/docs/kit/hooks#Server-hooks-handleFetch) hook allows you to modify (or replace) a `fetch` request that happens inside a `load` function that runs on the server (or during pre-rendering)
<div class="ts-block">
```dts
type HandleFetch = (input: {
event: RequestEvent;
request: Request;
fetch: typeof fetch;
}) => MaybePromise<Response>;
```
</div>
## HandleServerError
The server-side [`handleError`](/docs/kit/hooks#Shared-hooks-handleError) hook runs when an unexpected error is thrown while responding to a request.
If an unexpected error is thrown during loading or rendering, this function will be called with the error and the event.
Make sure that this function _never_ throws an error.
<div class="ts-block">
```dts
type HandleServerError = (input: {
error: unknown;
event: RequestEvent;
status: number;
message: string;
}) => MaybePromise<void | App.Error>;
```
</div>
## HttpError
The object returned by the [`error`](/docs/kit/@sveltejs-kit#error) function.
<div class="ts-block">
```dts
interface HttpError {/*…*/}
```
<div class="ts-block-property">
```dts
status: number;
```
<div class="ts-block-property-details">
The [HTTP status code](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#client_error_responses), in the range 400-599.
</div>
</div>
<div class="ts-block-property">
```dts
body: App.Error;
```
<div class="ts-block-property-details">
The content of the error.
</div>
</div></div>
## KitConfig
See the [configuration reference](/docs/kit/configuration) for details.
## LessThan
<div class="ts-block">
```dts
type LessThan<
TNumber extends number,
TArray extends any[] = []
> = TNumber extends TArray['length']
? TArray[number]
: LessThan<TNumber, [...TArray, TArray['length']]>;
```
</div>
## Load
The generic form of `PageLoad` and `LayoutLoad`. You should import those from `./$types` (see [generated types](/docs/kit/types#Generated-types))
rather than using `Load` directly.
<div class="ts-block">
```dts
type Load<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
InputData extends Record<string, unknown> | null = Record<
string,
any
> | null,
ParentData extends Record<string, unknown> = Record<
string,
any
>,
OutputData extends Record<
string,
unknown
> | void = Record<string, any> | void,
RouteId extends string | null = string | null
> = (
event: LoadEvent<Params, InputData, ParentData, RouteId>
) => MaybePromise<OutputData>;
```
</div>
## LoadEvent
The generic form of `PageLoadEvent` and `LayoutLoadEvent`. You should import those from `./$types` (see [generated types](/docs/kit/types#Generated-types))
rather than using `LoadEvent` directly.
<div class="ts-block">
```dts
interface LoadEvent<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
Data extends Record<string, unknown> | null = Record<
string,
any
> | null,
ParentData extends Record<string, unknown> = Record<
string,
any
>,
RouteId extends string | null = string | null
> extends NavigationEvent<Params, RouteId> {/*…*/}
```
<div class="ts-block-property">
```dts
fetch: typeof fetch;
```
<div class="ts-block-property-details">
`fetch` is equivalent to the [native `fetch` web API](https://developer.mozilla.org/en-US/docs/Web/API/fetch), with a few additional features:
- It can be used to make credentialed requests on the server, as it inherits the `cookie` and `authorization` headers for the page request.
- It can make relative requests on the server (ordinarily, `fetch` requires a URL with an origin when used in a server context).
- Internal requests (e.g. for `+server.js` routes) go directly to the handler function when running on the server, without the overhead of an HTTP call.
- During server-side rendering, the response will be captured and inlined into the rendered HTML by hooking into the `text` and `json` methods of the `Response` object. Note that headers will _not_ be serialized, unless explicitly included via [`filterSerializedResponseHeaders`](/docs/kit/hooks#Server-hooks-handle)
- During hydration, the response will be read from the HTML, guaranteeing consistency and preventing an additional network request.
You can learn more about making credentialed requests with cookies [here](/docs/kit/load#Cookies)
</div>
</div>
<div class="ts-block-property">
```dts
data: Data;
```
<div class="ts-block-property-details">
Contains the data returned by the route's server `load` function (in `+layout.server.js` or `+page.server.js`), if any.
</div>
</div>
<div class="ts-block-property">
```dts
setHeaders: (headers: Record<string, string>) => void;
```
<div class="ts-block-property-details">
If you need to set headers for the response, you can do so using the this method. This is useful if you want the page to be cached, for example:
```js
// @errors: 7031
/// file: src/routes/blog/+page.js
export async function load({ fetch, setHeaders }) {
const url = `https://cms.example.com/articles.json`;
const response = await fetch(url);
setHeaders({
age: response.headers.get('age'),
'cache-control': response.headers.get('cache-control')
});
return response.json();
}
```
Setting the same header multiple times (even in separate `load` functions) is an error — you can only set a given header once.
You cannot add a `set-cookie` header with `setHeaders` — use the [`cookies`](/docs/kit/@sveltejs-kit#Cookies) API in a server-only `load` function instead.
`setHeaders` has no effect when a `load` function runs in the browser.
</div>
</div>
<div class="ts-block-property">
```dts
parent: () => Promise<ParentData>;
```
<div class="ts-block-property-details">
`await parent()` returns data from parent `+layout.js` `load` functions.
Implicitly, a missing `+layout.js` is treated as a `({ data }) => data` function, meaning that it will return and forward data from parent `+layout.server.js` files.
Be careful not to introduce accidental waterfalls when using `await parent()`. If for example you only want to merge parent data into the returned output, call it _after_ fetching your other data.
</div>
</div>
<div class="ts-block-property">
```dts
depends: (...deps: Array<`${string}:${string}`>) => void;
```
<div class="ts-block-property-details">
This function declares that the `load` function has a _dependency_ on one or more URLs or custom identifiers, which can subsequently be used with [`invalidate()`](/docs/kit/$app-navigation#invalidate) to cause `load` to rerun.
Most of the time you won't need this, as `fetch` calls `depends` on your behalf — it's only necessary if you're using a custom API client that bypasses `fetch`.
URLs can be absolute or relative to the page being loaded, and must be [encoded](https://developer.mozilla.org/en-US/docs/Glossary/percent-encoding).
Custom identifiers have to be prefixed with one or more lowercase letters followed by a colon to conform to the [URI specification](https://www.rfc-editor.org/rfc/rfc3986.html).
The following example shows how to use `depends` to register a dependency on a custom identifier, which is `invalidate`d after a button click, making the `load` function rerun.
```js
// @errors: 7031
/// file: src/routes/+page.js
let count = 0;
export async function load({ depends }) {
depends('increase:count');
return { count: count++ };
}
```
```html
/// file: src/routes/+page.svelte
<script>
import { invalidate } from '$app/navigation';
let { data } = $props();
const increase = async () => {
await invalidate('increase:count');
}
</script>
<p>{data.count}<p>
<button on:click={increase}>Increase Count</button>
```
</div>
</div>
<div class="ts-block-property">
```dts
untrack: <T>(fn: () => T) => T;
```
<div class="ts-block-property-details">
Use this function to opt out of dependency tracking for everything that is synchronously called within the callback. Example:
```js
// @errors: 7031
/// file: src/routes/+page.server.js
export async function load({ untrack, url }) {
// Untrack url.pathname so that path changes don't trigger a rerun
if (untrack(() => url.pathname === '/')) {
return { message: 'Welcome!' };
}
}
```
</div>
</div></div>
## LoadProperties
<div class="ts-block">
```dts
type LoadProperties<
input extends Record<string, any> | void
> = input extends void
? undefined // needs to be undefined, because void will break intellisense
: input extends Record<string, any>
? input
: unknown;
```
</div>
## Navigation
<div class="ts-block">
```dts
interface Navigation {/*…*/}
```
<div class="ts-block-property">
```dts
from: NavigationTarget | null;
```
<div class="ts-block-property-details">
Where navigation was triggered from
</div>
</div>
<div class="ts-block-property">
```dts
to: NavigationTarget | null;
```
<div class="ts-block-property-details">
Where navigation is going to/has gone to
</div>
</div>
<div class="ts-block-property">
```dts
type: Exclude<NavigationType, 'enter'>;
```
<div class="ts-block-property-details">
The type of navigation:
- `form`: The user submitted a `<form>`
- `leave`: The app is being left either because the tab is being closed or a navigation to a different document is occurring
- `link`: Navigation was triggered by a link click
- `goto`: Navigation was triggered by a `goto(...)` call or a redirect
- `popstate`: Navigation was triggered by back/forward navigation
</div>
</div>
<div class="ts-block-property">
```dts
willUnload: boolean;
```
<div class="ts-block-property-details">
Whether or not the navigation will result in the page being unloaded (i.e. not a client-side navigation)
</div>
</div>
<div class="ts-block-property">
```dts
delta?: number;
```
<div class="ts-block-property-details">
In case of a history back/forward navigation, the number of steps to go back/forward
</div>
</div>
<div class="ts-block-property">
```dts
complete: Promise<void>;
```
<div class="ts-block-property-details">
A promise that resolves once the navigation is complete, and rejects if the navigation
fails or is aborted. In the case of a `willUnload` navigation, the promise will never resolve
</div>
</div></div>
## NavigationEvent
<div class="ts-block">
```dts
interface NavigationEvent<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
RouteId extends string | null = string | null
> {/*…*/}
```
<div class="ts-block-property">
```dts
params: Params;
```
<div class="ts-block-property-details">
The parameters of the current page - e.g. for a route like `/blog/[slug]`, a `{ slug: string }` object
</div>
</div>
<div class="ts-block-property">
```dts
route: {/*…*/}
```
<div class="ts-block-property-details">
Info about the current route
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
id: RouteId;
```
<div class="ts-block-property-details">
The ID of the current route - e.g. for `src/routes/blog/[slug]`, it would be `/blog/[slug]`
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```dts
url: URL;
```
<div class="ts-block-property-details">
The URL of the current page
</div>
</div></div>
## NavigationTarget
Information about the target of a specific navigation.
<div class="ts-block">
```dts
interface NavigationTarget {/*…*/}
```
<div class="ts-block-property">
```dts
params: Record<string, string> | null;
```
<div class="ts-block-property-details">
Parameters of the target page - e.g. for a route like `/blog/[slug]`, a `{ slug: string }` object.
Is `null` if the target is not part of the SvelteKit app (could not be resolved to a route).
</div>
</div>
<div class="ts-block-property">
```dts
route: { id: string | null };
```
<div class="ts-block-property-details">
Info about the target route
</div>
</div>
<div class="ts-block-property">
```dts
url: URL;
```
<div class="ts-block-property-details">
The URL that is navigated to
</div>
</div></div>
## NavigationType
- `enter`: The app has hydrated
- `form`: The user submitted a `<form>` with a GET method
- `leave`: The user is leaving the app by closing the tab or using the back/forward buttons to go to a different document
- `link`: Navigation was triggered by a link click
- `goto`: Navigation was triggered by a `goto(...)` call or a redirect
- `popstate`: Navigation was triggered by back/forward navigation
<div class="ts-block">
```dts
type NavigationType =
| 'enter'
| 'form'
| 'leave'
| 'link'
| 'goto'
| 'popstate';
```
</div>
## NumericRange
<div class="ts-block">
```dts
type NumericRange<
TStart extends number,
TEnd extends number
> = Exclude<TEnd | LessThan<TEnd>, LessThan<TStart>>;
```
</div>
## OnNavigate
The argument passed to [`onNavigate`](/docs/kit/$app-navigation#onNavigate) callbacks.
<div class="ts-block">
```dts
interface OnNavigate extends Navigation {/*…*/}
```
<div class="ts-block-property">
```dts
type: Exclude<NavigationType, 'enter' | 'leave'>;
```
<div class="ts-block-property-details">
The type of navigation:
- `form`: The user submitted a `<form>`
- `link`: Navigation was triggered by a link click
- `goto`: Navigation was triggered by a `goto(...)` call or a redirect
- `popstate`: Navigation was triggered by back/forward navigation
</div>
</div>
<div class="ts-block-property">
```dts
willUnload: false;
```
<div class="ts-block-property-details">
Since `onNavigate` callbacks are called immediately before a client-side navigation, they will never be called with a navigation that unloads the page.
</div>
</div></div>
## Page
The shape of the [`page`](/docs/kit/$app-state#page) reactive object and the [`$page`](/docs/kit/$app-stores) store.
<div class="ts-block">
```dts
interface Page<
Params extends Record<string, string> = Record<
string,
string
>,
RouteId extends string | null = string | null
> {/*…*/}
```
<div class="ts-block-property">
```dts
url: URL;
```
<div class="ts-block-property-details">
The URL of the current page.
</div>
</div>
<div class="ts-block-property">
```dts
params: Params;
```
<div class="ts-block-property-details">
The parameters of the current page - e.g. for a route like `/blog/[slug]`, a `{ slug: string }` object.
</div>
</div>
<div class="ts-block-property">
```dts
route: {/*…*/}
```
<div class="ts-block-property-details">
Info about the current route.
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
id: RouteId;
```
<div class="ts-block-property-details">
The ID of the current route - e.g. for `src/routes/blog/[slug]`, it would be `/blog/[slug]`.
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```dts
status: number;
```
<div class="ts-block-property-details">
HTTP status code of the current page.
</div>
</div>
<div class="ts-block-property">
```dts
error: App.Error | null;
```
<div class="ts-block-property-details">
The error object of the current page, if any. Filled from the `handleError` hooks.
</div>
</div>
<div class="ts-block-property">
```dts
data: App.PageData & Record<string, any>;
```
<div class="ts-block-property-details">
The merged result of all data from all `load` functions on the current page. You can type a common denominator through `App.PageData`.
</div>
</div>
<div class="ts-block-property">
```dts
state: App.PageState;
```
<div class="ts-block-property-details">
The page state, which can be manipulated using the [`pushState`](/docs/kit/$app-navigation#pushState) and [`replaceState`](/docs/kit/$app-navigation#replaceState) functions from `$app/navigation`.
</div>
</div>
<div class="ts-block-property">
```dts
form: any;
```
<div class="ts-block-property-details">
Filled only after a form submission. See [form actions](/docs/kit/form-actions) for more info.
</div>
</div></div>
## ParamMatcher
The shape of a param matcher. See [matching](/docs/kit/advanced-routing#Matching) for more info.
<div class="ts-block">
```dts
type ParamMatcher = (param: string) => boolean;
```
</div>
## PrerenderOption
<div class="ts-block">
```dts
type PrerenderOption = boolean | 'auto';
```
</div>
## Redirect
The object returned by the [`redirect`](/docs/kit/@sveltejs-kit#redirect) function
<div class="ts-block">
```dts
interface Redirect {/*…*/}
```
<div class="ts-block-property">
```dts
status: 300 | 301 | 302 | 303 | 304 | 305 | 306 | 307 | 308;
```
<div class="ts-block-property-details">
The [HTTP status code](https://developer.mozilla.org/en-US/docs/Web/HTTP/Status#redirection_messages), in the range 300-308.
</div>
</div>
<div class="ts-block-property">
```dts
location: string;
```
<div class="ts-block-property-details">
The location to redirect to.
</div>
</div></div>
## RequestEvent
<div class="ts-block">
```dts
interface RequestEvent<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
RouteId extends string | null = string | null
> {/*…*/}
```
<div class="ts-block-property">
```dts
cookies: Cookies;
```
<div class="ts-block-property-details">
Get or set cookies related to the current request
</div>
</div>
<div class="ts-block-property">
```dts
fetch: typeof fetch;
```
<div class="ts-block-property-details">
`fetch` is equivalent to the [native `fetch` web API](https://developer.mozilla.org/en-US/docs/Web/API/fetch), with a few additional features:
- It can be used to make credentialed requests on the server, as it inherits the `cookie` and `authorization` headers for the page request.
- It can make relative requests on the server (ordinarily, `fetch` requires a URL with an origin when used in a server context).
- Internal requests (e.g. for `+server.js` routes) go directly to the handler function when running on the server, without the overhead of an HTTP call.
- During server-side rendering, the response will be captured and inlined into the rendered HTML by hooking into the `text` and `json` methods of the `Response` object. Note that headers will _not_ be serialized, unless explicitly included via [`filterSerializedResponseHeaders`](/docs/kit/hooks#Server-hooks-handle)
- During hydration, the response will be read from the HTML, guaranteeing consistency and preventing an additional network request.
You can learn more about making credentialed requests with cookies [here](/docs/kit/load#Cookies)
</div>
</div>
<div class="ts-block-property">
```dts
getClientAddress: () => string;
```
<div class="ts-block-property-details">
The client's IP address, set by the adapter.
</div>
</div>
<div class="ts-block-property">
```dts
locals: App.Locals;
```
<div class="ts-block-property-details">
Contains custom data that was added to the request within the [`server handle hook`](/docs/kit/hooks#Server-hooks-handle).
</div>
</div>
<div class="ts-block-property">
```dts
params: Params;
```
<div class="ts-block-property-details">
The parameters of the current route - e.g. for a route like `/blog/[slug]`, a `{ slug: string }` object
</div>
</div>
<div class="ts-block-property">
```dts
platform: Readonly<App.Platform> | undefined;
```
<div class="ts-block-property-details">
Additional data made available through the adapter.
</div>
</div>
<div class="ts-block-property">
```dts
request: Request;
```
<div class="ts-block-property-details">
The original request object
</div>
</div>
<div class="ts-block-property">
```dts
route: {/*…*/}
```
<div class="ts-block-property-details">
Info about the current route
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
id: RouteId;
```
<div class="ts-block-property-details">
The ID of the current route - e.g. for `src/routes/blog/[slug]`, it would be `/blog/[slug]`
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```dts
setHeaders: (headers: Record<string, string>) => void;
```
<div class="ts-block-property-details">
If you need to set headers for the response, you can do so using the this method. This is useful if you want the page to be cached, for example:
```js
// @errors: 7031
/// file: src/routes/blog/+page.js
export async function load({ fetch, setHeaders }) {
const url = `https://cms.example.com/articles.json`;
const response = await fetch(url);
setHeaders({
age: response.headers.get('age'),
'cache-control': response.headers.get('cache-control')
});
return response.json();
}
```
Setting the same header multiple times (even in separate `load` functions) is an error — you can only set a given header once.
You cannot add a `set-cookie` header with `setHeaders` — use the [`cookies`](/docs/kit/@sveltejs-kit#Cookies) API instead.
</div>
</div>
<div class="ts-block-property">
```dts
url: URL;
```
<div class="ts-block-property-details">
The requested URL.
</div>
</div>
<div class="ts-block-property">
```dts
isDataRequest: boolean;
```
<div class="ts-block-property-details">
`true` if the request comes from the client asking for `+page/layout.server.js` data. The `url` property will be stripped of the internal information
related to the data request in this case. Use this property instead if the distinction is important to you.
</div>
</div>
<div class="ts-block-property">
```dts
isSubRequest: boolean;
```
<div class="ts-block-property-details">
`true` for `+server.js` calls coming from SvelteKit without the overhead of actually making an HTTP request. This happens when you make same-origin `fetch` requests on the server.
</div>
</div></div>
## RequestHandler
A `(event: RequestEvent) => Response` function exported from a `+server.js` file that corresponds to an HTTP verb (`GET`, `PUT`, `PATCH`, etc) and handles requests with that method.
It receives `Params` as the first generic argument, which you can skip by using [generated types](/docs/kit/types#Generated-types) instead.
<div class="ts-block">
```dts
type RequestHandler<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
RouteId extends string | null = string | null
> = (
event: RequestEvent<Params, RouteId>
) => MaybePromise<Response>;
```
</div>
## Reroute
<blockquote class="since note">
Available since 2.3.0
</blockquote>
The [`reroute`](/docs/kit/hooks#Universal-hooks-reroute) hook allows you to modify the URL before it is used to determine which route to render.
<div class="ts-block">
```dts
type Reroute = (event: { url: URL }) => void | string;
```
</div>
## ResolveOptions
<div class="ts-block">
```dts
interface ResolveOptions {/*…*/}
```
<div class="ts-block-property">
```dts
transformPageChunk?: (input: { html: string; done: boolean }) => MaybePromise<string | undefined>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `input` the html chunk and the info if this is the last chunk
</div>
Applies custom transforms to HTML. If `done` is true, it's the final chunk. Chunks are not guaranteed to be well-formed HTML
(they could include an element's opening tag but not its closing tag, for example)
but they will always be split at sensible boundaries such as `%sveltekit.head%` or layout/page components.
</div>
</div>
<div class="ts-block-property">
```dts
filterSerializedResponseHeaders?: (name: string, value: string) => boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `name` header name
- `value` header value
</div>
Determines which headers should be included in serialized responses when a `load` function loads a resource with `fetch`.
By default, none will be included.
</div>
</div>
<div class="ts-block-property">
```dts
preload?: (input: { type: 'font' | 'css' | 'js' | 'asset'; path: string }) => boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- `input` the type of the file and its path
</div>
Determines what should be added to the `<head>` tag to preload it.
By default, `js` and `css` files will be preloaded.
</div>
</div></div>
## RouteDefinition
<div class="ts-block">
```dts
interface RouteDefinition<Config = any> {/*…*/}
```
<div class="ts-block-property">
```dts
id: string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
api: {
methods: Array<HttpMethod | '*'>;
};
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
page: {
methods: Array<Extract<HttpMethod, 'GET' | 'POST'>>;
};
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
pattern: RegExp;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
prerender: PrerenderOption;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
segments: RouteSegment[];
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
methods: Array<HttpMethod | '*'>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
config: Config;
```
<div class="ts-block-property-details"></div>
</div></div>
## SSRManifest
<div class="ts-block">
```dts
interface SSRManifest {/*…*/}
```
<div class="ts-block-property">
```dts
appDir: string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
appPath: string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
assets: Set<string>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
mimeTypes: Record<string, string>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
_: {/*…*/}
```
<div class="ts-block-property-details">
private fields
<div class="ts-block-property-children"><div class="ts-block-property">
```dts
client: NonNullable<BuildData['client']>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
nodes: SSRNodeLoader[];
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
routes: SSRRoute[];
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
prerendered_routes: Set<string>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
matchers: () => Promise<Record<string, ParamMatcher>>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
server_assets: Record<string, number>;
```
<div class="ts-block-property-details">
A `[file]: size` map of all assets imported by server code
</div>
</div></div>
</div>
</div></div>
## ServerInit
<blockquote class="since note">
Available since 2.10.0
</blockquote>
The [`init`](/docs/kit/hooks#Shared-hooks-init) will be invoked before the server responds to its first request
<div class="ts-block">
```dts
type ServerInit = () => MaybePromise<void>;
```
</div>
## ServerInitOptions
<div class="ts-block">
```dts
interface ServerInitOptions {/*…*/}
```
<div class="ts-block-property">
```dts
env: Record<string, string>;
```
<div class="ts-block-property-details">
A map of environment variables
</div>
</div>
<div class="ts-block-property">
```dts
read?: (file: string) => ReadableStream;
```
<div class="ts-block-property-details">
A function that turns an asset filename into a `ReadableStream`. Required for the `read` export from `$app/server` to work
</div>
</div></div>
## ServerLoad
The generic form of `PageServerLoad` and `LayoutServerLoad`. You should import those from `./$types` (see [generated types](/docs/kit/types#Generated-types))
rather than using `ServerLoad` directly.
<div class="ts-block">
```dts
type ServerLoad<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
ParentData extends Record<string, any> = Record<
string,
any
>,
OutputData extends Record<string, any> | void = Record<
string,
any
> | void,
RouteId extends string | null = string | null
> = (
event: ServerLoadEvent<Params, ParentData, RouteId>
) => MaybePromise<OutputData>;
```
</div>
## ServerLoadEvent
<div class="ts-block">
```dts
interface ServerLoadEvent<
Params extends Partial<Record<string, string>> = Partial<
Record<string, string>
>,
ParentData extends Record<string, any> = Record<
string,
any
>,
RouteId extends string | null = string | null
> extends RequestEvent<Params, RouteId> {/*…*/}
```
<div class="ts-block-property">
```dts
parent: () => Promise<ParentData>;
```
<div class="ts-block-property-details">
`await parent()` returns data from parent `+layout.server.js` `load` functions.
Be careful not to introduce accidental waterfalls when using `await parent()`. If for example you only want to merge parent data into the returned output, call it _after_ fetching your other data.
</div>
</div>
<div class="ts-block-property">
```dts
depends: (...deps: string[]) => void;
```
<div class="ts-block-property-details">
This function declares that the `load` function has a _dependency_ on one or more URLs or custom identifiers, which can subsequently be used with [`invalidate()`](/docs/kit/$app-navigation#invalidate) to cause `load` to rerun.
Most of the time you won't need this, as `fetch` calls `depends` on your behalf — it's only necessary if you're using a custom API client that bypasses `fetch`.
URLs can be absolute or relative to the page being loaded, and must be [encoded](https://developer.mozilla.org/en-US/docs/Glossary/percent-encoding).
Custom identifiers have to be prefixed with one or more lowercase letters followed by a colon to conform to the [URI specification](https://www.rfc-editor.org/rfc/rfc3986.html).
The following example shows how to use `depends` to register a dependency on a custom identifier, which is `invalidate`d after a button click, making the `load` function rerun.
```js
// @errors: 7031
/// file: src/routes/+page.js
let count = 0;
export async function load({ depends }) {
depends('increase:count');
return { count: count++ };
}
```
```html
/// file: src/routes/+page.svelte
<script>
import { invalidate } from '$app/navigation';
let { data } = $props();
const increase = async () => {
await invalidate('increase:count');
}
</script>
<p>{data.count}<p>
<button on:click={increase}>Increase Count</button>
```
</div>
</div>
<div class="ts-block-property">
```dts
untrack: <T>(fn: () => T) => T;
```
<div class="ts-block-property-details">
Use this function to opt out of dependency tracking for everything that is synchronously called within the callback. Example:
```js
// @errors: 7031
/// file: src/routes/+page.js
export async function load({ untrack, url }) {
// Untrack url.pathname so that path changes don't trigger a rerun
if (untrack(() => url.pathname === '/')) {
return { message: 'Welcome!' };
}
}
```
</div>
</div></div>
## Snapshot
The type of `export const snapshot` exported from a page or layout component.
<div class="ts-block">
```dts
interface Snapshot<T = any> {/*…*/}
```
<div class="ts-block-property">
```dts
capture: () => T;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
restore: (snapshot: T) => void;
```
<div class="ts-block-property-details"></div>
</div></div>
## SubmitFunction
<div class="ts-block">
```dts
type SubmitFunction<
Success extends
| Record<string, unknown>
| undefined = Record<string, any>,
Failure extends
| Record<string, unknown>
| undefined = Record<string, any>
> = (input: {
action: URL;
formData: FormData;
formElement: HTMLFormElement;
controller: AbortController;
submitter: HTMLElement | null;
cancel: () => void;
}) => MaybePromise<
| void
| ((opts: {
formData: FormData;
formElement: HTMLFormElement;
action: URL;
result: ActionResult<Success, Failure>;
/**
* Call this to get the default behavior of a form submission response.
* @param options Set `reset: false` if you don't want the `<form>` values to be reset after a successful submission.
* @param invalidateAll Set `invalidateAll: false` if you don't want the action to call `invalidateAll` after submission.
*/
update: (options?: {
reset?: boolean;
invalidateAll?: boolean;
}) => Promise<void>;
}) => void)
>;
```
</div>
## Transport
<blockquote class="since note">
Available since 2.11.0
</blockquote>
The [`transport`](/docs/kit/hooks#Universal-hooks-transport) hook allows you to transport custom types across the server/client boundary.
Each transporter has a pair of `encode` and `decode` functions. On the server, `encode` determines whether a value is an instance of the custom type and, if so, returns a non-falsy encoding of the value which can be an object or an array (or `false` otherwise).
In the browser, `decode` turns the encoding back into an instance of the custom type.
```ts
import type { Transport } from '@sveltejs/kit';
declare class MyCustomType {
data: any
}
// hooks.js
export const transport: Transport = {
MyCustomType: {
encode: (value) => value instanceof MyCustomType && [value.data],
decode: ([data]) => new MyCustomType(data)
}
};
```
<div class="ts-block">
```dts
type Transport = Record<string, Transporter>;
```
</div>
## Transporter
A member of the [`transport`](/docs/kit/hooks#Universal-hooks-transport) hook.
<div class="ts-block">
```dts
interface Transporter<
T = any,
U = Exclude<
any,
false | 0 | '' | null | undefined | typeof NaN
>
> {/*…*/}
```
<div class="ts-block-property">
```dts
encode: (value: T) => false | U;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
decode: (data: U) => T;
```
<div class="ts-block-property-details"></div>
</div></div>
## Private types
The following are referenced by the public types documented above, but cannot be imported directly:
## AdapterEntry
<div class="ts-block">
```dts
interface AdapterEntry {/*…*/}
```
<div class="ts-block-property">
```dts
id: string;
```
<div class="ts-block-property-details">
A string that uniquely identifies an HTTP service (e.g. serverless function) and is used for deduplication.
For example, `/foo/a-[b]` and `/foo/[c]` are different routes, but would both
be represented in a Netlify _redirects file as `/foo/:param`, so they share an ID
</div>
</div>
<div class="ts-block-property">
```dts
filter(route: RouteDefinition): boolean;
```
<div class="ts-block-property-details">
A function that compares the candidate route with the current route to determine
if it should be grouped with the current route.
Use cases:
- Fallback pages: `/foo/[c]` is a fallback for `/foo/a-[b]`, and `/[...catchall]` is a fallback for all routes
- Grouping routes that share a common `config`: `/foo` should be deployed to the edge, `/bar` and `/baz` should be deployed to a serverless function
</div>
</div>
<div class="ts-block-property">
```dts
complete(entry: { generateManifest(opts: { relativePath: string }): string }): MaybePromise<void>;
```
<div class="ts-block-property-details">
A function that is invoked once the entry has been created. This is where you
should write the function to the filesystem and generate redirect manifests.
</div>
</div></div>
## Csp
<div class="ts-block">
```dts
namespace Csp {
type ActionSource = 'strict-dynamic' | 'report-sample';
type BaseSource =
| 'self'
| 'unsafe-eval'
| 'unsafe-hashes'
| 'unsafe-inline'
| 'wasm-unsafe-eval'
| 'none';
type CryptoSource =
`${'nonce' | 'sha256' | 'sha384' | 'sha512'}-${string}`;
type FrameSource =
| HostSource
| SchemeSource
| 'self'
| 'none';
type HostNameScheme = `${string}.${string}` | 'localhost';
type HostSource =
`${HostProtocolSchemes}${HostNameScheme}${PortScheme}`;
type HostProtocolSchemes = `${string}://` | '';
type HttpDelineator = '/' | '?' | '#' | '\\';
type PortScheme = `:${number}` | '' | ':*';
type SchemeSource =
| 'http:'
| 'https:'
| 'data:'
| 'mediastream:'
| 'blob:'
| 'filesystem:';
type Source =
| HostSource
| SchemeSource
| CryptoSource
| BaseSource;
type Sources = Source[];
}
```
</div>
## CspDirectives
<div class="ts-block">
```dts
interface CspDirectives {/*…*/}
```
<div class="ts-block-property">
```dts
'child-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'default-src'?: Array<Csp.Source | Csp.ActionSource>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'frame-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'worker-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'connect-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'font-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'img-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'manifest-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'media-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'object-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'prefetch-src'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'script-src'?: Array<Csp.Source | Csp.ActionSource>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'script-src-elem'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'script-src-attr'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'style-src'?: Array<Csp.Source | Csp.ActionSource>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'style-src-elem'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'style-src-attr'?: Csp.Sources;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'base-uri'?: Array<Csp.Source | Csp.ActionSource>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
sandbox?: Array<
| 'allow-downloads-without-user-activation'
| 'allow-forms'
| 'allow-modals'
| 'allow-orientation-lock'
| 'allow-pointer-lock'
| 'allow-popups'
| 'allow-popups-to-escape-sandbox'
| 'allow-presentation'
| 'allow-same-origin'
| 'allow-scripts'
| 'allow-storage-access-by-user-activation'
| 'allow-top-navigation'
| 'allow-top-navigation-by-user-activation'
>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'form-action'?: Array<Csp.Source | Csp.ActionSource>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'frame-ancestors'?: Array<Csp.HostSource | Csp.SchemeSource | Csp.FrameSource>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'navigate-to'?: Array<Csp.Source | Csp.ActionSource>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'report-uri'?: string[];
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'report-to'?: string[];
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'require-trusted-types-for'?: Array<'script'>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'trusted-types'?: Array<'none' | 'allow-duplicates' | '*' | string>;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'upgrade-insecure-requests'?: boolean;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
'require-sri-for'?: Array<'script' | 'style' | 'script style'>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span>
</div>
</div>
</div>
<div class="ts-block-property">
```dts
'block-all-mixed-content'?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span>
</div>
</div>
</div>
<div class="ts-block-property">
```dts
'plugin-types'?: Array<`${string}/${string}` | 'none'>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span>
</div>
</div>
</div>
<div class="ts-block-property">
```dts
referrer?: Array<
| 'no-referrer'
| 'no-referrer-when-downgrade'
| 'origin'
| 'origin-when-cross-origin'
| 'same-origin'
| 'strict-origin'
| 'strict-origin-when-cross-origin'
| 'unsafe-url'
| 'none'
>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag deprecated">deprecated</span>
</div>
</div>
</div></div>
## HttpMethod
<div class="ts-block">
```dts
type HttpMethod =
| 'GET'
| 'HEAD'
| 'POST'
| 'PUT'
| 'DELETE'
| 'PATCH'
| 'OPTIONS';
```
</div>
## Logger
<div class="ts-block">
```dts
interface Logger {/*…*/}
```
<div class="ts-block-property">
```dts
(msg: string): void;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
success(msg: string): void;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
error(msg: string): void;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
warn(msg: string): void;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
minor(msg: string): void;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
info(msg: string): void;
```
<div class="ts-block-property-details"></div>
</div></div>
## MaybePromise
<div class="ts-block">
```dts
type MaybePromise<T> = T | Promise<T>;
```
</div>
## PrerenderEntryGeneratorMismatchHandler
<div class="ts-block">
```dts
interface PrerenderEntryGeneratorMismatchHandler {/*…*/}
```
<div class="ts-block-property">
```dts
(details: { generatedFromId: string; entry: string; matchedId: string; message: string }): void;
```
<div class="ts-block-property-details"></div>
</div></div>
## PrerenderEntryGeneratorMismatchHandlerValue
<div class="ts-block">
```dts
type PrerenderEntryGeneratorMismatchHandlerValue =
| 'fail'
| 'warn'
| 'ignore'
| PrerenderEntryGeneratorMismatchHandler;
```
</div>
## PrerenderHttpErrorHandler
<div class="ts-block">
```dts
interface PrerenderHttpErrorHandler {/*…*/}
```
<div class="ts-block-property">
```dts
(details: {
status: number;
path: string;
referrer: string | null;
referenceType: 'linked' | 'fetched';
message: string;
}): void;
```
<div class="ts-block-property-details"></div>
</div></div>
## PrerenderHttpErrorHandlerValue
<div class="ts-block">
```dts
type PrerenderHttpErrorHandlerValue =
| 'fail'
| 'warn'
| 'ignore'
| PrerenderHttpErrorHandler;
```
</div>
## PrerenderMap
<div class="ts-block">
```dts
type PrerenderMap = Map<string, PrerenderOption>;
```
</div>
## PrerenderMissingIdHandler
<div class="ts-block">
```dts
interface PrerenderMissingIdHandler {/*…*/}
```
<div class="ts-block-property">
```dts
(details: { path: string; id: string; referrers: string[]; message: string }): void;
```
<div class="ts-block-property-details"></div>
</div></div>
## PrerenderMissingIdHandlerValue
<div class="ts-block">
```dts
type PrerenderMissingIdHandlerValue =
| 'fail'
| 'warn'
| 'ignore'
| PrerenderMissingIdHandler;
```
</div>
## PrerenderOption
<div class="ts-block">
```dts
type PrerenderOption = boolean | 'auto';
```
</div>
## Prerendered
<div class="ts-block">
```dts
interface Prerendered {/*…*/}
```
<div class="ts-block-property">
```dts
pages: Map<
string,
{
/** The location of the .html file relative to the output directory */
file: string;
}
>;
```
<div class="ts-block-property-details">
A map of `path` to `{ file }` objects, where a path like `/foo` corresponds to `foo.html` and a path like `/bar/` corresponds to `bar/index.html`.
</div>
</div>
<div class="ts-block-property">
```dts
assets: Map<
string,
{
/** The MIME type of the asset */
type: string;
}
>;
```
<div class="ts-block-property-details">
A map of `path` to `{ type }` objects.
</div>
</div>
<div class="ts-block-property">
```dts
redirects: Map<
string,
{
status: number;
location: string;
}
>;
```
<div class="ts-block-property-details">
A map of redirects encountered during prerendering.
</div>
</div>
<div class="ts-block-property">
```dts
paths: string[];
```
<div class="ts-block-property-details">
An array of prerendered paths (without trailing slashes, regardless of the trailingSlash config)
</div>
</div></div>
## RequestOptions
<div class="ts-block">
```dts
interface RequestOptions {/*…*/}
```
<div class="ts-block-property">
```dts
getClientAddress(): string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
platform?: App.Platform;
```
<div class="ts-block-property-details"></div>
</div></div>
## RouteSegment
<div class="ts-block">
```dts
interface RouteSegment {/*…*/}
```
<div class="ts-block-property">
```dts
content: string;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
dynamic: boolean;
```
<div class="ts-block-property-details"></div>
</div>
<div class="ts-block-property">
```dts
rest: boolean;
```
<div class="ts-block-property-details"></div>
</div></div>
## TrailingSlash
<div class="ts-block">
```dts
type TrailingSlash = 'never' | 'always' | 'ignore';
```
</div>
# @sveltejs/kit/hooks
```js
// @noErrors
import { sequence } from '@sveltejs/kit/hooks';
```
## sequence
A helper function for sequencing multiple `handle` calls in a middleware-like manner.
The behavior for the `handle` options is as follows:
- `transformPageChunk` is applied in reverse order and merged
- `preload` is applied in forward order, the first option "wins" and no `preload` options after it are called
- `filterSerializedResponseHeaders` behaves the same as `preload`
```js
// @errors: 7031
/// file: src/hooks.server.js
import { sequence } from '@sveltejs/kit/hooks';
/** @type {import('@sveltejs/kit').Handle} */
async function first({ event, resolve }) {
console.log('first pre-processing');
const result = await resolve(event, {
transformPageChunk: ({ html }) => {
// transforms are applied in reverse order
console.log('first transform');
return html;
},
preload: () => {
// this one wins as it's the first defined in the chain
console.log('first preload');
return true;
}
});
console.log('first post-processing');
return result;
}
/** @type {import('@sveltejs/kit').Handle} */
async function second({ event, resolve }) {
console.log('second pre-processing');
const result = await resolve(event, {
transformPageChunk: ({ html }) => {
console.log('second transform');
return html;
},
preload: () => {
console.log('second preload');
return true;
},
filterSerializedResponseHeaders: () => {
// this one wins as it's the first defined in the chain
console.log('second filterSerializedResponseHeaders');
return true;
}
});
console.log('second post-processing');
return result;
}
export const handle = sequence(first, second);
```
The example above would print:
```
first pre-processing
first preload
second pre-processing
second filterSerializedResponseHeaders
second transform
first transform
second post-processing
first post-processing
```
<div class="ts-block">
```dts
function sequence(
...handlers: import('@sveltejs/kit').Handle[]
): import('@sveltejs/kit').Handle;
```
</div>
# @sveltejs/kit/node/polyfills
```js
// @noErrors
import { installPolyfills } from '@sveltejs/kit/node/polyfills';
```
## installPolyfills
Make various web APIs available as globals:
- `crypto`
- `File`
<div class="ts-block">
```dts
function installPolyfills(): void;
```
</div>
# @sveltejs/kit/node
```js
// @noErrors
import {
createReadableStream,
getRequest,
setResponse
} from '@sveltejs/kit/node';
```
## createReadableStream
<blockquote class="since note">
Available since 2.4.0
</blockquote>
Converts a file on disk to a readable stream
<div class="ts-block">
```dts
function createReadableStream(file: string): ReadableStream;
```
</div>
## getRequest
<div class="ts-block">
```dts
function getRequest({
request,
base,
bodySizeLimit
}: {
request: import('http').IncomingMessage;
base: string;
bodySizeLimit?: number;
}): Promise<Request>;
```
</div>
## setResponse
<div class="ts-block">
```dts
function setResponse(
res: import('http').ServerResponse,
response: Response
): Promise<void>;
```
</div>
# @sveltejs/kit/vite
```js
// @noErrors
import { sveltekit } from '@sveltejs/kit/vite';
```
## sveltekit
Returns the SvelteKit Vite plugins.
<div class="ts-block">
```dts
function sveltekit(): Promise<import('vite').Plugin[]>;
```
</div>
# $app/environment
```js
// @noErrors
import { browser, building, dev, version } from '$app/environment';
```
## browser
`true` if the app is running in the browser.
<div class="ts-block">
```dts
const browser: boolean;
```
</div>
## building
SvelteKit analyses your app during the `build` step by running it. During this process, `building` is `true`. This also applies during prerendering.
<div class="ts-block">
```dts
const building: boolean;
```
</div>
## dev
Whether the dev server is running. This is not guaranteed to correspond to `NODE_ENV` or `MODE`.
<div class="ts-block">
```dts
const dev: boolean;
```
</div>
## version
The value of `config.kit.version.name`.
<div class="ts-block">
```dts
const version: string;
```
</div>
# $app/forms
```js
// @noErrors
import { applyAction, deserialize, enhance } from '$app/forms';
```
## applyAction
This action updates the `form` property of the current page with the given data and updates `page.status`.
In case of an error, it redirects to the nearest error page.
<div class="ts-block">
```dts
function applyAction<
Success extends Record<string, unknown> | undefined,
Failure extends Record<string, unknown> | undefined
>(
result: import('@sveltejs/kit').ActionResult<
Success,
Failure
>
): Promise<void>;
```
</div>
## deserialize
Use this function to deserialize the response from a form submission.
Usage:
```js
// @errors: 7031
import { deserialize } from '$app/forms';
async function handleSubmit(event) {
const response = await fetch('/form?/action', {
method: 'POST',
body: new FormData(event.target)
});
const result = deserialize(await response.text());
// ...
}
```
<div class="ts-block">
```dts
function deserialize<
Success extends Record<string, unknown> | undefined,
Failure extends Record<string, unknown> | undefined
>(
result: string
): import('@sveltejs/kit').ActionResult<Success, Failure>;
```
</div>
## enhance
This action enhances a `<form>` element that otherwise would work without JavaScript.
The `submit` function is called upon submission with the given FormData and the `action` that should be triggered.
If `cancel` is called, the form will not be submitted.
You can use the abort `controller` to cancel the submission in case another one starts.
If a function is returned, that function is called with the response from the server.
If nothing is returned, the fallback will be used.
If this function or its return value isn't set, it
- falls back to updating the `form` prop with the returned data if the action is on the same page as the form
- updates `page.status`
- resets the `<form>` element and invalidates all data in case of successful submission with no redirect response
- redirects in case of a redirect response
- redirects to the nearest error page in case of an unexpected error
If you provide a custom function with a callback and want to use the default behavior, invoke `update` in your callback.
It accepts an options object
- `reset: false` if you don't want the `<form>` values to be reset after a successful submission
- `invalidateAll: false` if you don't want the action to call `invalidateAll` after submission
<div class="ts-block">
```dts
function enhance<
Success extends Record<string, unknown> | undefined,
Failure extends Record<string, unknown> | undefined
>(
form_element: HTMLFormElement,
submit?: import('@sveltejs/kit').SubmitFunction<
Success,
Failure
>
): {
destroy(): void;
};
```
</div>
# $app/navigation
```js
// @noErrors
import {
afterNavigate,
beforeNavigate,
disableScrollHandling,
goto,
invalidate,
invalidateAll,
onNavigate,
preloadCode,
preloadData,
pushState,
replaceState
} from '$app/navigation';
```
## afterNavigate
A lifecycle function that runs the supplied `callback` when the current component mounts, and also whenever we navigate to a URL.
`afterNavigate` must be called during a component initialization. It remains active as long as the component is mounted.
<div class="ts-block">
```dts
function afterNavigate(
callback: (
navigation: import('@sveltejs/kit').AfterNavigate
) => void
): void;
```
</div>
## beforeNavigate
A navigation interceptor that triggers before we navigate to a URL, whether by clicking a link, calling `goto(...)`, or using the browser back/forward controls.
Calling `cancel()` will prevent the navigation from completing. If `navigation.type === 'leave'` — meaning the user is navigating away from the app (or closing the tab) — calling `cancel` will trigger the native browser unload confirmation dialog. In this case, the navigation may or may not be cancelled depending on the user's response.
When a navigation isn't to a SvelteKit-owned route (and therefore controlled by SvelteKit's client-side router), `navigation.to.route.id` will be `null`.
If the navigation will (if not cancelled) cause the document to unload — in other words `'leave'` navigations and `'link'` navigations where `navigation.to.route === null` — `navigation.willUnload` is `true`.
`beforeNavigate` must be called during a component initialization. It remains active as long as the component is mounted.
<div class="ts-block">
```dts
function beforeNavigate(
callback: (
navigation: import('@sveltejs/kit').BeforeNavigate
) => void
): void;
```
</div>
## disableScrollHandling
If called when the page is being updated following a navigation (in `onMount` or `afterNavigate` or an action, for example), this disables SvelteKit's built-in scroll handling.
This is generally discouraged, since it breaks user expectations.
<div class="ts-block">
```dts
function disableScrollHandling(): void;
```
</div>
## goto
Allows you to navigate programmatically to a given route, with options such as keeping the current element focused.
Returns a Promise that resolves when SvelteKit navigates (or fails to navigate, in which case the promise rejects) to the specified `url`.
For external URLs, use `window.location = url` instead of calling `goto(url)`.
<div class="ts-block">
```dts
function goto(
url: string | URL,
opts?:
| {
replaceState?: boolean | undefined;
noScroll?: boolean | undefined;
keepFocus?: boolean | undefined;
invalidateAll?: boolean | undefined;
invalidate?:
| (string | URL | ((url: URL) => boolean))[]
| undefined;
state?: App.PageState | undefined;
}
| undefined
): Promise<void>;
```
</div>
## invalidate
Causes any `load` functions belonging to the currently active page to re-run if they depend on the `url` in question, via `fetch` or `depends`. Returns a `Promise` that resolves when the page is subsequently updated.
If the argument is given as a `string` or `URL`, it must resolve to the same URL that was passed to `fetch` or `depends` (including query parameters).
To create a custom identifier, use a string beginning with `[a-z]+:` (e.g. `custom:state`) — this is a valid URL.
The `function` argument can be used define a custom predicate. It receives the full `URL` and causes `load` to rerun if `true` is returned.
This can be useful if you want to invalidate based on a pattern instead of a exact match.
```ts
// Example: Match '/path' regardless of the query parameters
import { invalidate } from '$app/navigation';
invalidate((url) => url.pathname === '/path');
```
<div class="ts-block">
```dts
function invalidate(
resource: string | URL | ((url: URL) => boolean)
): Promise<void>;
```
</div>
## invalidateAll
Causes all `load` functions belonging to the currently active page to re-run. Returns a `Promise` that resolves when the page is subsequently updated.
<div class="ts-block">
```dts
function invalidateAll(): Promise<void>;
```
</div>
## onNavigate
A lifecycle function that runs the supplied `callback` immediately before we navigate to a new URL except during full-page navigations.
If you return a `Promise`, SvelteKit will wait for it to resolve before completing the navigation. This allows you to — for example — use `document.startViewTransition`. Avoid promises that are slow to resolve, since navigation will appear stalled to the user.
If a function (or a `Promise` that resolves to a function) is returned from the callback, it will be called once the DOM has updated.
`onNavigate` must be called during a component initialization. It remains active as long as the component is mounted.
<div class="ts-block">
```dts
function onNavigate(
callback: (
navigation: import('@sveltejs/kit').OnNavigate
) => MaybePromise<(() => void) | void>
): void;
```
</div>
## preloadCode
Programmatically imports the code for routes that haven't yet been fetched.
Typically, you might call this to speed up subsequent navigation.
You can specify routes by any matching pathname such as `/about` (to match `src/routes/about/+page.svelte`) or `/blog/*` (to match `src/routes/blog/[slug]/+page.svelte`).
Unlike `preloadData`, this won't call `load` functions.
Returns a Promise that resolves when the modules have been imported.
<div class="ts-block">
```dts
function preloadCode(pathname: string): Promise<void>;
```
</div>
## preloadData
Programmatically preloads the given page, which means
1. ensuring that the code for the page is loaded, and
2. calling the page's load function with the appropriate options.
This is the same behaviour that SvelteKit triggers when the user taps or mouses over an `<a>` element with `data-sveltekit-preload-data`.
If the next navigation is to `href`, the values returned from load will be used, making navigation instantaneous.
Returns a Promise that resolves with the result of running the new route's `load` functions once the preload is complete.
<div class="ts-block">
```dts
function preloadData(href: string): Promise<
| {
type: 'loaded';
status: number;
data: Record<string, any>;
}
| {
type: 'redirect';
location: string;
}
>;
```
</div>
## pushState
Programmatically create a new history entry with the given `page.state`. To use the current URL, you can pass `''` as the first argument. Used for [shallow routing](/docs/kit/shallow-routing).
<div class="ts-block">
```dts
function pushState(
url: string | URL,
state: App.PageState
): void;
```
</div>
## replaceState
Programmatically replace the current history entry with the given `page.state`. To use the current URL, you can pass `''` as the first argument. Used for [shallow routing](/docs/kit/shallow-routing).
<div class="ts-block">
```dts
function replaceState(
url: string | URL,
state: App.PageState
): void;
```
</div>
# $app/paths
```js
// @noErrors
import { assets, base, resolveRoute } from '$app/paths';
```
## assets
An absolute path that matches [`config.kit.paths.assets`](/docs/kit/configuration#paths).
> [!NOTE] If a value for `config.kit.paths.assets` is specified, it will be replaced with `'/_svelte_kit_assets'` during `vite dev` or `vite preview`, since the assets don't yet live at their eventual URL.
<div class="ts-block">
```dts
let assets:
| ''
| `https://${string}`
| `http://${string}`
| '/_svelte_kit_assets';
```
</div>
## base
A string that matches [`config.kit.paths.base`](/docs/kit/configuration#paths).
Example usage: `<a href="{base}/your-page">Link</a>`
<div class="ts-block">
```dts
let base: '' | `/${string}`;
```
</div>
## resolveRoute
Populate a route ID with params to resolve a pathname.
```js
// @errors: 7031
import { resolveRoute } from '$app/paths';
resolveRoute(
`/blog/[slug]/[...somethingElse]`,
{
slug: 'hello-world',
somethingElse: 'something/else'
}
); // `/blog/hello-world/something/else`
```
<div class="ts-block">
```dts
function resolveRoute(
id: string,
params: Record<string, string | undefined>
): string;
```
</div>
# $app/server
```js
// @noErrors
import { read } from '$app/server';
```
## read
<blockquote class="since note">
Available since 2.4.0
</blockquote>
Read the contents of an imported asset from the filesystem
```js
// @errors: 7031
import { read } from '$app/server';
import somefile from './somefile.txt';
const asset = read(somefile);
const text = await asset.text();
```
<div class="ts-block">
```dts
function read(asset: string): Response;
```
</div>
# $app/state
SvelteKit makes three read-only state objects available via the `$app/state` module — `page`, `navigating` and `updated`.
> [!NOTE]
> This module was added in 2.12. If you're using an earlier version of SvelteKit, use [`$app/stores`]($app-stores) instead.
```js
// @noErrors
import { navigating, page, updated } from '$app/state';
```
## navigating
A read-only object representing an in-progress navigation, with `from`, `to`, `type` and (if `type === 'popstate'`) `delta` properties.
Values are `null` when no navigation is occurring, or during server rendering.
<div class="ts-block">
```dts
const navigating:
| import('@sveltejs/kit').Navigation
| {
from: null;
to: null;
type: null;
willUnload: null;
delta: null;
complete: null;
};
```
</div>
## page
A read-only reactive object with information about the current page, serving several use cases:
- retrieving the combined `data` of all pages/layouts anywhere in your component tree (also see [loading data](/docs/kit/load))
- retrieving the current value of the `form` prop anywhere in your component tree (also see [form actions](/docs/kit/form-actions))
- retrieving the page state that was set through `goto`, `pushState` or `replaceState` (also see [goto](/docs/kit/$app-navigation#goto) and [shallow routing](/docs/kit/shallow-routing))
- retrieving metadata such as the URL you're on, the current route and its parameters, and whether or not there was an error
```svelte
<!--- file: +layout.svelte --->
<script>
import { page } from '$app/state';
</script>
<p>Currently at {page.url.pathname}</p>
{#if page.error}
<span class="red">Problem detected</span>
{:else}
<span class="small">All systems operational</span>
{/if}
```
Changes to `page` are available exclusively with runes. (The legacy reactivity syntax will not reflect any changes)
```svelte
<!--- file: +page.svelte --->
<script>
import { page } from '$app/state';
const id = $derived(page.params.id); // This will correctly update id for usage on this page
$: badId = page.params.id; // Do not use; will never update after initial load
</script>
```
On the server, values can only be read during rendering (in other words _not_ in e.g. `load` functions). In the browser, the values can be read at any time.
<div class="ts-block">
```dts
const page: import('@sveltejs/kit').Page;
```
</div>
## updated
A read-only reactive value that's initially `false`. If [`version.pollInterval`](/docs/kit/configuration#version) is a non-zero value, SvelteKit will poll for new versions of the app and update `current` to `true` when it detects one. `updated.check()` will force an immediate check, regardless of polling.
<div class="ts-block">
```dts
const updated: {
get current(): boolean;
check(): Promise<boolean>;
};
```
</div>
# $app/stores
This module contains store-based equivalents of the exports from [`$app/state`]($app-state). If you're using SvelteKit 2.12 or later, use that module instead.
```js
// @noErrors
import { getStores, navigating, page, updated } from '$app/stores';
```
## getStores
<div class="ts-block">
```dts
function getStores(): {
page: typeof page;
navigating: typeof navigating;
updated: typeof updated;
};
```
</div>
## navigating
<blockquote class="tag deprecated note">
Use `navigating` from `$app/state` instead (requires Svelte 5, [see docs for more info](/docs/kit/migrating-to-sveltekit-2#SvelteKit-2.12:-$app-stores-deprecated))
</blockquote>
A readable store.
When navigating starts, its value is a `Navigation` object with `from`, `to`, `type` and (if `type === 'popstate'`) `delta` properties.
When navigating finishes, its value reverts to `null`.
On the server, this store can only be subscribed to during component initialization. In the browser, it can be subscribed to at any time.
<div class="ts-block">
```dts
const navigating: import('svelte/store').Readable<
import('@sveltejs/kit').Navigation | null
>;
```
</div>
## page
<blockquote class="tag deprecated note">
Use `page` from `$app/state` instead (requires Svelte 5, [see docs for more info](/docs/kit/migrating-to-sveltekit-2#SvelteKit-2.12:-$app-stores-deprecated))
</blockquote>
A readable store whose value contains page data.
On the server, this store can only be subscribed to during component initialization. In the browser, it can be subscribed to at any time.
<div class="ts-block">
```dts
const page: import('svelte/store').Readable<
import('@sveltejs/kit').Page
>;
```
</div>
## updated
<blockquote class="tag deprecated note">
Use `updated` from `$app/state` instead (requires Svelte 5, [see docs for more info](/docs/kit/migrating-to-sveltekit-2#SvelteKit-2.12:-$app-stores-deprecated))
</blockquote>
A readable store whose initial value is `false`. If [`version.pollInterval`](/docs/kit/configuration#version) is a non-zero value, SvelteKit will poll for new versions of the app and update the store value to `true` when it detects one. `updated.check()` will force an immediate check, regardless of polling.
On the server, this store can only be subscribed to during component initialization. In the browser, it can be subscribed to at any time.
<div class="ts-block">
```dts
const updated: import('svelte/store').Readable<boolean> & {
check(): Promise<boolean>;
};
```
</div>
# $env/dynamic/private
This module provides access to runtime environment variables, as defined by the platform you're running on. For example if you're using [`adapter-node`](https://github.com/sveltejs/kit/tree/main/packages/adapter-node) (or running [`vite preview`](/docs/kit/cli)), this is equivalent to `process.env`. This module only includes variables that _do not_ begin with [`config.kit.env.publicPrefix`](/docs/kit/configuration#env) _and do_ start with [`config.kit.env.privatePrefix`](/docs/kit/configuration#env) (if configured).
This module cannot be imported into client-side code.
Dynamic environment variables cannot be used during prerendering.
```ts
import { env } from '$env/dynamic/private';
console.log(env.DEPLOYMENT_SPECIFIC_VARIABLE);
```
> In `dev`, `$env/dynamic` always includes environment variables from `.env`. In `prod`, this behavior will depend on your adapter.
# $env/dynamic/public
Similar to [`$env/dynamic/private`](/docs/kit/$env-dynamic-private), but only includes variables that begin with [`config.kit.env.publicPrefix`](/docs/kit/configuration#env) (which defaults to `PUBLIC_`), and can therefore safely be exposed to client-side code.
Note that public dynamic environment variables must all be sent from the server to the client, causing larger network requests — when possible, use `$env/static/public` instead.
Dynamic environment variables cannot be used during prerendering.
```ts
import { env } from '$env/dynamic/public';
console.log(env.PUBLIC_DEPLOYMENT_SPECIFIC_VARIABLE);
```
# $env/static/private
Environment variables [loaded by Vite](https://vitejs.dev/guide/env-and-mode.html#env-files) from `.env` files and `process.env`. Like [`$env/dynamic/private`](/docs/kit/$env-dynamic-private), this module cannot be imported into client-side code. This module only includes variables that _do not_ begin with [`config.kit.env.publicPrefix`](/docs/kit/configuration#env) _and do_ start with [`config.kit.env.privatePrefix`](/docs/kit/configuration#env) (if configured).
_Unlike_ [`$env/dynamic/private`](/docs/kit/$env-dynamic-private), the values exported from this module are statically injected into your bundle at build time, enabling optimisations like dead code elimination.
```ts
import { API_KEY } from '$env/static/private';
```
Note that all environment variables referenced in your code should be declared (for example in an `.env` file), even if they don't have a value until the app is deployed:
```
MY_FEATURE_FLAG=""
```
You can override `.env` values from the command line like so:
```bash
MY_FEATURE_FLAG="enabled" npm run dev
```
# $env/static/public
Similar to [`$env/static/private`](/docs/kit/$env-static-private), except that it only includes environment variables that begin with [`config.kit.env.publicPrefix`](/docs/kit/configuration#env) (which defaults to `PUBLIC_`), and can therefore safely be exposed to client-side code.
Values are replaced statically at build time.
```ts
import { PUBLIC_BASE_URL } from '$env/static/public';
```
# $lib
SvelteKit automatically makes files under `src/lib` available using the `$lib` import alias. You can change which directory this alias points to in your [config file](configuration#files).
```svelte
<!--- file: src/lib/Component.svelte --->
A reusable component
```
```svelte
<!--- file: src/routes/+page.svelte --->
<script>
import Component from '$lib/Component.svelte';
</script>
<Component />
```
# $service-worker
```js
// @noErrors
import { base, build, files, prerendered, version } from '$service-worker';
```
This module is only available to [service workers](/docs/kit/service-workers).
## base
The `base` path of the deployment. Typically this is equivalent to `config.kit.paths.base`, but it is calculated from `location.pathname` meaning that it will continue to work correctly if the site is deployed to a subdirectory.
Note that there is a `base` but no `assets`, since service workers cannot be used if `config.kit.paths.assets` is specified.
<div class="ts-block">
```dts
const base: string;
```
</div>
## build
An array of URL strings representing the files generated by Vite, suitable for caching with `cache.addAll(build)`.
During development, this is an empty array.
<div class="ts-block">
```dts
const build: string[];
```
</div>
## files
An array of URL strings representing the files in your static directory, or whatever directory is specified by `config.kit.files.assets`. You can customize which files are included from `static` directory using [`config.kit.serviceWorker.files`](/docs/kit/configuration)
<div class="ts-block">
```dts
const files: string[];
```
</div>
## prerendered
An array of pathnames corresponding to prerendered pages and endpoints.
During development, this is an empty array.
<div class="ts-block">
```dts
const prerendered: string[];
```
</div>
## version
See [`config.kit.version`](/docs/kit/configuration#version). It's useful for generating unique cache names inside your service worker, so that a later deployment of your app can invalidate old caches.
<div class="ts-block">
```dts
const version: string;
```
</div>
# Configuration
Your project's configuration lives in a `svelte.config.js` file at the root of your project. As well as SvelteKit, this config object is used by other tooling that integrates with Svelte such as editor extensions.
```js
/// file: svelte.config.js
// @filename: ambient.d.ts
declare module '@sveltejs/adapter-auto' {
const plugin: () => import('@sveltejs/kit').Adapter;
export default plugin;
}
// @filename: index.js
// ---cut---
import adapter from '@sveltejs/adapter-auto';
/** @type {import('@sveltejs/kit').Config} */
const config = {
kit: {
adapter: adapter()
}
};
export default config;
```
## Config
<div class="ts-block">
```dts
interface Config {/*…*/}
```
<div class="ts-block-property">
```dts
compilerOptions?: CompileOptions;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `{}`
</div>
Options passed to [`svelte.compile`](/docs/svelte/svelte-compiler#CompileOptions).
</div>
</div>
<div class="ts-block-property">
```dts
extensions?: string[];
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `[".svelte"]`
</div>
List of file extensions that should be treated as Svelte files.
</div>
</div>
<div class="ts-block-property">
```dts
kit?: KitConfig;
```
<div class="ts-block-property-details">
SvelteKit options
</div>
</div>
<div class="ts-block-property">
```dts
preprocess?: any;
```
<div class="ts-block-property-details">
Preprocessor options, if any. Preprocessing can alternatively also be done through Vite's preprocessor capabilities.
</div>
</div>
<div class="ts-block-property">
```dts
vitePlugin?: PluginOptions;
```
<div class="ts-block-property-details">
`vite-plugin-svelte` plugin options.
</div>
</div>
<div class="ts-block-property">
```dts
[key: string]: any;
```
<div class="ts-block-property-details">
Any additional options required by tooling that integrates with Svelte.
</div>
</div></div>
## KitConfig
The `kit` property configures SvelteKit, and can have the following properties:
## adapter
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `undefined`
</div>
Your [adapter](/docs/kit/adapters) is run when executing `vite build`. It determines how the output is converted for different platforms.
<div class="ts-block-property-children">
</div>
## alias
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `{}`
</div>
An object containing zero or more aliases used to replace values in `import` statements. These aliases are automatically passed to Vite and TypeScript.
```js
// @errors: 7031
/// file: svelte.config.js
/** @type {import('@sveltejs/kit').Config} */
const config = {
kit: {
alias: {
// this will match a file
'my-file': 'path/to/my-file.js',
// this will match a directory and its contents
// (`my-directory/x` resolves to `path/to/my-directory/x`)
'my-directory': 'path/to/my-directory',
// an alias ending /* will only match
// the contents of a directory, not the directory itself
'my-directory/*': 'path/to/my-directory/*'
}
}
};
```
> [!NOTE] The built-in `$lib` alias is controlled by `config.kit.files.lib` as it is used for packaging.
> [!NOTE] You will need to run `npm run dev` to have SvelteKit automatically generate the required alias configuration in `jsconfig.json` or `tsconfig.json`.
<div class="ts-block-property-children">
</div>
## appDir
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"_app"`
</div>
The directory where SvelteKit keeps its stuff, including static assets (such as JS and CSS) and internally-used routes.
If `paths.assets` is specified, there will be two app directories — `${paths.assets}/${appDir}` and `${paths.base}/${appDir}`.
<div class="ts-block-property-children">
</div>
## csp
<div class="ts-block-property-bullets">
</div>
[Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy) configuration. CSP helps to protect your users against cross-site scripting (XSS) attacks, by limiting the places resources can be loaded from. For example, a configuration like this...
```js
// @errors: 7031
/// file: svelte.config.js
/** @type {import('@sveltejs/kit').Config} */
const config = {
kit: {
csp: {
directives: {
'script-src': ['self']
},
// must be specified with either the `report-uri` or `report-to` directives, or both
reportOnly: {
'script-src': ['self'],
'report-uri': ['/']
}
}
}
};
export default config;
```
...would prevent scripts loading from external sites. SvelteKit will augment the specified directives with nonces or hashes (depending on `mode`) for any inline styles and scripts it generates.
To add a nonce for scripts and links manually included in `src/app.html`, you may use the placeholder `%sveltekit.nonce%` (for example `<script nonce="%sveltekit.nonce%">`).
When pages are prerendered, the CSP header is added via a `<meta http-equiv>` tag (note that in this case, `frame-ancestors`, `report-uri` and `sandbox` directives will be ignored).
> [!NOTE] When `mode` is `'auto'`, SvelteKit will use nonces for dynamically rendered pages and hashes for prerendered pages. Using nonces with prerendered pages is insecure and therefore forbidden.
> [!NOTE] Note that most [Svelte transitions](/tutorial/svelte/transition) work by creating an inline `<style>` element. If you use these in your app, you must either leave the `style-src` directive unspecified or add `unsafe-inline`.
If this level of configuration is insufficient and you have more dynamic requirements, you can use the [`handle` hook](/docs/kit/hooks#Server-hooks-handle) to roll your own CSP.
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
mode?: 'hash' | 'nonce' | 'auto';
```
<div class="ts-block-property-details">
Whether to use hashes or nonces to restrict `<script>` and `<style>` elements. `'auto'` will use hashes for prerendered pages, and nonces for dynamically rendered pages.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
directives?: CspDirectives;
```
<div class="ts-block-property-details">
Directives that will be added to `Content-Security-Policy` headers.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
reportOnly?: CspDirectives;
```
<div class="ts-block-property-details">
Directives that will be added to `Content-Security-Policy-Report-Only` headers.
</div>
</div>
</div>
## csrf
<div class="ts-block-property-bullets">
</div>
Protection against [cross-site request forgery (CSRF)](https://owasp.org/www-community/attacks/csrf) attacks.
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
checkOrigin?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `true`
</div>
Whether to check the incoming `origin` header for `POST`, `PUT`, `PATCH`, or `DELETE` form submissions and verify that it matches the server's origin.
To allow people to make `POST`, `PUT`, `PATCH`, or `DELETE` requests with a `Content-Type` of `application/x-www-form-urlencoded`, `multipart/form-data`, or `text/plain` to your app from other origins, you will need to disable this option. Be careful!
</div>
</div>
</div>
## embedded
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `false`
</div>
Whether or not the app is embedded inside a larger app. If `true`, SvelteKit will add its event listeners related to navigation etc on the parent of `%sveltekit.body%` instead of `window`, and will pass `params` from the server rather than inferring them from `location.pathname`.
Note that it is generally not supported to embed multiple SvelteKit apps on the same page and use client-side SvelteKit features within them (things such as pushing to the history state assume a single instance).
<div class="ts-block-property-children">
</div>
## env
<div class="ts-block-property-bullets">
</div>
Environment variable configuration
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
dir?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"."`
</div>
The directory to search for `.env` files.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
publicPrefix?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"PUBLIC_"`
</div>
A prefix that signals that an environment variable is safe to expose to client-side code. See [`$env/static/public`](/docs/kit/$env-static-public) and [`$env/dynamic/public`](/docs/kit/$env-dynamic-public). Note that Vite's [`envPrefix`](https://vitejs.dev/config/shared-options.html#envprefix) must be set separately if you are using Vite's environment variable handling - though use of that feature should generally be unnecessary.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
privatePrefix?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `""`
- <span class="tag since">available since</span> v1.21.0
</div>
A prefix that signals that an environment variable is unsafe to expose to client-side code. Environment variables matching neither the public nor the private prefix will be discarded completely. See [`$env/static/private`](/docs/kit/$env-static-private) and [`$env/dynamic/private`](/docs/kit/$env-dynamic-private).
</div>
</div>
</div>
## files
<div class="ts-block-property-bullets">
</div>
Where to find various files within your project.
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
assets?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"static"`
</div>
a place to put static files that should have stable URLs and undergo no processing, such as `favicon.ico` or `manifest.json`
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
hooks?: {/*…*/}
```
<div class="ts-block-property-details">
<div class="ts-block-property-children"><div class="ts-block-property">
```ts
// @noErrors
client?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/hooks.client"`
</div>
The location of your client [hooks](/docs/kit/hooks).
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
server?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/hooks.server"`
</div>
The location of your server [hooks](/docs/kit/hooks).
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
universal?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/hooks"`
- <span class="tag since">available since</span> v2.3.0
</div>
The location of your universal [hooks](/docs/kit/hooks).
</div>
</div></div>
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
lib?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/lib"`
</div>
your app's internal library, accessible throughout the codebase as `$lib`
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
params?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/params"`
</div>
a directory containing [parameter matchers](/docs/kit/advanced-routing#Matching)
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
routes?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/routes"`
</div>
the files that define the structure of your app (see [Routing](/docs/kit/routing))
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
serviceWorker?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/service-worker"`
</div>
the location of your service worker's entry point (see [Service workers](/docs/kit/service-workers))
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
appTemplate?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/app.html"`
</div>
the location of the template for HTML responses
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
errorTemplate?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"src/error.html"`
</div>
the location of the template for fallback error responses
</div>
</div>
</div>
## inlineStyleThreshold
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `0`
</div>
Inline CSS inside a `<style>` block at the head of the HTML. This option is a number that specifies the maximum length of a CSS file in UTF-16 code units, as specified by the [String.length](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/length) property, to be inlined. All CSS files needed for the page and smaller than this value are merged and inlined in a `<style>` block.
> [!NOTE] This results in fewer initial requests and can improve your [First Contentful Paint](https://web.dev/first-contentful-paint) score. However, it generates larger HTML output and reduces the effectiveness of browser caches. Use it advisedly.
<div class="ts-block-property-children">
</div>
## moduleExtensions
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `[".js", ".ts"]`
</div>
An array of file extensions that SvelteKit will treat as modules. Files with extensions that match neither `config.extensions` nor `config.kit.moduleExtensions` will be ignored by the router.
<div class="ts-block-property-children">
</div>
## outDir
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `".svelte-kit"`
</div>
The directory that SvelteKit writes files to during `dev` and `build`. You should exclude this directory from version control.
<div class="ts-block-property-children">
</div>
## output
<div class="ts-block-property-bullets">
</div>
Options related to the build output format
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
preloadStrategy?: 'modulepreload' | 'preload-js' | 'preload-mjs';
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"modulepreload"`
- <span class="tag since">available since</span> v1.8.4
</div>
SvelteKit will preload the JavaScript modules needed for the initial page to avoid import 'waterfalls', resulting in faster application startup. There
are three strategies with different trade-offs:
- `modulepreload` - uses `<link rel="modulepreload">`. This delivers the best results in Chromium-based browsers, in Firefox 115+, and Safari 17+. It is ignored in older browsers.
- `preload-js` - uses `<link rel="preload">`. Prevents waterfalls in Chromium and Safari, but Chromium will parse each module twice (once as a script, once as a module). Causes modules to be requested twice in Firefox. This is a good setting if you want to maximise performance for users on iOS devices at the cost of a very slight degradation for Chromium users.
- `preload-mjs` - uses `<link rel="preload">` but with the `.mjs` extension which prevents double-parsing in Chromium. Some static webservers will fail to serve .mjs files with a `Content-Type: application/javascript` header, which will cause your application to break. If that doesn't apply to you, this is the option that will deliver the best performance for the largest number of users, until `modulepreload` is more widely supported.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
bundleStrategy?: 'split' | 'single' | 'inline';
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `'split'`
- <span class="tag since">available since</span> v2.13.0
</div>
The bundle strategy option affects how your app's JavaScript and CSS files are loaded.
- If `'split'`, splits the app up into multiple .js/.css files so that they are loaded lazily as the user navigates around the app. This is the default, and is recommended for most scenarios.
- If `'single'`, creates just one .js bundle and one .css file containing code for the entire app.
- If `'inline'`, inlines all JavaScript and CSS of the entire app into the HTML. The result is usable without a server (i.e. you can just open the file in your browser).
When using `'split'`, you can also adjust the bundling behaviour by setting [`output.experimentalMinChunkSize`](https://rollupjs.org/configuration-options/#output-experimentalminchunksize) and [`output.manualChunks`](https://rollupjs.org/configuration-options/#output-manualchunks) inside your Vite config's [`build.rollupOptions`](https://vite.dev/config/build-options.html#build-rollupoptions).
If you want to inline your assets, you'll need to set Vite's [`build.assetsInlineLimit`](https://vite.dev/config/build-options.html#build-assetsinlinelimit) option to an appropriate size then import your assets through Vite.
```js
// @errors: 7031
/// file: vite.config.js
import { sveltekit } from '@sveltejs/kit/vite';
import { defineConfig } from 'vite';
export default defineConfig({
plugins: [sveltekit()],
build: {
// inline all imported assets
assetsInlineLimit: Infinity
}
});
```
```svelte
/// file: src/routes/+layout.svelte
<script>
// import the asset through Vite
import favicon from './favicon.png';
</script>
<svelte:head>
<!-- this asset will be inlined as a base64 URL -->
<link rel="icon" href={favicon} />
</svelte:head>
```
</div>
</div>
</div>
## paths
<div class="ts-block-property-bullets">
</div>
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
assets?: '' | `http://${string}` | `https://${string}`;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `""`
</div>
An absolute path that your app's files are served from. This is useful if your files are served from a storage bucket of some kind.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
base?: '' | `/${string}`;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `""`
</div>
A root-relative path that must start, but not end with `/` (e.g. `/base-path`), unless it is the empty string. This specifies where your app is served from and allows the app to live on a non-root path. Note that you need to prepend all your root-relative links with the base value or they will point to the root of your domain, not your `base` (this is how the browser works). You can use [`base` from `$app/paths`](/docs/kit/$app-paths#base) for that: `<a href="{base}/your-page">Link</a>`. If you find yourself writing this often, it may make sense to extract this into a reusable component.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
relative?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `true`
- <span class="tag since">available since</span> v1.9.0
</div>
Whether to use relative asset paths.
If `true`, `base` and `assets` imported from `$app/paths` will be replaced with relative asset paths during server-side rendering, resulting in more portable HTML.
If `false`, `%sveltekit.assets%` and references to build artifacts will always be root-relative paths, unless `paths.assets` is an external URL
[Single-page app](/docs/kit/single-page-apps) fallback pages will always use absolute paths, regardless of this setting.
If your app uses a `<base>` element, you should set this to `false`, otherwise asset URLs will incorrectly be resolved against the `<base>` URL rather than the current page.
In 1.0, `undefined` was a valid value, which was set by default. In that case, if `paths.assets` was not external, SvelteKit would replace `%sveltekit.assets%` with a relative path and use relative paths to reference build artifacts, but `base` and `assets` imported from `$app/paths` would be as specified in your config.
</div>
</div>
</div>
## prerender
<div class="ts-block-property-bullets">
</div>
See [Prerendering](/docs/kit/page-options#prerender).
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
concurrency?: number;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `1`
</div>
How many pages can be prerendered simultaneously. JS is single-threaded, but in cases where prerendering performance is network-bound (for example loading content from a remote CMS) this can speed things up by processing other tasks while waiting on the network response.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
crawl?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `true`
</div>
Whether SvelteKit should find pages to prerender by following links from `entries`.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
entries?: Array<'*' | `/${string}`>;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `["*"]`
</div>
An array of pages to prerender, or start crawling from (if `crawl: true`). The `*` string includes all routes containing no required `[parameters]` with optional parameters included as being empty (since SvelteKit doesn't know what value any parameters should have).
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
handleHttpError?: PrerenderHttpErrorHandlerValue;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"fail"`
- <span class="tag since">available since</span> v1.15.7
</div>
How to respond to HTTP errors encountered while prerendering the app.
- `'fail'` — fail the build
- `'ignore'` - silently ignore the failure and continue
- `'warn'` — continue, but print a warning
- `(details) => void` — a custom error handler that takes a `details` object with `status`, `path`, `referrer`, `referenceType` and `message` properties. If you `throw` from this function, the build will fail
```js
// @errors: 7031
/// file: svelte.config.js
/** @type {import('@sveltejs/kit').Config} */
const config = {
kit: {
prerender: {
handleHttpError: ({ path, referrer, message }) => {
// ignore deliberate link to shiny 404 page
if (path === '/not-found' && referrer === '/blog/how-we-built-our-404-page') {
return;
}
// otherwise fail the build
throw new Error(message);
}
}
}
};
```
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
handleMissingId?: PrerenderMissingIdHandlerValue;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"fail"`
- <span class="tag since">available since</span> v1.15.7
</div>
How to respond when hash links from one prerendered page to another don't correspond to an `id` on the destination page.
- `'fail'` — fail the build
- `'ignore'` - silently ignore the failure and continue
- `'warn'` — continue, but print a warning
- `(details) => void` — a custom error handler that takes a `details` object with `path`, `id`, `referrers` and `message` properties. If you `throw` from this function, the build will fail
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
handleEntryGeneratorMismatch?: PrerenderEntryGeneratorMismatchHandlerValue;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"fail"`
- <span class="tag since">available since</span> v1.16.0
</div>
How to respond when an entry generated by the `entries` export doesn't match the route it was generated from.
- `'fail'` — fail the build
- `'ignore'` - silently ignore the failure and continue
- `'warn'` — continue, but print a warning
- `(details) => void` — a custom error handler that takes a `details` object with `generatedFromId`, `entry`, `matchedId` and `message` properties. If you `throw` from this function, the build will fail
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
origin?: string;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"http://sveltekit-prerender"`
</div>
The value of `url.origin` during prerendering; useful if it is included in rendered content.
</div>
</div>
</div>
## router
<div class="ts-block-property-bullets">
</div>
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
type?: 'pathname' | 'hash';
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"pathname"`
- <span class="tag since">available since</span> v2.14.0
</div>
What type of client-side router to use.
- `'pathname'` is the default and means the current URL pathname determines the route
- `'hash'` means the route is determined by `location.hash`. In this case, SSR and prerendering are disabled. This is only recommended if `pathname` is not an option, for example because you don't control the webserver where your app is deployed.
It comes with some caveats: you can't use server-side rendering (or indeed any server logic), and you have to make sure that the links in your app all start with #/, or they won't work. Beyond that, everything works exactly like a normal SvelteKit app.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
resolution?: 'client' | 'server';
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `"client"`
- <span class="tag since">available since</span> v2.17.0
</div>
How to determine which route to load when navigating to a new page.
By default, SvelteKit will serve a route manifest to the browser.
When navigating, this manifest is used (along with the `reroute` hook, if it exists) to determine which components to load and which `load` functions to run.
Because everything happens on the client, this decision can be made immediately. The drawback is that the manifest needs to be
loaded and parsed before the first navigation can happen, which may have an impact if your app contains many routes.
Alternatively, SvelteKit can determine the route on the server. This means that for every navigation to a path that has not yet been visited, the server will be asked to determine the route.
This has several advantages:
- The client does not need to load the routing manifest upfront, which can lead to faster initial page loads
- The list of routes is hidden from public view
- The server has an opportunity to intercept each navigation (for example through a middleware), enabling (for example) A/B testing opaque to SvelteKit
The drawback is that for unvisited paths, resolution will take slightly longer (though this is mitigated by [preloading](/docs/kit/link-options#data-sveltekit-preload-data)).
> [!NOTE] When using server-side route resolution and prerendering, the resolution is prerendered along with the route itself.
</div>
</div>
</div>
## serviceWorker
<div class="ts-block-property-bullets">
</div>
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
register?: boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `true`
</div>
Whether to automatically register the service worker, if it exists.
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
files?(filepath: string): boolean;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `(filename) => !/\.DS_Store/.test(filename)`
</div>
Determine which files in your `static` directory will be available in `$service-worker.files`.
</div>
</div>
</div>
## typescript
<div class="ts-block-property-bullets">
</div>
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
config?: (config: Record<string, any>) => Record<string, any> | void;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `(config) => config`
- <span class="tag since">available since</span> v1.3.0
</div>
A function that allows you to edit the generated `tsconfig.json`. You can mutate the config (recommended) or return a new one.
This is useful for extending a shared `tsconfig.json` in a monorepo root, for example.
</div>
</div>
</div>
## version
<div class="ts-block-property-bullets">
</div>
Client-side navigation can be buggy if you deploy a new version of your app while people are using it. If the code for the new page is already loaded, it may have stale content; if it isn't, the app's route manifest may point to a JavaScript file that no longer exists.
SvelteKit helps you solve this problem through version management.
If SvelteKit encounters an error while loading the page and detects that a new version has been deployed (using the `name` specified here, which defaults to a timestamp of the build) it will fall back to traditional full-page navigation.
Not all navigations will result in an error though, for example if the JavaScript for the next page is already loaded. If you still want to force a full-page navigation in these cases, use techniques such as setting the `pollInterval` and then using `beforeNavigate`:
```html
/// file: +layout.svelte
<script>
import { beforeNavigate } from '$app/navigation';
import { updated } from '$app/state';
beforeNavigate(({ willUnload, to }) => {
if (updated.current && !willUnload && to?.url) {
location.href = to.url.href;
}
});
</script>
```
If you set `pollInterval` to a non-zero value, SvelteKit will poll for new versions in the background and set the value of [`updated.current`](/docs/kit/$app-state#updated) `true` when it detects one.
<div class="ts-block-property-children">
<div class="ts-block-property">
```ts
// @noErrors
name?: string;
```
<div class="ts-block-property-details">
The current app version string. If specified, this must be deterministic (e.g. a commit ref rather than `Math.random()` or `Date.now().toString()`), otherwise defaults to a timestamp of the build.
For example, to use the current commit hash, you could do use `git rev-parse HEAD`:
```js
// @errors: 7031
/// file: svelte.config.js
import * as child_process from 'node:child_process';
export default {
kit: {
version: {
name: child_process.execSync('git rev-parse HEAD').toString().trim()
}
}
};
```
</div>
</div>
<div class="ts-block-property">
```ts
// @noErrors
pollInterval?: number;
```
<div class="ts-block-property-details">
<div class="ts-block-property-bullets">
- <span class="tag">default</span> `0`
</div>
The interval in milliseconds to poll for version changes. If this is `0`, no polling occurs.
</div>
</div>
</div>
# Command Line Interface
SvelteKit projects use [Vite](https://vitejs.dev), meaning you'll mostly use its CLI (albeit via `npm run dev/build/preview` scripts):
- `vite dev` — start a development server
- `vite build` — build a production version of your app
- `vite preview` — run the production version locally
However SvelteKit includes its own CLI for initialising your project:
## svelte-kit sync
`svelte-kit sync` creates the `tsconfig.json` and all generated types (which you can import as `./$types` inside routing files) for your project. When you create a new project, it is listed as the `prepare` script and will be run automatically as part of the npm lifecycle, so you should not ordinarily have to run this command.
# Types
## Generated types
The `RequestHandler` and `Load` types both accept a `Params` argument allowing you to type the `params` object. For example this endpoint expects `foo`, `bar` and `baz` params:
```js
/// file: src/routes/[foo]/[bar]/[baz]/+page.server.js
// @errors: 2355 2322 1360
/** @type {import('@sveltejs/kit').RequestHandler<{
foo: string;
bar: string;
baz: string
}>} */
export async function GET({ params }) {
// ...
}
```
Needless to say, this is cumbersome to write out, and less portable (if you were to rename the `[foo]` directory to `[qux]`, the type would no longer reflect reality).
To solve this problem, SvelteKit generates `.d.ts` files for each of your endpoints and pages:
```ts
/// file: .svelte-kit/types/src/routes/[foo]/[bar]/[baz]/$types.d.ts
/// link: true
import type * as Kit from '@sveltejs/kit';
type RouteParams = {
foo: string;
bar: string;
baz: string;
};
export type PageServerLoad = Kit.ServerLoad<RouteParams>;
export type PageLoad = Kit.Load<RouteParams>;
```
These files can be imported into your endpoints and pages as siblings, thanks to the [`rootDirs`](https://www.typescriptlang.org/tsconfig#rootDirs) option in your TypeScript configuration:
```js
/// file: src/routes/[foo]/[bar]/[baz]/+page.server.js
// @filename: $types.d.ts
import type * as Kit from '@sveltejs/kit';
type RouteParams = {
foo: string;
bar: string;
baz: string;
}
export type PageServerLoad = Kit.ServerLoad<RouteParams>;
// @filename: index.js
// @errors: 2355
// ---cut---
/** @type {import('./$types').PageServerLoad} */
export async function GET({ params }) {
// ...
}
```
```js
/// file: src/routes/[foo]/[bar]/[baz]/+page.js
// @filename: $types.d.ts
import type * as Kit from '@sveltejs/kit';
type RouteParams = {
foo: string;
bar: string;
baz: string;
}
export type PageLoad = Kit.Load<RouteParams>;
// @filename: index.js
// @errors: 2355
// ---cut---
/** @type {import('./$types').PageLoad} */
export async function load({ params, fetch }) {
// ...
}
```
The return types of the load functions are then available through the `$types` module as `PageData` and `LayoutData` respectively, while the union of the return values of all `Actions` is available as `ActionData`. Starting with version 2.16.0, two additional helper types are provided. `PageProps` defines `data: PageData`, as well as `form: ActionData`, when there are actions defined. `LayoutProps` defines `data: LayoutData`, as well as `children: Snippet`:
```svelte
<!--- file: src/routes/+page.svelte --->
<script>
/** @type {import('./$types').PageProps} */
let { data, form } = $props();
</script>
```
> [!LEGACY]
> Before 2.16.0:
> ```svelte
> <!--- file: src/routes/+page.svelte --->
> <script>
> /** @type {{ data: import('./$types').PageData, form: import('./$types').ActionData }} */
> let { data, form } = $props();
> </script>
> ```
>
> Using Svelte 4:
> ```svelte
> <!--- file: src/routes/+page.svelte --->
> <script>
> /** @type {import('./$types').PageData} */
> export let data;
> /** @type {import('./$types').ActionData} */
> export let form;
> </script>
> ```
> [!NOTE] For this to work, your own `tsconfig.json` or `jsconfig.json` should extend from the generated `.svelte-kit/tsconfig.json` (where `.svelte-kit` is your [`outDir`](configuration#outDir)):
>
> `{ "extends": "./.svelte-kit/tsconfig.json" }`
### Default tsconfig.json
The generated `.svelte-kit/tsconfig.json` file contains a mixture of options. Some are generated programmatically based on your project configuration, and should generally not be overridden without good reason:
```json
/// file: .svelte-kit/tsconfig.json
{
"compilerOptions": {
"paths": {
"$lib": ["../src/lib"],
"$lib/*": ["../src/lib/*"]
},
"rootDirs": ["..", "./types"]
},
"include": [
"ambient.d.ts",
"non-ambient.d.ts",
"./types/**/$types.d.ts",
"../vite.config.js",
"../vite.config.ts",
"../src/**/*.js",
"../src/**/*.ts",
"../src/**/*.svelte",
"../tests/**/*.js",
"../tests/**/*.ts",
"../tests/**/*.svelte"
],
"exclude": [
"../node_modules/**",
"../src/service-worker.js",
"../src/service-worker/**/*.js",
"../src/service-worker.ts",
"../src/service-worker/**/*.ts",
"../src/service-worker.d.ts",
"../src/service-worker/**/*.d.ts"
]
}
```
Others are required for SvelteKit to work properly, and should also be left untouched unless you know what you're doing:
```json
/// file: .svelte-kit/tsconfig.json
{
"compilerOptions": {
// this ensures that types are explicitly
// imported with `import type`, which is
// necessary as Svelte/Vite cannot
// otherwise compile components correctly
"verbatimModuleSyntax": true,
// Vite compiles one TypeScript module
// at a time, rather than compiling
// the entire module graph
"isolatedModules": true,
// Tell TS it's used only for type-checking
"noEmit": true,
// This ensures both `vite build`
// and `svelte-package` work correctly
"lib": ["esnext", "DOM", "DOM.Iterable"],
"moduleResolution": "bundler",
"module": "esnext",
"target": "esnext"
}
}
```
## $lib
This is a simple alias to `src/lib`, or whatever directory is specified as [`config.kit.files.lib`](configuration#files). It allows you to access common components and utility modules without `../../../../` nonsense.
### $lib/server
A subdirectory of `$lib`. SvelteKit will prevent you from importing any modules in `$lib/server` into client-side code. See [server-only modules](server-only-modules).
## app.d.ts
The `app.d.ts` file is home to the ambient types of your apps, i.e. types that are available without explicitly importing them.
Always part of this file is the `App` namespace. This namespace contains several types that influence the shape of certain SvelteKit features you interact with.
## Error
Defines the common shape of expected and unexpected errors. Expected errors are thrown using the `error` function. Unexpected errors are handled by the `handleError` hooks which should return this shape.
<div class="ts-block">
```dts
interface Error {/*…*/}
```
<div class="ts-block-property">
```dts
message: string;
```
<div class="ts-block-property-details"></div>
</div></div>
## Locals
The interface that defines `event.locals`, which can be accessed in server [hooks](/docs/kit/hooks) (`handle`, and `handleError`), server-only `load` functions, and `+server.js` files.
<div class="ts-block">
```dts
interface Locals {}
```
</div>
## PageData
Defines the common shape of the [page.data state](/docs/kit/$app-state#page) and [$page.data store](/docs/kit/$app-stores#page) - that is, the data that is shared between all pages.
The `Load` and `ServerLoad` functions in `./$types` will be narrowed accordingly.
Use optional properties for data that is only present on specific pages. Do not add an index signature (`[key: string]: any`).
<div class="ts-block">
```dts
interface PageData {}
```
</div>
## PageState
The shape of the `page.state` object, which can be manipulated using the [`pushState`](/docs/kit/$app-navigation#pushState) and [`replaceState`](/docs/kit/$app-navigation#replaceState) functions from `$app/navigation`.
<div class="ts-block">
```dts
interface PageState {}
```
</div>
## Platform
If your adapter provides [platform-specific context](/docs/kit/adapters#Platform-specific-context) via `event.platform`, you can specify it here.
<div class="ts-block">
```dts
interface Platform {}
```
</div>
# Start of the Svelte CLI documentation
# Overview
The command line interface (CLI), `sv`, is a toolkit for creating and maintaining Svelte applications.
## Usage
The easiest way to run `sv` is with [`npx`](https://docs.npmjs.com/cli/v8/commands/npx) (or the equivalent command if you're using a different package manager — for example, `pnpx` if you're using [pnpm](https://pnpm.io/)):
```bash
npx sv <command> <args>
```
If you're inside a project where `sv` is already installed, this will use the local installation, otherwise it will download the latest version and run it without installing it, which is particularly useful for [`sv create`](sv-create).
## Acknowledgements
Thank you to [Christopher Brown](https://github.com/chbrown) who originally owned the `sv` name on npm for graciously allowing it to be used for the Svelte CLI. You can find the original `sv` package at [`@chbrown/sv`](https://www.npmjs.com/package/@chbrown/sv).
# sv create
`sv create` sets up a new SvelteKit project, with options to [setup additional functionality](sv-add#Official-add-ons).
## Usage
```bash
npx sv create [options] [path]
```
## Options
### `--template <name>`
Which project template to use:
- `minimal` — barebones scaffolding for your new app
- `demo` — showcase app with a word guessing game that works without JavaScript
- `library` — template for a Svelte library, set up with `svelte-package`
### `--types <option>`
Whether and how to add typechecking to the project:
- `ts` — default to `.ts` files and use `lang="ts"` for `.svelte` components
- `jsdoc` — use [JSDoc syntax](https://www.typescriptlang.org/docs/handbook/jsdoc-supported-types.html) for types
### `--no-types`
Prevent typechecking from being added. Not recommended!
### `--no-add-ons`
Run the command without the interactive add-ons prompt
### `--no-install`
Skip dependency installation
<!-- ## Programmatic interface
```js
// TODO: this gives type checking errors in the docs site when not commented out. Need to release sv, install it in the site, and uncomment this.
// import { create } from 'sv';
// // todo: check if this is right
// create(cwd, {
// // add your options here
// // todo: list available option
// });
```
-->
# sv add
`sv add` updates an existing project with new functionality.
## Usage
```bash
npx sv add
```
```bash
npx sv add [add-ons]
```
You can select multiple space-separated add-ons from [the list below](#Official-add-ons), or you can use the interactive prompt.
## Options
- `-C`, `--cwd` — path to the root of your Svelte(Kit) project
- `--no-preconditions` — skip checking preconditions <!-- TODO what does this mean? -->
- `--no-install` — skip dependency installation
## Official add-ons
<!-- TODO this should be a separate section, each of these should have their own page -->
- `drizzle`
- `eslint`
- `sveltekit-adapter`
- `lucia`
- `mdsvex`
- `paraglide`
- `playwright`
- `prettier`
- `storybook`
- `tailwindcss`
- `vitest`
# sv check
`sv check` finds errors and warnings in your project, such as:
- unused CSS
- accessibility hints
- JavaScript/TypeScript compiler errors
Requires Node 16 or later.
## Installation
You will need to have the `svelte-check` package installed in your project:
```bash
npm i -D svelte-check
```
## Usage
```bash
npx sv check
```
## Options
### `--workspace <path>`
Path to your workspace. All subdirectories except `node_modules` and those listed in `--ignore` are checked.
### `--output <format>`
How to display errors and warnings. See [machine-readable output](#Machine-readable-output).
- `human`
- `human-verbose`
- `machine`
- `machine-verbose`
### `--watch`
Keeps the process alive and watches for changes.
### `--preserveWatchOutput`
Prevents the screen from being cleared in watch mode.
### `--tsconfig <path>`
Pass a path to a `tsconfig` or `jsconfig` file. The path can be relative to the workspace path or absolute. Doing this means that only files matched by the `files`/`include`/`exclude` pattern of the config file are diagnosed. It also means that errors from TypeScript and JavaScript files are reported. If not given, will traverse upwards from the project directory looking for the next `jsconfig`/`tsconfig.json` file.
### `--no-tsconfig`
Use this if you only want to check the Svelte files found in the current directory and below and ignore any `.js`/`.ts` files (they will not be type-checked)
### `--ignore <paths>`
Files/folders to ignore, relative to workspace root. Paths should be comma-separated and quoted. Example:
```bash
npx sv check --ignore "dist,build"
```
<!-- TODO what the hell does this mean? is it possible to use --tsconfig AND --no-tsconfig? if so what would THAT mean? -->
Only has an effect when used in conjunction with `--no-tsconfig`. When used in conjunction with `--tsconfig`, this will only have effect on the files watched, not on the files that are diagnosed, which is then determined by the `tsconfig.json`.
### `--fail-on-warnings`
If provided, warnings will cause `sv check` to exit with an error code.
### `--compiler-warnings <warnings>`
A quoted, comma-separated list of `code:behaviour` pairs where `code` is a [compiler warning code](../svelte/compiler-warnings) and `behaviour` is either `ignore` or `error`:
```bash
npx sv check --compiler-warnings "css_unused_selector:ignore,a11y_missing_attribute:error"
```
### `--diagnostic-sources <sources>`
A quoted, comma-separated list of sources that should run diagnostics on your code. By default, all are active:
<!-- TODO would be nice to have a clearer definition of what these are -->
- `js` (includes TypeScript)
- `svelte`
- `css`
Example:
```bash
npx sv check --diagnostic-sources "js,svelte"
```
### `--threshold <level>`
Filters the diagnostics:
- `warning` (default) — both errors and warnings are shown
- `error` — only errors are shown
## Troubleshooting
[See the language-tools documentation](https://github.com/sveltejs/language-tools/blob/master/docs/README.md) for more information on preprocessor setup and other troubleshooting.
## Machine-readable output
Setting the `--output` to `machine` or `machine-verbose` will format output in a way that is easier to read
by machines, e.g. inside CI pipelines, for code quality checks, etc.
Each row corresponds to a new record. Rows are made up of columns that are separated by a
single space character. The first column of every row contains a timestamp in milliseconds
which can be used for monitoring purposes. The second column gives us the "row type", based
on which the number and types of subsequent columns may differ.
The first row is of type `START` and contains the workspace folder (wrapped in quotes). Example:
```
1590680325583 START "/home/user/language-tools/packages/language-server/test/plugins/typescript/testfiles"
```
Any number of `ERROR` or `WARNING` records may follow. Their structure is identical and depends on the output argument.
If the argument is `machine` it will tell us the filename, the starting line and column numbers, and the error message. The filename is relative to the workspace directory. The filename and the message are both wrapped in quotes. Example:
```
1590680326283 ERROR "codeactions.svelte" 1:16 "Cannot find module 'blubb' or its corresponding type declarations."
1590680326778 WARNING "imported-file.svelte" 0:37 "Component has unused export property 'prop'. If it is for external reference only, please consider using `export const prop`"
```
If the argument is `machine-verbose` it will tell us the filename, the starting line and column numbers, the ending line and column numbers, the error message, the code of diagnostic, the human-friendly description of the code and the human-friendly source of the diagnostic (eg. svelte/typescript). The filename is relative to the workspace directory. Each diagnostic is represented as an [ndjson](https://en.wikipedia.org/wiki/JSON_streaming#Newline-Delimited_JSON) line prefixed by the timestamp of the log. Example:
```
1590680326283 {"type":"ERROR","fn":"codeaction.svelte","start":{"line":1,"character":16},"end":{"line":1,"character":23},"message":"Cannot find module 'blubb' or its corresponding type declarations.","code":2307,"source":"js"}
1590680326778 {"type":"WARNING","filename":"imported-file.svelte","start":{"line":0,"character":37},"end":{"line":0,"character":51},"message":"Component has unused export property 'prop'. If it is for external reference only, please consider using `export
const prop`","code":"unused-export-let","source":"svelte"}
```
The output concludes with a `COMPLETED` message that summarizes total numbers of files, errors and warnings that were encountered during the check. Example:
```
1590680326807 COMPLETED 20 FILES 21 ERRORS 1 WARNINGS 3 FILES_WITH_PROBLEMS
```
If the application experiences a runtime error, this error will appear as a `FAILURE` record. Example:
```
1590680328921 FAILURE "Connection closed"
```
## Credits
- Vue's [VTI](https://github.com/vuejs/vetur/tree/master/vti) which laid the foundation for `svelte-check`
## FAQ
### Why is there no option to only check specific files (for example only staged files)?
`svelte-check` needs to 'see' the whole project for checks to be valid. Suppose you renamed a component prop but didn't update any of the places where the prop is used — the usage sites are all errors now, but you would miss them if checks only ran on changed files.
# sv migrate
`sv migrate` migrates Svelte(Kit) codebases. It delegates to the [`svelte-migrate`](https://www.npmjs.com/package/svelte-migrate) package.
Some migrations may annotate your codebase with tasks for completion that you can find by searching for `@migration`.
## Usage
```bash
npx sv migrate
```
You can also specify a migration directly via the CLI:
```bash
npx sv migrate [migration]
```
## Migrations
### `app-state`
Migrates `$app/stores` usage to `$app/state` in `.svelte` files. See the [migration guide](/docs/kit/migrating-to-sveltekit-2#SvelteKit-2.12:-$app-stores-deprecated) for more details.
### `svelte-5`
Upgrades a Svelte 4 app to use Svelte 5, and updates individual components to use [runes](../svelte/what-are-runes) and other Svelte 5 syntax ([see migration guide](../svelte/v5-migration-guide)).
### `self-closing-tags`
Replaces all the self-closing non-void elements in your `.svelte` files. See the [pull request](https://github.com/sveltejs/kit/pull/12128) for more details.
### `svelte-4`
Upgrades a Svelte 3 app to use Svelte 4 ([see migration guide](../svelte/v4-migration-guide)).
### `sveltekit-2`
Upgrades a SvelteKit 1 app to SvelteKit 2 ([see migration guide](../kit/migrating-to-sveltekit-2)).
### `package`
Upgrades a library using `@sveltejs/package` version 1 to version 2. See the [pull request](https://github.com/sveltejs/kit/pull/8922) for more details.
### `routes`
Upgrades a pre-release SvelteKit app to use the filesystem routing conventions in SvelteKit 1. See the [pull request](https://github.com/sveltejs/kit/discussions/5774) for more details.