# Svelte

> Svelte documentation (runes, template syntax, runtime) from sveltejs/svelte documentation/docs.

## [Svelte](https://docharvest.github.io/docs/svelte/)

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svelte

Svelte

Svelte

Svelte

## [Introduction](https://docharvest.github.io/docs/svelte/01-introduction/)

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svelte

Introduction

Svelte

Introduction

## [Overview](https://docharvest.github.io/docs/svelte/01-introduction/01-overview/)

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svelte

Overview

Svelte

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...

```
<!--- 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](https://docharvest.github.io/docs/svelte/01-introduction/02-getting-started/)

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svelte

Getting started

Svelte

Getting started

We recommend using [SvelteKit](../kit), which lets you [build almost anything](../kit/project-types). It's the official application framework from the Svelte team and powered by [Vite](https://vite.dev/). Create a new project with:

```
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 via [vite-plugin-svelte](https://github.com/sveltejs/vite-plugin-svelte) by running `npm create vite@latest` and selecting the `svelte` option (or, if working with an existing project, adding the plugin to your `vite.config.js` file). With this, `npm run build` will generate HTML, JS, and CSS files inside the `dist` directory. In most cases, you will probably need to [choose a routing library](/packages#routing) as well.

> \[!NOTE\] Vite is often used in standalone mode to build [single page apps (SPAs)](../kit/glossary#SPA), which you can also [build with SvelteKit](../kit/single-page-apps).

There are also [plugins for other bundlers](/packages#bundler-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/collection/editor-support-c85c080efc292a34) and tools as well.

You can also check your code from the command line using [`npx sv check`](https://svelte.dev/docs/cli/sv-check).

## 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](https://docharvest.github.io/docs/svelte/01-introduction/03-svelte-files/)

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svelte

.svelte files

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.

```
/// 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.

## `<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.

```
<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.

```
<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](https://docharvest.github.io/docs/svelte/01-introduction/04-svelte-js-files/)

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svelte

.svelte.js and .svelte.ts files

Svelte

.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 (though note that you [cannot export reassigned state]($state#Passing-state-across-modules)).

> \[!LEGACY\] This is a concept that didn't exist prior to Svelte 5

## [Runes](https://docharvest.github.io/docs/svelte/02-runes/)

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Runes

Svelte

Runes

## [What are runes?](https://docharvest.github.io/docs/svelte/02-runes/01-what-are-runes/)

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svelte

What are runes?

Svelte

What are runes?

> \[!NOTE\] **rune** /ruːn/ _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:

```
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.

## [$state](https://docharvest.github.io/docs/svelte/02-runes/02-$state/)

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svelte

$state

Svelte

$state

The `$state` rune allows you to create _reactive state_, which means that your UI _reacts_ when it changes.

```
<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.

State is proxified recursively until Svelte finds something other than an array or simple object (like a class or an object created with `Object.create`). In a case like this...

```
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:

```
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:

```
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. If you need to use your own proxy handlers in a state proxy, [you should wrap the object _after_ wrapping it in `$state`](https://svelte.dev/playground/hello-world?version=latest#H4sIAAAAAAAACpWR3WoDIRCFX2UqhWyIJL3erAulL9C7XnQLMe5ksbUqOpsfln33YuyGFNJC8UKdc2bOhw7Myk9kJXsJ0nttO9jcR5KEG9AWJDwHdzwxznbaYGTl68Do5JM_FRifuh-9X8Y9Gkq1rYx4q66cJbQUWcmqqIL2VDe2IYMEbvuOikBADi-GJDSkXG-phId0G-frye2DO2psQYDFQ0Ys8gQO350dUkEydEg82T0GOs0nsSG9g2IqgxACZueo2ZUlpdvoDC6N64qsg1QKY8T2bpZp8gpIfbCQ85Zn50Ud82HkeY83uDjspenxv3jXcSDyjPWf9L1vJf0GH666J-jLu1ery4dV257IWXBWGa0-xFDMQdTTn2ScxWKsn86ROsLwQxqrVR5QM84Ij8TKFD2-cUZSm4O2LSt30kQcvwCgCmfZnAIAAA==).

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:

```
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

Class instances are not proxied. Instead, you can use `$state` in class fields (whether public or private), or as the first assignment to a property immediately inside the `constructor`:

```
// @errors: 7006 2554
class Todo {
	done = $state(false);

	constructor(text) {
		this.text = $state(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`:

```
<button onclick={todo.reset}>
	reset
</button>
```

You can either use an inline function...

```
<button onclick=+++{() => todo.reset()}>+++
	reset
</button>
```

...or use an arrow function in the class definition:

```
// @errors: 7006 2554
class Todo {
	done = $state(false);

	constructor(text) {
		this.text = $state(text);
	}

	+++reset = () => {+++
		this.text = '';
		this.done = false;
	}
}
```

### Built-in classes

Svelte provides reactive implementations of built-in classes like `Set`, `Map`, `Date` and `URL` that can be imported from [`svelte/reactivity`](svelte-reactivity).

## `$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:

```
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).

As with `$state`, you can declare class fields using `$state.raw`.

## `$state.snapshot`

To take a static snapshot of a deeply reactive `$state` proxy, use `$state.snapshot`:

```
<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`.

If a value has a `toJSON` method, the snapshot will clone the value returned from `toJSON` instead of the original object.

## `$state.eager`

When state changes, it may not be reflected in the UI immediately if it is used by an `await` expression, because [updates are synchronized](await-expressions#Synchronized-updates).

In some cases, you may want to update the UI as soon as the state changes. For example, you might want to update a navigation bar when the user clicks on a link, so that they get visual feedback while waiting for the new page to load. To do this, use `$state.eager(value)`:

```
<nav>
	<a href="/" aria-current={$state.eager(pathname) === '/' ? 'page' : null}>home</a>
	<a href="/about" aria-current={$state.eager(pathname) === '/about' ? 'page' : null}>about</a>
</nav>
```

Use this feature sparingly, and only to provide feedback in response to user action — in general, allowing Svelte to coordinate updates will provide a better user experience.

## 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:

```
/// 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:

```
/// 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...

```
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...

```
/// 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.

## Passing state across modules

You can declare state in `.svelte.js` and `.svelte.ts` files, but you can only _export_ that state if it's not directly reassigned. In other words you can't do this:

```
/// file: state.svelte.js
export let count = $state(0);

export function increment() {
	count += 1;
}
```

That's because every reference to `count` is transformed by the Svelte compiler — the code above is roughly equivalent to this:

```
/// file: state.svelte.js (compiler output)
// @filename: index.ts
interface Signal<T> {
	value: T;
}

interface Svelte {
	state<T>(value?: T): Signal<T>;
	get<T>(source: Signal<T>): T;
	set<T>(source: Signal<T>, value: T): void;
}
declare const $: Svelte;
// ---cut---
export let count = $.state(0);

export function increment() {
	$.set(count, $.get(count) + 1);
}
```

> \[!NOTE\] You can see the code Svelte generates by clicking the 'JS Output' tab in the [playground](/playground).

Since the compiler only operates on one file at a time, if another file imports `count` Svelte doesn't know that it needs to wrap each reference in `$.get` and `$.set`:

```
// @filename: state.svelte.js
export let count = 0;

// @filename: index.js
// ---cut---
import { count } from './state.svelte.js';

console.log(typeof count); // 'object', not 'number'
```

This leaves you with two options for sharing state between modules — either don't reassign it...

```
// This is allowed — since we're updating
// `counter.count` rather than `counter`,
// Svelte doesn't wrap it in `$.state`
export const counter = $state({
	count: 0
});

export function increment() {
	counter.count += 1;
}
```

...or don't directly export it:

```
let count = $state(0);

export function getCount() {
	return count;
}

export function increment() {
	count += 1;
}
```

## [$derived](https://docharvest.github.io/docs/svelte/02-runes/03-$derived/)

Contents

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$derived

Svelte

$derived

Derived state is declared with the `$derived` rune:

```
<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.

```
<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.

In addition, if an expression contains an [`await`](await-expressions), Svelte transforms it such that any state _after_ the `await` is also tracked — in other words, in a case like this...

```
let a = Promise.resolve(1);
let b = 2;
// ---cut---
let total = $derived(await a + b);
```

...both `a` and `b` are tracked, even though `b` is only read once `a` has resolved, after the initial execution. (This does not apply to `await` in functions that are called by the expression, only the expression itself.)

To exempt a piece of state from being treated as a dependency, use [`untrack`](svelte#untrack).

## Overriding derived values

Derived expressions are recalculated when their dependencies change, but you can temporarily override their values by reassigning them (unless they are declared with `const`). This can be useful for things like _optimistic UI_, where a value is derived from the 'source of truth' (such as data from your server) but you'd like to show immediate feedback to the user:

```
<script>
	let { post, like } = $props();

	let likes = $derived(post.likes);

	async function onclick() {
		// increment the `likes` count immediately...
		likes += 1;

		// and tell the server, which will eventually update `post`
		try {
			await like();
		} catch {
			// failed! roll back the change
			likes -= 1;
		}
	}
</script>

<button {onclick}>🧡 {likes}</button>
```

> \[!NOTE\] Prior to Svelte 5.25, deriveds were read-only.

## Deriveds and reactivity

Unlike `$state`, which converts objects and arrays to [deeply reactive proxies]($state#Deep-state), `$derived` values are left as-is. For example, [in a case like this](/playground/untitled#H4sIAAAAAAAAE4VU22rjMBD9lUHd3aaQi9PdstS1A3t5XvpQ2Ic4D7I1iUUV2UjjNMX431eS7TRdSosxgjMzZ45mjt0yzffIYibvy0ojFJWqDKCQVBk2ZVup0LJ43TJ6rn2aBxw-FP2o67k9oCKP5dziW3hRaUJNjoYltjCyplWmM1JIIAn3FlL4ZIkTTtYez6jtj4w8WwyXv9GiIXiQxLVs9pfTMR7EuoSLIuLFbX7Z4930bZo_nBrD1bs834tlfvsBz9_SyX6PZXu9XaL4gOWn4sXjeyzftv4ZWfyxubpzxzg6LfD4MrooxELEosKCUPigQCMPKCZh0OtQE1iSxcsmdHuBvCiHZXALLXiN08EL3RRkaJ_kDVGle0HcSD5TPEeVtj67O4Nrg9aiSNtBY5oODJkrL5QsHtN2cgXp6nSJMWzpWWGasdlsGEMbzi5jPr5KFr0Ep7pdeM2-TCelCddIhDxAobi1jqF3cMaC1RKp64bAW9iFAmXGIHfd4wNXDabtOLN53w8W53VvJoZLh7xk4Rr3CoL-UNoLhWHrT1JQGcM17u96oES5K-kc2XOzkzqGCKL5De79OUTyyrg1zgwXsrEx3ESfx4Bz0M5UjVMHB24mw9SuXtXFoN13fYKOM1tyUT3FbvbWmSWCZX2Er-41u5xPoml45svRahl9Wb9aasbINJixDZwcPTbyTLZSUsAvrg_cPuCR7s782_WU8343Y72Qtlb8OYatwuOQvuN13M_hJKNfxann1v1U_B1KZ_D_mzhzhz24fw85CSz2irtN9w9HshBK7AQAAA==)...

```
// @errors: 7005
let items = $state([ /*...*/ ]);

let index = $state(0);
let selected = $derived(items[index]);
```

...you can change (or `bind:` to) properties of `selected` and it will affect the underlying `items` array. If `items` was _not_ deeply reactive, mutating `selected` would have no effect.

## Destructuring

If you use destructuring with a `$derived` declaration, the resulting variables will all be reactive — this...

```
function stuff() { return { a: 1, b: 2, c: 3 } }
// ---cut---
let { a, b, c } = $derived(stuff());
```

...is roughly equivalent to this:

```
function stuff() { return { a: 1, b: 2, c: 3 } }
// ---cut---
let _stuff = $derived(stuff());
let a = $derived(_stuff.a);
let b = $derived(_stuff.b);
let c = $derived(_stuff.c);
```

## 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`:

```
<script>
	let count = $state(0);
	let large = $derived(count > 10);
</script>

<button onclick={() => count++}>
	{large}
</button>
```

## [$effect](https://docharvest.github.io/docs/svelte/02-runes/04-$effect/)

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$effect

Svelte

$effect

Effects are functions that run when state updates, and can be used for things like calling third-party libraries, drawing on `<canvas>` elements, or making network requests. They only run in the browser, not during server-side rendering.

Generally speaking, you should _not_ update state inside effects, as it will make code more convoluted and will often lead to never-ending update cycles. If you find yourself doing so, see [when not to use `$effect`](#When-not-to-use-$effect) to learn about alternative approaches.

You can create an effect with the `$effect` rune ([demo](/playground/untitled#H4sIAAAAAAAAE31S246bMBD9lZF3pSRSAqTVvrCAVPUP2sdSKY4ZwJJjkD0hSVH-vbINuWxXfQH5zMyZc2ZmZLVUaFn6a2R06ZGlHmBrpvnBvb71fWQHVOSwPbf4GS46TajJspRlVhjZU1HqkhQSWPkHIYdXS5xw-Zas3ueI6FRn7qHFS11_xSRZhIxbFtcDtw7SJb1iXaOg5XIFeQGjzyPRaevYNOGZIJ8qogbpe8CWiy_VzEpTXiQUcvPDkSVrSNZz1UlW1N5eLcqmpdXUvaQ4BmqlhZNUCgxuzFHDqUWNAxrYeUM76AzsnOsdiJbrBp_71lKpn3RRbii-4P3f-IMsRxS-wcDV_bL4PmSdBa2wl7pKnbp8DMgVvJm8ZNskKRkEM_OzyOKQFkgqOYBQ3Nq89Ns0nbIl81vMFN-jKoLMTOr-SOBOJS-Z8f5Y6D1wdcR8dFqvEBdetK-PHwj-z-cH8oHPY54wRJ8Ys7iSQ3Bg3VA9azQbmC9k35kKzYa6PoVtfwbbKVnBixBiGn7Pq0rqJoUtHiCZwAM3jdTPWCVtr_glhVrhecIa3vuksJ_b7TqFs4DPyriSjd5IwoNNQaAmNI-ESfR2p8zimzvN1swdCkvJHPH6-_oX8o1SgcIDAAA=)):

```
<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"></canvas>
```

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.

> \[!NOTE\] If you're having difficulty understanding why your `$effect` is rerunning or is not running see [understanding dependencies](#Understanding-dependencies). Effects are triggered differently than the `$:` blocks you may be used to if coming from Svelte 4.

### Understanding lifecycle

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. 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 use `$effect` anywhere, not just at the top level of a component, as long as it is called while a parent effect is running.

> \[!NOTE\] Svelte uses effects internally to represent logic and expressions in your template — this is how `<h1>hello {name}!</h1>` updates when `name` changes.

An effect can return a _teardown function_ which will run immediately before the effect re-runs:

```
<!--- file: App.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 teardown function 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>
```

Teardown functions also run when the effect is destroyed, which happens when its parent is destroyed (for example, a component is unmounted) or the parent effect re-runs.

### 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 re-run.

If `$state` and `$derived` are used directly inside the `$effect` (for example, during creation of a [reactive class](https://svelte.dev/docs/svelte/$state#Classes)), those values will _not_ be treated as dependencies.

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=)):

```
// @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).)

```
<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 `condition` is `true` in the code snippet below, the code inside the `if` block will run and `color` will be evaluated. This means that changes to either `condition` or `color` [will cause the effect to re-run](/playground/untitled#H4sIAAAAAAAAE21RQW6DMBD8ytaNBJHaJFLViwNIVZ8RcnBgXVk1xsILTYT4e20TQg89IOPZ2fHM7siMaJBx9tmaWpFqjQNlAKXEihx7YVJpdIyfRkY3G4gB8Pi97cPanRtQU8AuwuF_eNUaQuPlOMtc1SlLRWlKUo1tOwJflUikQHZtA0klzCDc64Imx0ANn8bInV1CDhtHgjClrsftcSXotluLybOUb3g4JJHhOZs5WZpuIS9gjNqkJKQP5e2ClrR4SMdZ13E4xZ8zTPOTJU2A2uE_PQ9COCI926_hTVarIU4hu_REPlBrKq2q73ycrf1N-vS4TMUsulaVg3EtR8H9rFgsg8uUsT1B2F9eshigZHBRpuaD0D3mY8Qm2BfB5N2YyRzdNEYVDy0Ja-WsFjcOUuP1HvFLWA6H3XuHTUSmmDV2--0TXonxsKbp7G9C6R__NONS-MFNvxj_d6mBAgAA).

Conversely, if `condition` is `false`, `color` will not be evaluated, and the effect will _only_ re-run again when `condition` changes.

```
// @filename: ambient.d.ts
declare module 'canvas-confetti' {
	interface ConfettiOptions {
		colors: string[];
	}

	function confetti(opts?: ConfettiOptions): void;
	export default confetti;
}

// @filename: index.js
// ---cut---
import confetti from 'canvas-confetti';

let condition = $state(true);
let color = $state('#ff3e00');

$effect(() => {
	if (condition) {
		confetti({ colors: [color] });
	} else {
		confetti();
	}
});
```

## `$effect.pre`

In rare cases, you may need to run code _before_ the DOM updates. For this we can use the `$effect.pre` rune:

```
<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:

```
<!--- file: App.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.pending`

When using [`await`](await-expressions) in components, the `$effect.pending()` rune tells you how many promises are pending in the current [boundary](svelte-boundary), not including child boundaries:

```
<!--- file: App.svelte --->
<script>
	let a = $state(1);
	let b = $state(2);

	async function add(a, b) {
		await new Promise((f) => setTimeout(f, 500)); // artificial delay
		return a + b;
	}
</script>

<button onclick={() => a++}>a++</button>
<button onclick={() => b++}>b++</button>

<p>{a} + {b} = {await add(a, b)}</p>

{#if $effect.pending()}
	<p>pending promises: {$effect.pending()}</p>
{/if}
```

## `$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.

```
const destroy = $effect.root(() => {
	$effect(() => {
		// setup
	});

	return () => {
		// cleanup
	};
});

// later...
destroy();
```

## 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...

```
<script>
	let count = $state(0);
	let doubled = $state();

	// don't do this!
	$effect(() => {
		doubled = count * 2;
	});
</script>
```

...do this:

```
<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`.

If you're using an effect because you want to be able to reassign the derived value (to build an optimistic UI, for example) note that [deriveds can be directly overridden]($derived#Overriding-derived-values) as of Svelte 5.25.

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. Instead of using effects for this...

```
<!--- file: App.svelte --->
<script>
	const 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>

<style>
	label {
		display: flex;
		gap: 0.5em;
	}
</style>
```

...use `oninput` callbacks or — better still — [function bindings](bind#Function-bindings) where possible:

```
<!--- file: App.svelte --->
<script>
	const total = 100;
	let spent = $state(0);
	let left = $derived(total - spent);

+++	function updateLeft(left) {
		spent = total - left;
	}+++
</script>

<label>
	<input type="range" bind:value={spent} max={total} />
	{spent}/{total} spent
</label>

<label>
	<input type="range" +++bind:value={() => left, updateLeft}+++ max={total} />
	{left}/{total} left
</label>

<style>
	label {
		display: flex;
		gap: 0.5em;
	}
</style>
```

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).

## [$props](https://docharvest.github.io/docs/svelte/02-runes/05-$props/)

Contents

svelte

$props

Svelte

$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:

```
<!--- 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...

```
<!--- 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:

```
<!--- 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 (or the value is `undefined`):

```
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`:

```
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:

```
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:

```
<!--- file: App.svelte --->
<script>
	import Child from './Child.svelte';

	let count = $state(0);
</script>

<button onclick={() => (count += 1)}>
	clicks (parent): {count}
</button>

<Child {count} />
```

```
<!--- 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:

```
<!--- file: App.svelte --->
<script>
	import Child from './Child.svelte';
</script>

<Child object={{ count: 0 }} />
```

```
<!--- 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:

```
<!--- file: App.svelte --->
<script>
	import Child from './Child.svelte';

	let object = $state({count: 0});
</script>

<Child {object} />
```

```
<!--- 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:

```
<!--- file: App.svelte --->
<script>
	import Child from './Child.svelte';
</script>

<Child />
```

```
<!--- 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...

```
<script lang="ts">
	let { adjective }: { adjective: string } = $props();
</script>
```

...while in JSDoc you can do this:

```
<script>
	/** @type {{ adjective: string }} */
	let { adjective } = $props();
</script>
```

You can, of course, separate the type declaration from the annotation:

```
<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))

If your component exposes [snippet](snippet) props like `children`, these should be typed using the `Snippet` interface imported from `'svelte'` — see [Typing snippets](snippet#Typing-snippets) for examples.

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`.

```
<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>
```

## [$bindable](https://docharvest.github.io/docs/svelte/02-runes/06-$bindable/)

Contents

svelte

$bindable

Svelte

$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 — overuse can make your data flow unpredictable and your components harder to maintain — 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:

```
/// 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==)):

```
/// file: 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:

```
/// file: FancyInput.svelte
let { value = $bindable('fallback'), ...props } = $props();
```

## [$inspect](https://docharvest.github.io/docs/svelte/02-runes/07-$inspect/)

Contents

svelte

$inspect

Svelte

$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:

```
<!--- file: App.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} />
```

On updates, a stack trace will be printed, making it easy to find the origin of a state change (unless you're in the playground, due to technical limitations).

## $inspect(...).with

`$inspect(...)` returns an object with a `with` method, which you can invoke with a callback that will then be invoked instead of `console.log`. The first argument to the callback is either `"init"` or `"update"`; subsequent arguments are the values passed to `$inspect`:

```
<!--- file: App.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>
```

## $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.

```
<script>
	import { doSomeWork } from './elsewhere';

	$effect(() => {
		+++// $inspect.trace must be the first statement of a function body+++
		+++$inspect.trace();+++
		doSomeWork();
	});
</script>
```

`$inspect.trace` takes an optional first argument which will be used as the label.

## [$host](https://docharvest.github.io/docs/svelte/02-runes/08-$host/)

Contents

svelte

$host

Svelte

$host

When compiling a component as a [custom element](custom-elements), 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)):

```
/// 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>
```

```
/// 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>
```

## [Template syntax](https://docharvest.github.io/docs/svelte/03-template-syntax/)

Contents

svelte

Template syntax

Svelte

Template syntax

## [Basic markup](https://docharvest.github.io/docs/svelte/03-template-syntax/01-basic-markup/)

Contents

svelte

Basic markup

Svelte

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_.

```
<script>
	import Widget from './Widget.svelte';
</script>

<div>
	<Widget />
</div>
```

## Element attributes

By default, attributes work exactly like their HTML counterparts.

```
<div class="foo">
	<button disabled>can't touch this</button>
</div>
```

As in HTML, values may be unquoted.

```
<input type=checkbox />
```

Attribute values can contain JavaScript expressions.

```
<a href="page/{p}">page {p}</a>
```

Or they can _be_ JavaScript expressions.

```
<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`).

```
<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:
> 
> ```
> <button disabled="{number !== 42}">...</button>
> ```

When the attribute name and value match (`name={name}`), they can be replaced with `{name}`.

```
<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.

```
<Widget foo={bar} answer={42} text="hello" />
```

## Spread attributes

_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. Order matters — if `things.a` exists it will take precedence over `a="b"`, while `c="d"` would take precedence over `things.c`:

```
<Widget a="b" {...things} c="d" />
```

## 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:

```
<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.

```
{expression}
```

Expressions that are `null` or `undefined` will be omitted; all others are [coerced to strings](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#string_coercion).

Curly braces can be included in a Svelte template by using their [HTML entity](https://developer.mozilla.org/docs/Glossary/Entity) strings: `&lbrace;`, `&lcub;`, or `&#123;` for `{` and `&rbrace;`, `&rcub;`, or `&#125;` 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.

```
<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.

```
{@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.

```
<!-- 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-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.

```
<!--
@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 ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/02-if/)

Contents

svelte

{#if ...}

Svelte

{#if ...}

```
<!--- copy: false  --->
{#if expression}...{/if}
```

```
<!--- copy: false  --->
{#if expression}...{:else if expression}...{/if}
```

```
<!--- copy: false  --->
{#if expression}...{:else}...{/if}
```

Content that is conditionally rendered can be wrapped in an if block.

```
{#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.

```
{#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 ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/03-each/)

Contents

svelte

{#each ...}

Svelte

{#each ...}

```
<!--- copy: false  --->
{#each expression as name}...{/each}
```

```
<!--- 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`. (Internally, they are converted to arrays with [`Array.from`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/from).)

If the value is `null` or `undefined`, it is treated the same as an empty array (which will cause [else blocks](#Else-blocks) to be rendered, where applicable).

```
<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:

```
{#each items as item, i}
	<li>{i + 1}: {item.name} x {item.qty}</li>
{/each}
```

## Keyed each blocks

```
<!--- copy: false  --->
{#each expression as name (key)}...{/each}
```

```
<!--- 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 intelligently update the list when data changes by inserting, moving and deleting items, rather than adding or removing items at the end and updating the state in the middle.

The key can be any object, but strings and numbers are recommended since they allow identity to persist when the objects themselves change.

```
{#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.

```
{#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

```
<!--- copy: false  --->
{#each expression}...{/each}
```

```
<!--- copy: false  --->
{#each expression, index}...{/each}
```

In case you just want to render something `n` times, you can omit the `as` part:

```
<!--- file: App.svelte --->
<div class="chess-board">
	{#each { length: 8 }, rank}
		{#each { length: 8 }, file}
			<div class:black={(rank + file) % 2 === 1}></div>
		{/each}
	{/each}
</div>

<style>
	.chess-board {
		display: grid;
		grid-template-columns: repeat(8, 1fr);
		grid-template-rows: repeat(8, 1fr);
		border: 1px solid black;
		aspect-ratio: 1;

		.black {
			background: black;
		}
	}
</style>
```

## Else blocks

```
<!--- 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.

```
{#each todos as todo}
	<p>{todo.text}</p>
{:else}
	<p>No tasks today!</p>
{/each}
```

## [{#key ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/04-key/)

Contents

svelte

{#key ...}

Svelte

{#key ...}

```
<!--- 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:

```
{#key value}
	<Component />
{/key}
```

It's also useful if you want a transition to play whenever a value changes:

```
{#key value}
	<div transition:fade>{value}</div>
{/key}
```

## [{#await ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/05-await/)

Contents

svelte

{#await ...}

Svelte

{#await ...}

```
<!--- copy: false  --->
{#await expression}...{:then name}...{:catch name}...{/await}
```

```
<!--- copy: false  --->
{#await expression}...{:then name}...{/await}
```

```
<!--- copy: false  --->
{#await expression then name}...{/await}
```

```
<!--- 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.

```
{#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).

```
{#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.

```
{#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.

```
{#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:
> 
> ```
> {#await import('./Component.svelte') then { default: Component }}
> 	<Component />
> {/await}
> ```

## [{#snippet ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/06-snippet/)

Contents

svelte

{#snippet ...}

Svelte

{#snippet ...}

```
<!--- copy: false  --->
{#snippet name()}...{/snippet}
```

```
<!--- 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=)...

```
{#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=):

```
{#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...

```
<!--- file: App.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):

```
<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:

```
<!--- file: App.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

### Explicit props

Within the template, snippets are values just like any other. As such, they can be passed to components as props:

```
<!--- file: App.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}+++ />
```

```
<!--- file: Table.svelte --->
<script>
	let { data, header, row } = $props();
</script>

<table>
	{#if header}
		<thead>
			<tr>{@render header()}</tr>
		</thead>
	{/if}

	<tbody>
		{#each data as d}
			<tr>{@render row(d)}</tr>
		{/each}
	</tbody>
</table>

<style>
	table {
		text-align: left;
		border-spacing: 0;
	}

	tbody tr:nth-child(2n+1) {
		background: ButtonFace;
	}

	table :global(th), table :global(td) {
		padding: 0.5em;
	}
</style>
```

Think about it like passing content instead of data to a component. The concept is similar to slots in web components.

### Implicit props

As an authoring convenience, snippets declared directly _inside_ a component implicitly become props _on_ the component:

```
<!--- file: App.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>

<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>
```

```
<!--- file: Table.svelte --->
<script>
	let { data, header, row } = $props();
</script>

<table>
	{#if header}
		<thead>
			<tr>{@render header()}</tr>
		</thead>
	{/if}

	<tbody>
		{#each data as d}
			<tr>{@render row(d)}</tr>
		{/each}
	</tbody>
</table>

<style>
	table {
		text-align: left;
		border-spacing: 0;
	}

	tbody tr:nth-child(2n+1) {
		background: ButtonFace;
	}

	table :global(th), table :global(td) {
		padding: 0.5em;
	}
</style>
```

### Implicit `children` snippet

Any content inside the component tags that is _not_ a snippet declaration implicitly becomes part of the `children` snippet:

```
<!--- file: App.svelte --->
<script>
	import Button from './Button.svelte';
</script>

<Button>click me</Button>
```

```
<!--- 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

### Optional snippet props

You can declare snippet props as being optional. You can either use optional chaining to not render anything if the snippet isn't set...

```
<script>
    let { children } = $props();
</script>

{@render children?.()}
```

...or use an `#if` block to render fallback content:

```
<script>
    let { children } = $props();
</script>

{#if children}
    {@render children()}
{:else}
    fallback content
{/if}
```

## Typing snippets

Snippets implement the `Snippet` interface imported from `'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:

```
<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):

```
<!--- file: App.svelte --->
<script>
	import { add } from './snippets.svelte';
</script>

{@render add(1, 2)}
```

```
<!--- file: snippets.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 so slots have been deprecated in Svelte 5.

## [{@render ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/07-@render/)

Contents

svelte

{@render ...}

Svelte

{@render ...}

To render a [snippet](snippet), use a `{@render ...}` tag.

```
{#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:

```
{@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:

```
{@render children?.()}
```

Alternatively, use an [`{#if ...}`](if) block with an `:else` clause to render fallback content:

```
{#if children}
	{@render children()}
{:else}
	<p>fallback content</p>
{/if}
```

## [{@html ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/08-@html/)

Contents

svelte

{@html ...}

Svelte

{@html ...}

To inject raw HTML into your component, use the `{@html ...}` tag:

```
<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:

```
{@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:

```
<article>
	{@html content}
</article>

<style>
	article {
		a { color: hotpink }
		img { width: 100% }
	}
</style>
```

Instead, use the `:global` modifier to target everything inside the `<article>`:

```
<style>
	article +++:global+++ {
		a { color: hotpink }
		img { width: 100% }
	}
</style>
```

## [{@attach ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/09-@attach/)

Contents

svelte

{@attach ...}

Svelte

{@attach ...}

Attachments are functions that run in an [effect]($effect) when an element is mounted to the DOM or when [state]($state) read inside the function updates.

Optionally, they can return a function that is called before the attachment re-runs, or after the element is later removed from the DOM.

> \[!NOTE\] Attachments are available in Svelte 5.29 and newer.

```
<!--- file: App.svelte --->
<script>
	/** @type {import('svelte/attachments').Attachment} */
	function myAttachment(element) {
		console.log(element.nodeName); // 'DIV'

		return () => {
			console.log('cleaning up');
		};
	}
</script>

<div {@attach myAttachment}>...</div>
```

An element can have any number of attachments.

## Attachment factories

A useful pattern is for a function, such as `tooltip` in this example, to _return_ an attachment ([demo](/playground/untitled#H4sIAAAAAAAAE3VT0XLaMBD8lavbDiaNCUlbHhTItG_5h5AH2T5ArdBppDOEMv73SkbGJGnH47F9t3un3TsfMyO3mInsh2SW1Sa7zlZKo8_E0zHjg42pGAjxBPxp7cTvUHOMldLjv-IVGUbDoUw295VTlh-WZslqa8kxsLL2ACtHWxh175NffnQfAAGikSGxYQGfPEvGfPSIWtOH0TiBVo2pWJEBJtKhQp4YYzjG9JIdcuMM5IZqHMPioY8vOSA997zQoevf4a7heO7cdp34olRiTGr07OhwH1IdoO2A7dLMbwahZq6MbRhKZWqxk7rBxTGVbuHmhCgb5qDgmIx_J6XtHHukHTrYYqx_YpzYng8aO4RYayql7hU-1ZJl0akqHBE_D9KLolwL-Dibzc7iSln9XjtqTF1UpMkJ2EmXR-BgQErsN4pxIJKr0RVO1qrxAqaTO4fbc9bKulZm3cfDY3aZDgvFGErWjmzhN7KmfX5rXyDeX8Pt1mU-hXjdBOrtuB97vK4GPUtmJ41XcRMEGDLD8do0nJ73zhUhSlyRw0t3vPqD8cjfLs-axiFgNBrkUd9Ulp50c-GLxlXAVlJX-ffpZyiSn7H0eLCUySZQcQdXlxj4El0Yv_FZvIKElqqGTruVLhzu7VRKCh22_5toOyxsWqLwwzK-cCbYNdg-hy-p9D7sbiZWUnts_wLUOF3CJgQAAA==)):

```
<!--- file: App.svelte --->
<script>
	import tippy from 'tippy.js';

	let content = $state('Hello!');

	/**
	 * @param {string} content
	 * @returns {import('svelte/attachments').Attachment}
	 */
	function tooltip(content) {
		return (element) => {
			const tooltip = tippy(element, { content });
			return tooltip.destroy;
		};
	}
</script>

<input bind:value={content} />

<button {@attach tooltip(content)}>
	Hover me
</button>
```

Since the `tooltip(content)` expression runs inside an [effect]($effect), the attachment will be destroyed and recreated whenever `content` changes. The same thing would happen for any state read _inside_ the attachment function when it first runs. (If this isn't what you want, see [Controlling when attachments re-run](#Controlling-when-attachments-re-run).)

## Inline attachments

Attachments can also be created inline ([demo](/playground/untitled#H4sIAAAAAAAAE71Wf3OaWBT9KoyTTnW3MS-I3dYmnWXVtnRAazRJzbozRSQEApiRhwKO333vuY8m225m_9yZGOT9OPfcc84D943UTfxGr_G7K6Xr3TVeNW7D2M8avT_3DVk-YAoDNF4vNB8e2tnWjyXGlm7mPzfurVPpp5JgGmeZtwkf5PtFupCxLzVvHa832rl2lElX-s2Xm2DZFNqp_hs-rZetd4v07ORpT3qmQHu7MF2td0BZp8k6z_xkvfXP902_pZ2_1_aYWEiqm0kN8I4r79qbdZ6umnq3q_2iNf22F4dE6qt2oimwdpim_uY6XMm7Fuo-IQT_iTD_CeGTHwZ38ieIJUFQRxirR1Xf39Dw0X5z0I72Af4tD61vvPNwWKQnqmfPTbduhsEd2J3vO_oBd3dc6fF2X7umNdWGf0vBRhSS6qoV7cCXfTXWfKmvWG61_si_vfU92Wz-E4RhsLhNIYinsox9QKGVd8-tuACCeKXRX12P-T_eKf7fhTq0Hvt-f3ailtSeoxJHRo1-58NoPe1UiBc1hkL8Yeh45y_vQ3mcuNl9T8s3cXPRWLnS7YWJG_gn2Tb4tUjid8jua-PVl08j_ab8I14mH8Llx0s5Tz5Err4ql52r_GYg0mVy1bEGZuD0ze64b5TWYFiM-16wSuJ4JT5vfVpDcztrcG_YkRU4s6HxufzDWF4XuVeJ1P10IbzBemt3Vp1V2e04ZXfrJd7Wicyd039brRIv_RIVu_nXi7X1cfL2sy66ztToUp1TO7qJ7NlwZ0f30pld5qNSVE5o6PbMojFHjgZB7oSicPpGteyLclQap7SvY0dXtM_LR1NT2JFHey3aaxa0VxCeYJ7RMHemoiCcgPZV9pR7o7kgcOjeGliYk9hjDZx8FAq6enwlTPSZj_vYPw9Il64dXdIY8ZmapzwfEd8-1ZyaxWhqkIZOibXUd-6Upqi1pD4uMicCV1GA_7zi73UN8BaF4sC8peJtMjfmjbHZBFwq5ov50qRaE0l96NZggnW4KqypYRAW-uhSz9ADvklwJF2J-5W0Z5fQPBhDX92R6I_0IFxRgDftge4l4dP-gH1hjD7uqU6fsOEZ9UNrCdPB-nys6uXgY6O3ZMd9sy5T9PghqrWHdjo4jB51CgLiKJaDYYA-7WgYONf1FbjkI-mE3EAfUY_rijfuJ_CVPaR50oe9JF7Q0pI8Dw3osxxYHdYPGbp2CnwHF8KvwJv2wEv0Z3ilQI6U9uwbZxbYJXvEmjjQjjCHkvNLvNg3yhzXQd1olamsT4IRrZmX0MUDpwL7R8zzHj7pSh9hPHFSHjLezKqAST51uC5zmtQ87skDUaneLokT5RbXkPWSYz53Abgjc8_o4KFGUZ-Hgv2Z1l5OTYM9D-HfUD0L-EwxH5wRnIG61gS-khfgY1bq7IAP_DA4l5xRuh9xlm8yGjutc8t-wHtkhWv3hc7aqGwiK5KzgvM5xRkZYn193uEln-su55j1GaIv7oM4iPrsVHiG0Dx7TR9-1lBfqFdwfvSd5LNL5xyZVp5NoHFZ57FkfiF6vKs4k5zvIfrX5xX6MXmt0gM5MTu8DjnhukrHHzTRd3jm0dma0_f_x5cxP9f4jBdqHvmbq2fUjzqcKh2Cp-yWj9ntcHanXmBXxhu7Q--eyjhfNFpaV7zgz4nWEUb7zUOhpevjjf_gu_KZ99pxFlZ-T3sttkmYqrco_26q35v0Ewzv5EZPbnL_8BfduWGMnyyN3q0bZ_7hb_7KG_L4CQAA)):

```
<!--- file: App.svelte --->
<canvas
	width={32}
	height={32}
	{@attach (canvas) => {
		const context = canvas.getContext('2d');

		$effect(() => {
			context.fillStyle = color;
			context.fillRect(0, 0, canvas.width, canvas.height);
		});
	}}
></canvas>
```

> \[!NOTE\] The nested effect runs whenever `color` changes, while the outer effect (where `canvas.getContext(...)` is called) only runs once, since it doesn't read any reactive state.

## Conditional attachments

Falsy values like `false` or `undefined` are treated as no attachment, enabling conditional usage:

```
<div {@attach enabled && myAttachment}>...</div>
```

## Passing attachments to components

When used on a component, `{@attach ...}` will create a prop whose key is a [`Symbol`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Symbol). If the component then [spreads](/tutorial/svelte/spread-props) props onto an element, the element will receive those attachments.

This allows you to create _wrapper components_ that augment elements ([demo](/playground/untitled#H4sIAAAAAAAAE3VUS3ObMBD-KxvajnFqsJM2PhA7TXrKob31FjITAbKtRkiMtDhJPfz3LiAMdpxhGJvdb1_fPnaeYjn3Iu-WIbJ04028lZDcetHDzsO3olbVApI74F1RhHbLJdayhFl-Sp5qhVwhufEWNjWiwJtYxSjyQhsEFEXxBiujcxg1_8O_dnQ9APwsEbVyiHDafjrvDZCgkiO4MLCEzxYZcn90z6XUZ6OxA61KlaIgV6i1pFC-sxjDrlbHaDiWRoGvdMbHsLzp5DES0mJnRxGaRBvcBHb7yFUTCQeunEWYcYtGv12TqgFUDbCK1WLaM6IWQhUlQiJUFm2ZLPly51xXMG0Rjoyd69C7UqqG2nu95QZyXvtvLVpri2-SN4hoLXXCZFfhQ8aQBU1VgdEaH_vSgyBZR_BpPp_vi0tY-rw2ulRZkGqpTQRbZvwa2BPgFC8bgbw31CbjJjAsE6WNYBZeGp7vtQXLMqHWnZx-5kM1TR5ycpkZXQR2wzL94l8Ur1C_3-g168SfQf1MyfRi3LW9fs77emJEw5QV9SREoLTq06tcczq7d6xEUcJX2vAhO1b843XK34e5unZEMBr15ekuKEusluWAF8lXhE2ZTP2r2RcIHJ-163FPKerCgYJLOB9i4GvNwviI5-gAQiFFBk3tBTOU3HFXEk0R8o86WvUD64aINhv5K3oRmpJXkw8uxMG6Hh6JY9X7OwGSqfUy9tDG3sHNoEi0d_d_fv9qndxRU0VClFqo3KVo3U655Hnt1PXB3Qra2Y2QGdEwgTAMCxopsoxOe6SD0gD8movDhT0LAnhqlE8gVCpLWnRoV7OJCkFAwEXitrYL1W7p7pbiE_P7XH6E_rihODm5s52XtiH9Ekaw0VgI9exadWL1uoEYjPtg2672k5szsxbKyWB2fdT0w5Y_0hcT8oXOlRetmLS8-g-6TLXXQgYAAA==)):

```
<!--- file: Button.svelte --->
<script>
	/** @type {import('svelte/elements').HTMLButtonAttributes} */
	let { children, ...props } = $props();
</script>

<!-- `props` includes attachments -->
<button {...props}>
	{@render children?.()}
</button>
```

```
<!--- file: App.svelte --->
<script>
	import tippy from 'tippy.js';
	import Button from './Button.svelte';

	let content = $state('Hello!');

	/**
	 * @param {string} content
	 * @returns {import('svelte/attachments').Attachment}
	 */
	function tooltip(content) {
		return (element) => {
			const tooltip = tippy(element, { content });
			return tooltip.destroy;
		};
	}
</script>

<input bind:value={content} />

<Button {@attach tooltip(content)}>
	Hover me
</Button>
```

## Controlling when attachments re-run

Attachments, unlike [actions](use), are fully reactive: `{@attach foo(bar)}` will re-run on changes to `foo` _or_ `bar` (or any state read inside `foo`):

```
// @errors: 7006 2304 2552
function foo(bar) {
	return (node) => {
		veryExpensiveSetupWork(node);
		update(node, bar);
	};
}
```

In the rare case that this is a problem (for example, if `foo` does expensive and unavoidable setup work) consider passing the data inside a function and reading it in a child effect:

```
// @errors: 7006 2304 2552
function foo(+++getBar+++) {
	return (node) => {
		veryExpensiveSetupWork(node);

+++		$effect(() => {
			update(node, getBar());
		});+++
	}
}
```

## Creating attachments programmatically

To add attachments to an object that will be spread onto a component or element, use [`createAttachmentKey`](svelte-attachments#createAttachmentKey).

## Converting actions to attachments

If you're using a library that only provides actions, you can convert them to attachments with [`fromAction`](svelte-attachments#fromAction), allowing you to (for example) use them with components.

## [{@const ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/10-@const/)

Contents

svelte

{@const ...}

Svelte

{@const ...}

> \[!NOTE\] `{@const x = y}` is legacy syntax — use [`{const x = $derived(y)}`](declaration-tags) instead

The `{@const ...}` tag defines a local constant.

```
{#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 ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/11-@debug/)

Contents

svelte

{@debug ...}

Svelte

{@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.

```
<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).

```
<!-- 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.

## [{let/const ...}](https://docharvest.github.io/docs/svelte/03-template-syntax/11-declaration-tags/)

Contents

svelte

{let/const ...}

Svelte

{let/const ...}

Declaration tags define local variables inside markup with `const` or `let`:

```
<!--- file: App.svelte --->
<script>
	let boxes = [{ width: 10, height: 10 }, { width: 15, height: 15 }];
</script>

{#each boxes as box}
	{const area = box.width * box.height}
	{const label = `${box.width} ⨉ ${box.height} = ${area}`}

	<p>{label}</p>
{/each}
```

> \[!NOTE\] Declaration tags are available since Svelte 5.56.

> \[!NOTE\] The [`{@const ...}`](@const) syntax is considered legacy — use declaration tags instead.

When values should be reactive, you can use `$state` and `$derived`:

```
<!--- file: App.svelte --->
<script>
	let user = $state({ name: 'Svelte' });
	let editing = $state(false);
</script>

<p>Hello {user.name}</p>
<button onclick={() => editing = true}>edit name</button>

{#if editing}
	{let name = $state(user.name)}
	{const greeting = $derived(`Hello ${name}`)}

	<hr>
	<input bind:value={name} />
	<p>{greeting}</p>

	<button onclick={() => {
		user.name = name;
		editing = false;
	}}>save</button>
{/if}
```

Declaration tags can be used anywhere inside the component. They can reference values declared outside themselves (for example in the `<script>` tag or in `{#each ...}` blocks) and are 'visible' to everything in the same lexical scope (i.e. siblings, and children of those siblings):

```
<!--- file: App.svelte --->
{const hello = 'hello'}
{hello} <!-- 'hello' -->
<div>
	{const hello = 'hi'}
	{hello} <!-- 'hi' -->
	<div>
		{hello} <!-- 'hi' -->
	</div>
</div>
{hello} <!-- 'hello' -->
```

## [12 Bind](https://docharvest.github.io/docs/svelte/03-template-syntax/12-bind/)

Contents

svelte

12 Bind

Svelte

12 Bind

## --- title: 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_](https://press.rebus.community/programmingfundamentals/chapter/lvalue-and-rvalue/) (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:

```
<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:

```
<input bind:value={
	() => value,
	(v) => value = v.toLowerCase()}
/>
```

In the case of readonly bindings like [dimension bindings](#Dimensions), the `get` value should be `null`:

```
<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:

```
<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:

```
<!--- file: App.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`.

```
<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 inputs can be bound with `bind:checked`:

```
<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`.

```
<script>
	let checked = $state(true);
</script>

<form>
	<input type="checkbox" bind:checked defaultChecked={true}>
	<input type="reset" value="Reset">
</form>
```

> \[!NOTE\] Use `bind:group` for radio inputs instead of `bind:checked`.

## `<input bind:indeterminate>`

Checkboxes can be in an [indeterminate](https://developer.mozilla.org/en-US/docs/Web/API/HTMLInputElement/indeterminate) state, independently of whether they are checked or unchecked:

```
<script>
	let checked = $state(false);
	let indeterminate = $state(true);
</script>

<form>
	<input type="checkbox" bind:checked bind:indeterminate>

	{#if indeterminate}
		waiting...
	{:else if checked}
		checked
	{:else}
		unchecked
	{/if}
</form>
```

## `<input bind:group>`

Inputs that work together can use `bind:group`:

```
<!--- file: App.svelte --->
<script>
	let tortilla = $state('Plain');

	/** @type {string[]} */
	let fillings = $state([]);
</script>

<h1>Customize your burrito</h1>

<!-- grouped radio inputs are mutually exclusive -->
<label><input type="radio" bind:group={tortilla} value="Plain" /> Plain</label>
<label><input type="radio" bind:group={tortilla} value="Whole wheat" /> Whole wheat</label>
<label><input type="radio" bind:group={tortilla} value="Spinach" /> Spinach</label>

<!-- grouped checkbox inputs populate an array -->
<label><input type="checkbox" bind:group={fillings} value="Rice" /> Rice</label>
<label><input type="checkbox" bind:group={fillings} value="Beans" /> Beans</label>
<label><input type="checkbox" bind:group={fillings} value="Cheese" /> Cheese</label>
<label><input type="checkbox" bind:group={fillings} value="Guac (extra)" /> Guac (extra)</label>

<p>Tortilla: {tortilla}</p>
<p>Fillings: {fillings.join(', ') || 'None'}</p>

<style>
	label {
		display: block;
	}
</style>
```

> \[!NOTE\] `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.

```
<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).

```
<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>`.

```
<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.

```
<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`.

```
<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 six 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)
-   [`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)
-   [`played`](https://developer.mozilla.org/en-US/docs/Web/API/HTMLMediaElement/played)

```
<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.

```
<details bind:open={isOpen}>
	<summary>How do you comfort a JavaScript bug?</summary>
	<p>You console it.</p>
</details>
```

## `window` and `document`

To bind to properties of `window` and `document`, see [`<svelte:window>`](svelte-window) and [`<svelte:document>`](svelte-document).

## 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).

```
<div contenteditable="true" bind:innerHTML={html}></div>
```

## 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)
-   [`contentRect`](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserverEntry/contentRect)
-   [`contentBoxSize`](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserverEntry/contentBoxSize)
-   [`borderBoxSize`](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserverEntry/borderBoxSize)
-   [`devicePixelContentBoxSize`](https://developer.mozilla.org/en-US/docs/Web/API/ResizeObserverEntry/devicePixelContentBoxSize)

```
<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`. Note that CSS transformations do not trigger `ResizeObserver` callbacks.

## bind:this

```
<!--- 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:

```
<script>
	/** @type {HTMLCanvasElement} */
	let canvas;

	$effect(() => {
		const ctx = canvas.getContext('2d');
		drawStuff(ctx);
	});
</script>

<canvas bind:this={canvas}></canvas>
```

Components also support `bind:this`, allowing you to interact with component instances programmatically.

```
<!--- file: App.svelte --->
<ShoppingCart bind:this={cart} />

<button onclick={() => cart.empty()}> Empty shopping cart </button>
```

```
<!--- file: ShoppingCart.svelte --->
<script>
	// All instance exports are available on the instance object
	export function empty() {
		// ...
	}
</script>
```

> \[!NOTE\] In case of using [the function bindings](#Function-bindings), the getter is required to ensure that the correct value is nullified on component or element destruction.

## bind:_property_ for components

```
bind:property={variable}
```

You can bind to component props using the same syntax as for elements.

```
<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:

```
<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:

```
<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:](https://docharvest.github.io/docs/svelte/03-template-syntax/13-use/)

Contents

svelte

use:

Svelte

use:

> \[!NOTE\] In Svelte 5.29 and newer, consider using [attachments](@attach) instead, as they are more flexible and composable.

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:

```
<!--- 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:

```
<!--- 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:

```
<!--- 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:](https://docharvest.github.io/docs/svelte/03-template-syntax/14-transition/)

Contents

svelte

transition:

Svelte

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.

```
<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}
```

## 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.

```
{#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}
```

## Built-in transitions

A selection of built-in transitions can be imported from the [`svelte/transition`](svelte-transition) module.

## Transition parameters

Transitions can have parameters.

(The double `{{curlies}}` aren't a special syntax; this is an object literal inside an expression tag.)

```
{#if visible}
	<div transition:fade={{ duration: 2000 }}>fades in and out over two seconds</div>
{/if}
```

## Custom transition functions

```
/// 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.

```
<!--- 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.

```
<!--- 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`

```
{#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:](https://docharvest.github.io/docs/svelte/03-template-syntax/15-in-and-out/)

Contents

svelte

in: and out:

Svelte

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.

```
<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:](https://docharvest.github.io/docs/svelte/03-template-syntax/16-animate/)

Contents

svelte

animate:

Svelte

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).

```
<!-- 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.)

```
{#each list as item, index (item)}
	<li animate:flip={{ delay: 500 }}>{item}</li>
{/each}
```

## Custom animation functions

```
/// 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.

```
<!--- 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.

```
<!--- 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:](https://docharvest.github.io/docs/svelte/03-template-syntax/17-style/)

Contents

svelte

style:

Svelte

style:

The `style:` directive provides a shorthand for setting multiple styles on an element.

```
<!-- These are equivalent -->
<div style:color="red">...</div>
<div style="color: red;">...</div>
```

The value can contain arbitrary expressions:

```
<div style:color={myColor}>...</div>
```

The shorthand form is allowed:

```
<div style:color>...</div>
```

Multiple styles can be set on a single element:

```
<div style:color style:width="12rem" style:background-color={darkMode ? 'black' : 'white'}>...</div>
```

To mark a style as important, use the `|important` modifier:

```
<div style:color|important="red">...</div>
```

When `style:` directives are combined with `style` attributes, the directives will take precedence, even over `!important` properties:

```
<div style:color="red" style="color: blue">This will be red</div>
<div style:color="red" style="color: blue !important">This will still be red</div>
```

You can set CSS custom properties:

```
<div style:--columns={columns}>...</div>
```

## [class](https://docharvest.github.io/docs/svelte/03-template-syntax/18-class/)

Contents

svelte

class

Svelte

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:

```
<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:

```
<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:

```
<!-- 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:

```
<!--- 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:

```
<!--- 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>
```

Since Svelte 5.19, 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:

```
<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.

```
<!-- 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:

```
<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.

## [await](https://docharvest.github.io/docs/svelte/03-template-syntax/19-await-expressions/)

Contents

svelte

await

Svelte

await

As of Svelte 5.36, you can use the `await` keyword inside your components in three places where it was previously unavailable:

-   at the top level of your component's `<script>`
-   inside `$derived(...)` declarations
-   inside your markup

This feature is currently experimental, and you must opt in by adding the `experimental.async` option wherever you [configure](/docs/kit/configuration) Svelte, usually `svelte.config.js`:

```
/// file: svelte.config.js
export default {
	compilerOptions: {
		experimental: {
			async: true
		}
	}
};
```

The experimental flag will be removed in Svelte 6.

## Synchronized updates

When an `await` expression depends on a particular piece of state, changes to that state will not be reflected in the UI until the asynchronous work has completed, so that the UI is not left in an inconsistent state. In other words, in an example like this...

```
<!--- file: App.svelte --->
<script>
	let a = $state(1);
	let b = $state(2);

	async function add(a, b) {
		await new Promise((f) => setTimeout(f, 500)); // artificial delay
		return a + b;
	}
</script>

<input type="number" bind:value={a}>
<input type="number" bind:value={b}>

<p>{a} + {b} = {await add(a, b)}</p>
```

...if you increment `a`, the contents of the `<p>` will _not_ immediately update to read this —

```
<p>2 + 2 = 3</p>
```

— instead, the text will update to `2 + 2 = 4` when `add(a, b)` resolves.

Updates can overlap — a fast update will be reflected in the UI while an earlier slow update is still ongoing.

## Concurrency

Svelte will do as much asynchronous work as it can in parallel. For example if you have two `await` expressions in your markup...

```
<p>{await one(x)}</p>
<p>{await two(y)}</p>
```

...both functions will run at the same time, as they are independent expressions, even though they are _visually_ sequential.

This does not apply to sequential `await` expressions inside your `<script>` or inside async functions — these run like any other asynchronous JavaScript. An exception is that independent `$derived` expressions will update independently, even though they will run sequentially when they are first created:

```
/** @param {number} x */
async function one(x) { return x; }
/** @param {number} y */
async function two(y) { return y; }
let x = $state(1);
let y = $state(2);
// ---cut---
// `b` will not be created until `a` has resolved,
// but once created they will update independently
// even if `x` and `y` update simultaneously
let a = $derived(await one(x));
let b = $derived(await two(y));
```

> \[!NOTE\] If you write code like this, expect Svelte to give you an [`await_waterfall`](runtime-warnings#Client-warnings-await_waterfall) warning

## Indicating loading states

To render placeholder UI, you can wrap content in a `<svelte:boundary>` with a [`pending`](svelte-boundary#Properties-pending) snippet. This will be shown when the boundary is first created, but not for subsequent updates, which are globally coordinated.

After the contents of a boundary have resolved for the first time and have replaced the `pending` snippet, you can detect subsequent async work with [`$effect.pending()`]($effect#$effect.pending). This is what you would use to display a "we're asynchronously validating your input" spinner next to a form field, for example.

You can also use [`settled()`](svelte#settled) to get a promise that resolves when the current update is complete:

```
let color = 'red';
let answer = -1;
let updating = false;
// ---cut---
import { tick, settled } from 'svelte';

async function onclick() {
	updating = true;

	// without this, the change to `updating` will be
	// grouped with the other changes, meaning it
	// won't be reflected in the UI
	await tick();

	color = 'octarine';
	answer = 42;

	await settled();

	// any updates affected by `color` or `answer`
	// have now been applied
	updating = false;
}
```

## Error handling

Errors in `await` expressions will bubble to the nearest [error boundary](svelte-boundary).

## Server-side rendering

Svelte supports asynchronous server-side rendering (SSR) with the `render(...)` API. To use it, simply await the return value:

```
/// file: server.js
import { render } from 'svelte/server';
import App from './App.svelte';

const { head, body } = +++await+++ render(App);
```

> \[!NOTE\] If you're using a framework like SvelteKit, this is done on your behalf.

If a `<svelte:boundary>` with a `pending` snippet is encountered during SSR, that snippet will be rendered while the rest of the content is ignored. All `await` expressions encountered outside boundaries with `pending` snippets will resolve and render their contents prior to `await render(...)` returning.

> \[!NOTE\] In the future, we plan to add a streaming implementation that renders the content in the background.

## Forking

The [`fork(...)`](svelte#fork) API, added in 5.42, makes it possible to run `await` expressions that you _expect_ to happen in the near future. This is mainly intended for frameworks like SvelteKit to implement preloading when (for example) users signal an intent to navigate.

```
<script>
	import { fork } from 'svelte';
	import Menu from './Menu.svelte';

	let open = $state(false);

	/** @type {import('svelte').Fork | null} */
	let pending = null;

	function preload() {
		pending ??= fork(() => {
			open = true;
		});
	}

	function discard() {
		pending?.discard();
		pending = null;
	}
</script>

<button
	onfocusin={preload}
	onfocusout={discard}
	onpointerenter={preload}
	onpointerleave={discard}
	onclick={() => {
		pending?.commit();
		pending = null;

		// in case `pending` didn't exist
		// (if it did, this is a no-op)
		open = true;
	}}
>open menu</button>

{#if open}
	<!-- any async work inside this component will start
	     as soon as the fork is created -->
	<Menu onclose={() => open = false} />
{/if}
```

## Caveats

As an experimental feature, the details of how `await` is handled (and related APIs like `$effect.pending()`) are subject to breaking changes outside of a semver major release, though we intend to keep such changes to a bare minimum.

## Breaking changes

Effects run in a slightly different order when the `experimental.async` option is `true`. Specifically, _block_ effects like `{#if ...}` and `{#each ...}` now run before an `$effect.pre` or `beforeUpdate` in the same component, which means that in [very rare situations](/playground/untitled?#H4sIAAAAAAAAE22R3VLDIBCFX2WLvUhnTHsf0zre-Q7WmfwtFV2BgU1rJ5N3F0jaOuoVcPbw7VkYhK4_URTiGYkMnIyjDjLsFGO3EvdCKkIvipdB8NlGXxSCPt96snbtj0gctab2-J_eGs2oOWBE6VunLO_2es-EDKZ5x5ZhC0vPNWM2gHXGouNzAex6hHH1cPHil_Lsb95YT9VQX6KUAbS2DrNsBdsdDFHe8_XSYjH1SrhELTe3MLpsemajweiWVPuxHSbKNd-8eQTdE0EBf4OOaSg2hwNhhE_ABB_ulJzjj9FULvIcqgm5vnAqUB7wWFMfhuugQWkcAr8hVD-mq8D12kOep24J_IszToOXdveGDsuNnZwbJUNlXsKnhJdhUcTo42s41YpOSneikDV5HL8BktM6yRcCAAA=) it is possible to update a block that should no longer exist, but only if you update state inside an effect, [which you should avoid]($effect#When-not-to-use-$effect).

## [Styling](https://docharvest.github.io/docs/svelte/04-styling/)

Contents

svelte

Styling

Svelte

Styling

## [Scoped styles](https://docharvest.github.io/docs/svelte/04-styling/01-scoped-styles/)

Contents

svelte

Scoped styles

Svelte

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`).

```
<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:

```
<style>
	.bouncy {
		animation: bounce 10s;
	}

	/* these keyframes are only accessible inside this component */
	@keyframes bounce {
		/* ... */
	}
</style>
```

## [Global styles](https://docharvest.github.io/docs/svelte/04-styling/02-global-styles/)

Contents

svelte

Global styles

Svelte

Global styles

## :global(...)

To apply styles to a single selector globally, use the `:global(...)` modifier:

```
<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.

```
<style>
	@keyframes -global-my-animation-name {
		/* code goes here */
	}
</style>
```

## :global

To apply styles to a group of selectors globally, create a `:global {...}` block:

```
<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](https://docharvest.github.io/docs/svelte/04-styling/03-custom-properties/)

Contents

svelte

Custom properties

Svelte

Custom properties

You can pass CSS custom properties — both static and dynamic — to components:

```
<Slider
	bind:value
	min={0}
	max={100}
	--track-color="black"
	--thumb-color="rgb({r} {g} {b})"
/>
```

The above code essentially desugars to this:

```
<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:

```
<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):

```
<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 · docharvest](https://docharvest.github.io/docs/svelte/04-styling/04-nested-style-elements/)



## [Special elements](https://docharvest.github.io/docs/svelte/05-special-elements/)

Contents

svelte

Special elements

Svelte

Special elements

## [svelte:boundary](https://docharvest.github.io/docs/svelte/05-special-elements/01-svelte-boundary/)

Contents

svelte

<svelte:boundary>

Svelte

<svelte:boundary>

```
<svelte:boundary onerror={handler}>...</svelte:boundary>
```

> \[!NOTE\] This feature was added in 5.3.0

Boundaries allow you to 'wall off' parts of your app, so that you can:

-   provide UI that should be shown when [`await`](await-expressions) expressions are first resolving
-   handle errors that occur during rendering or while running effects, and provide UI that should be rendered when an error happens

If a boundary handles an error (with a `failed` snippet or `onerror` handler, or both) its existing content will be removed.

> \[!NOTE\] 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 more of the following must be provided.

### `pending`

This snippet will be shown when the boundary is first created, and will remain visible until all the [`await`](await-expressions) expressions inside the boundary have resolved ([demo](/playground/untitled#H4sIAAAAAAAAE21QQW6DQAz8ytY9BKQVpFdKkPqDHnorPWzAaSwt3tWugUaIv1eE0KpKD5as8YxnNBOw6RAKKOOAVrA4up5bEy6VGknOyiO3xJ8qMnmPAhpOZDFC8T6BXPyiXADQ258X77P1FWg4moj_4Y1jQZZ49W0CealqruXUcyPkWLVozQXbZDC2R606spYiNo7bqA7qab_fp2paFLUElD6wYhzVa3AdRUySgNHZAVN1qDZaLRHljTp0vSTJ9XJjrSbpX5f0eZXN6zLXXOa_QfmurIVU-moyoyH5ib87o7XuYZfOZe6vnGWmx1uZW7lJOq9upa-sMwuUZdkmmfIbfQ1xZwwaBL8ECgk9zh8axJAdiVsoTsZGnL8Bg4tX_OMBAAA=)):

```
<svelte:boundary>
	<p>{await delayed('hello!')}</p>

	{#snippet pending()}
		<p>loading...</p>
	{/snippet}
</svelte:boundary>
```

The `pending` snippet will _not_ be shown for subsequent async updates — for these, you can use [`$effect.pending()`]($effect#$effect.pending).

> \[!NOTE\] In the [playground](/playground), your app is rendered inside a boundary with an empty pending snippet, so that you can use `await` without having to create one.

### `failed`

If a `failed` snippet is provided, it will be rendered when an error is thrown inside the boundary, with the `error` and a `reset` function that recreates the contents ([demo](/playground/hello-world#H4sIAAAAAAAAE3VRy26DMBD8lS2tFCIh6JkAUlWp39Cq9EBg06CAbdlLArL87zWGKk8ORnhmd3ZnrD1WtOjFXqKO2BDGW96xqpBD5gXerm5QefG39mgQY9EIWHxueRMinLosti0UPsJLzggZKTeilLWgLGc51a3gkuCjKQ7DO7cXZotgJ3kLqzC6hmex1SZnSXTWYHcrj8LJjWTk0PHoZ8VqIdCOKayPykcpuQxAokJaG1dGybYj4gw4K5u6PKTasSbjXKgnIDlA8VvUdo-pzonraBY2bsH7HAl78mKSHZpgIcuHjq9jXSpZSLixRlveKYQUXhQVhL6GPobXAAb7BbNeyvNUs4qfRg3OnELLj5hqH9eQZqCnoBwR9lYcQxuVXeBzc8kMF8yXY4yNJ5oGiUzP_aaf_waTRGJib5_Ad3P_vbCuaYxzeNpbU0eUMPAOKh7Yw1YErgtoXyuYlPLzc10_xo_5A91zkQL_AgAA)):

```
<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: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:boundary onerror={(e) => report(e)}>
	...
</svelte:boundary>
```

...or using `error` and `reset` outside the boundary itself:

```
<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.

## Using `transformError`

By default, error boundaries have no effect on the server — if an error occurs during rendering, the render as a whole will fail.

Since 5.51 you can control this behaviour for boundaries with a `failed` snippet, by calling [`render(...)`](imperative-component-api#render) with a `transformError` function.

> \[!NOTE\] If you're using Svelte via a framework such as SvelteKit, you most likely don't have direct access to the `render(...)` call — the framework must configure `transformError` on your behalf. SvelteKit will add support for this in the near future, via the [`handleError`](../kit/hooks#Shared-hooks-handleError) hook.

The `transformError` function must return a JSON-stringifiable object which will be used to render the `failed` snippet. This object will be serialized and used to hydrate the snippet in the browser:

```
// @errors: 1005
import { render } from 'svelte/server';
import App from './App.svelte';

const { head, body } = await render(App, {
	transformError: (error) => {
		// log the original error, with the stack trace...
		console.error(error);

		// ...and return a sanitized user-friendly error
		// to display in the `failed` snippet
		return {
			message: 'An error occurred!'
		};
	};
});
```

If `transformError` throws (or rethrows) an error, `render(...)` as a whole will fail with that error.

> \[!NOTE\] Errors that occur during server-side rendering can contain sensitive information in the `message` and `stack`. It's recommended to redact these rather than sending them unaltered to the browser.

If the boundary has an `onerror` handler, it will be called upon hydration with the deserialized error object.

The [`mount`](imperative-component-api#mount) and [`hydrate`](imperative-component-api#hydrate) functions also accept a `transformError` option, which defaults to the identity function. As with `render`, this function transforms a render-time error before it is passed to a `failed` snippet or `onerror` handler.

## [svelte:window](https://docharvest.github.io/docs/svelte/05-special-elements/02-svelte-window/)

Contents

svelte

<svelte:window>

Svelte

<svelte:window>

```
<svelte:window onevent={handler} />
```

```
<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.

```
<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: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](https://docharvest.github.io/docs/svelte/05-special-elements/03-svelte-document/)

Contents

svelte

<svelte:document>

Svelte

<svelte:document>

```
<svelte:document onevent={handler} />
```

```
<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 [attachments](@attach) 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:document onvisibilitychange={handleVisibilityChange} {@attach someAttachment} />
```

You can also bind to the following properties:

-   `activeElement`
-   `fullscreenElement`
-   `pointerLockElement`
-   `visibilityState`

All are readonly.

## [svelte:body](https://docharvest.github.io/docs/svelte/05-special-elements/04-svelte-body/)

Contents

svelte

<svelte:body>

Svelte

<svelte:body>

```
<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 at the top level of your component and must never be inside a block or element.

```
<svelte:body onmouseenter={handleMouseenter} onmouseleave={handleMouseleave} use:someAction />
```

## [svelte:head](https://docharvest.github.io/docs/svelte/05-special-elements/05-svelte-head/)

Contents

svelte

<svelte:head>

Svelte

<svelte:head>

```
<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:head>
	<title>Hello world!</title>
	<meta name="description" content="This is where the description goes for SEO" />
</svelte:head>
```

## [svelte:element](https://docharvest.github.io/docs/svelte/05-special-elements/06-svelte-element/)

Contents

svelte

<svelte:element>

Svelte

<svelte:element>

```
<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:

```
<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: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](https://docharvest.github.io/docs/svelte/05-special-elements/07-svelte-options/)

Contents

svelte

<svelte:options>

Svelte

<svelte:options>

```
<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:options customElement="my-custom-element" />
```

## [Runtime](https://docharvest.github.io/docs/svelte/06-runtime/)

Contents

svelte

Runtime

Svelte

Runtime

## [Stores](https://docharvest.github.io/docs/svelte/06-runtime/01-stores/)

Contents

svelte

Stores

Svelte

Stores

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.

```
<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

```
/// file: state.svelte.js
export const userState = $state({
	name: 'name',
	/* ... */
});
```

```
<!--- 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.

```
/// 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.

```
/// 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`.

```
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.

```
// @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`.

```
// @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.

```
// @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.

```
// @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.

```
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.

```
// @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

```
// @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](https://docharvest.github.io/docs/svelte/06-runtime/02-context/)

Contents

svelte

Context

Svelte

Context

Context allows components to access values owned by parent components without passing them down as props (potentially through many layers of intermediate components, known as 'prop-drilling').

By creating a `[get, set]` pair of functions with `createContext`, you can set the context in a parent component and get it in a child component:

```
<!--- file: App.svelte --->
<script>
	import Parent from './Parent.svelte';
	import Child from './Child.svelte';
</script>

<Parent>
	<Child />
</Parent>
```

```
<!--- file: Parent.svelte --->
<script>
	import { setUserContext } from './context';

	let { children } = $props();

	setUserContext({ name: 'world' });
</script>

{@render children()}
```

```
<!--- file: Child.svelte --->
<script>
	import { getUserContext } from './context';

	const user = getUserContext();
</script>

<h1>hello {user.name}, inside Child.svelte</h1>
```

```
/// file: context.ts
import { createContext } from 'svelte';

interface User {
	name: string;
}

export const [getUserContext, setUserContext] = createContext<User>();
```

> \[!NOTE\] `createContext` was added in version 5.40. If you are using an earlier version of Svelte, you must use `setContext` and `getContext` instead.

This is particularly useful when `Parent.svelte` is not directly aware of `Child.svelte`, but instead renders it as part of a `children` [snippet](snippet) as shown above.

## `setContext` and `getContext`

As an alternative to `createContext`, you can use `setContext` and `getContext` directly. The parent component sets context with `setContext(key, value)`...

```
<!--- file: Parent.svelte --->
<script>
	import { setContext } from 'svelte';

	setContext('my-context', 'hello from Parent.svelte');
</script>
```

...and the child retrieves it with `getContext`:

```
<!--- file: Child.svelte --->
<script>
	import { getContext } from 'svelte';

	const message = getContext('my-context');
</script>

<h1>{message}, inside Child.svelte</h1>
```

The key (`'my-context'`, in the example above) and the context itself can be any JavaScript value.

> \[!NOTE\] `createContext` is preferred since it provides better type safety and makes it unnecessary to use keys.

In addition to [`setContext`](svelte#setContext) and [`getContext`](svelte#getContext), Svelte exposes [`hasContext`](svelte#hasContext) and [`getAllContexts`](svelte#getAllContexts) functions.

## Using context with state

You can store reactive state in context...

```
<!--- file: App.svelte --->
<script>
	import { setCounter } from './context.ts';
	import Child from './Child.svelte';

	let counter = $state({
		count: 0
	});

	setCounter(counter);
</script>

<button onclick={() => counter.count += 1}>
	increment
</button>

<Child />
<Child />
<Child />

<button onclick={() => counter.count = 0}>
	reset
</button>
```

```
<!--- file: Child.svelte --->
<script>
	import { getCounter } from './context.ts';

	const counter = getCounter();
</script>

<p>{counter.count}</p>
```

```
/// file: context.ts
import { createContext } from 'svelte';

interface Counter {
	count: number;
}

export const [getCounter, setCounter] = createContext<Counter>();
```

...though note that if you _reassign_ `counter` instead of updating it, you will 'break the link' — in other words instead of this...

```
<button onclick={() => counter = { count: 0 } }>
	reset
</button>
```

...you must do this:

```
<button onclick={() => +++counter.count = 0+++}>
	reset
</button>
```

Svelte will warn you if you get it wrong.

Similarly, to pass primitive values through context, use functions as described in [Passing state into functions]($state#Passing-state-into-functions).

## Component testing

When writing [component tests](testing#Unit-and-component-tests-with-Vitest-Component-testing), it can be useful to create a wrapper component that sets the context in order to check the behaviour of a component that uses it. As of version 5.49, you can do this sort of thing:

```
import { mount, unmount } from 'svelte';
import { expect, test } from 'vitest';
import { setUserContext } from './context';
import MyComponent from './MyComponent.svelte';

test('MyComponent', () => {
	function Wrapper(...args) {
		setUserContext({ name: 'Bob' });
		return MyComponent(...args);
	}

	const component = mount(Wrapper, {
		target: document.body
	});

	expect(document.body.innerHTML).toBe('<h1>Hello Bob!</h1>');

	unmount(component);
});
```

This approach also works with [`hydrate`](imperative-component-api#hydrate) and [`render`](imperative-component-api#render).

## Replacing global state

When you have state shared by many different components, you might be tempted to put it in its own module and just import it wherever it's needed:

```
/// file: state.svelte.js
export const myGlobalState = $state({
	user: {
		// ...
	}
	// ...
});
```

In many cases this is perfectly fine, but there is a risk: if you mutate the state during server-side rendering (which is discouraged, but entirely possible!)...

```
<!--- file: App.svelte --->
<script>
	import { myGlobalState } from './state.svelte.js';

	let { data } = $props();

	if (data.user) {
		myGlobalState.user = data.user;
	}
</script>
```

...then the data may be accessible by the _next_ user. Context solves this problem because it is not shared between requests.

## [Lifecycle hooks](https://docharvest.github.io/docs/svelte/06-runtime/03-lifecycle-hooks/)

Contents

svelte

Lifecycle hooks

Svelte

Lifecycle hooks

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.

```
<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.

```
<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` is _synchronous_. `async` functions always return a `Promise`.

## `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.

```
<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.

```
<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.

```
<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==)

```
<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() {
		theme = theme === 'dark' ? 'light' : 'dark';
	}
</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](https://docharvest.github.io/docs/svelte/06-runtime/04-imperative-component-api/)

Contents

svelte

Imperative component API

Svelte

Imperative component API

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:

```
// @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:

```
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:

```
// @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:

```
// @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.

## [Hydratable data](https://docharvest.github.io/docs/svelte/06-runtime/05-hydratable/)

Contents

svelte

Hydratable data

Svelte

Hydratable data

In Svelte, when you want to render asynchronous content data on the server, you can simply `await` it. This is great! However, it comes with a pitfall: when hydrating that content on the client, Svelte has to redo the asynchronous work, which blocks hydration for however long it takes:

```
<script>
  import { getUser } from 'my-database-library';

  // This will get the user on the server, render the user's name into the h1,
  // and then, during hydration on the client, it will get the user _again_,
  // blocking hydration until it's done.
  const user = await getUser();
</script>

<h1>{user.name}</h1>
```

That's silly, though. If we've already done the hard work of getting the data on the server, we don't want to get it again during hydration on the client. `hydratable` is a low-level API built to solve this problem. You probably won't need this very often — it will be used behind the scenes by whatever datafetching library you use. For example, it powers [remote functions in SvelteKit](/docs/kit/remote-functions).

To fix the example above:

```
<script>
  import { hydratable } from 'svelte';
  import { getUser } from 'my-database-library';

  // During server rendering, this will serialize and stash the result of `getUser`, associating
  // it with the provided key and baking it into the `head` content. During hydration, it will
  // look for the serialized version, returning it instead of running `getUser`. After hydration
  // is done, if it's called again, it'll simply invoke `getUser`.
  const user = await hydratable('user', () => getUser());
</script>

<h1>{user.name}</h1>
```

This API can also be used to provide access to random or time-based values that are stable between server rendering and hydration. For example, to get a random number that doesn't update on hydration:

```
import { hydratable } from 'svelte';
const rand = hydratable('random', () => Math.random());
```

If you're a library author, be sure to prefix the keys of your `hydratable` values with the name of your library so that your keys don't conflict with other libraries.

## Serialization

All data returned from a `hydratable` function must be serializable. But this doesn't mean you're limited to JSON — Svelte uses [`devalue`](https://npmjs.com/package/devalue), which can serialize all sorts of things including `Map`, `Set`, `URL`, and `BigInt`. Check the documentation page for a full list. In addition to these, thanks to some Svelte magic, you can also fearlessly use promises:

```
<script>
  import { hydratable } from 'svelte';
  const promises = hydratable('random', () => {
    return {
      one: Promise.resolve(1),
      two: Promise.resolve(2)
    }
  });
</script>

{await promises.one}
{await promises.two}
```

## CSP

`hydratable` adds an inline `<script>` block to the `head` returned from `render`. If you're using [Content Security Policy](https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/CSP) (CSP), this script will likely fail to run. You can provide a `nonce` to `render`:

```
/// file: server.js
import { render } from 'svelte/server';
import App from './App.svelte';
// ---cut---
const nonce = crypto.randomUUID();

const { head, body } = await render(App, {
	csp: { nonce }
});
```

This will add the `nonce` to the script block, on the assumption that you will later add the same nonce to the CSP header of the document that contains it:

```
/// file: server.js
let response = new Response();
let nonce = 'xyz123';
// ---cut---
response.headers.set(
  'Content-Security-Policy',
  `script-src 'nonce-${nonce}'`
 );
```

It's essential that a `nonce` — which, British slang definition aside, means 'number used once' — is only used when dynamically server rendering an individual response.

If instead you are generating static HTML ahead of time, you must use hashes instead:

```
/// file: server.js
import { render } from 'svelte/server';
import App from './App.svelte';
// ---cut---
const { head, body, hashes } = await render(App, {
	csp: { hash: true }
});
```

`hashes.script` will be an array of strings like `["sha256-abcd123"]`. As with `nonce`, the hashes should be used in your CSP header:

```
/// file: server.js
let response = new Response();
let hashes = { script: ['sha256-xyz123'] };
// ---cut---
response.headers.set(
  'Content-Security-Policy',
  `script-src ${hashes.script.map((hash) => `'${hash}'`).join(' ')}`
 );
```

We recommend using `nonce` over hash if you can, as `hash` will interfere with streaming SSR in the future.

## [Misc](https://docharvest.github.io/docs/svelte/07-misc/)

Contents

svelte

Misc

Svelte

Misc

## [Best practices](https://docharvest.github.io/docs/svelte/07-misc/01-best-practices/)

Contents

svelte

Best practices

Svelte

Best practices

This document outlines some best practices that will help you write fast, robust Svelte apps. It is also available as a `svelte-core-bestpractices` skill for your agents.

## `$state`

Only use the `$state` rune for variables that should be _reactive_ — in other words, variables that cause an `$effect`, `$derived` or template expression to update. Everything else can be a normal variable.

Objects and arrays (`$state({...})` or `$state([...])`) are made deeply reactive, meaning mutation will trigger updates. This has a trade-off: in exchange for fine-grained reactivity, the objects must be proxied, which has performance overhead. In cases where you're dealing with large objects that are only ever reassigned (rather than mutated), use `$state.raw` instead. This is often the case with API responses, for example.

## `$derived`

To compute something from state, use `$derived` rather than `$effect`:

```
// @errors: 2451
let num = 0;
// ---cut---
// do this
let square = $derived(num * num);

// don't do this
let square;

$effect(() => {
	square = num * num;
});
```

> \[!NOTE\] `$derived` is given an expression, _not_ a function. If you need to use a function (because the expression is complex, for example) use `$derived.by`.

Deriveds are writable — you can assign to them, just like `$state`, except that they will re-evaluate when their expression changes.

If the derived expression is an object or array, it will be returned as-is — it is _not_ made deeply reactive. You can, however, use `$state` inside `$derived.by` in the rare cases that you need this.

## `$effect`

Effects are an escape hatch and should mostly be avoided. In particular, avoid updating state inside effects.

-   If you need to sync state to an external library such as D3, it is often neater to use [`{@attach ...}`](@attach)
-   If you need to run some code in response to user interaction, put the code directly in an event handler or use a [function binding](bind#Function-bindings) as appropriate
-   If you need to log values for debugging purposes, use [`$inspect`]($inspect)
-   If you need to observe something external to Svelte, use [`createSubscriber`](svelte-reactivity#createSubscriber)

Never wrap the contents of an effect in `if (browser) {...}` or similar — effects do not run on the server.

## `$props`

Treat props as though they will change. For example, values that depend on props should usually use `$derived`:

```
// @errors: 2451
let { type } = $props();

// do this
let color = $derived(type === 'danger' ? 'red' : 'green');

// don't do this — `color` will not update if `type` changes
let color = type === 'danger' ? 'red' : 'green';
```

## `$inspect.trace`

`$inspect.trace` is a debugging tool for reactivity. If something is not updating properly or running more than it should you can add `$inspect.trace(label)` as the first line of an `$effect` or `$derived.by` (or any function they call) to trace their dependencies and discover which one triggered an update.

## Events

Any element attribute starting with `on` is treated as an event listener:

```
<button onclick={() => {...}}>click me</button>

<!-- attribute shorthand also works -->
<button {onclick}>...</button>

<!-- so do spread attributes -->
<button {...props}>...</button>
```

If you need to attach listeners to `window` or `document` you can use `<svelte:window>` and `<svelte:document>`:

```
<svelte:window onkeydown={...} />
<svelte:document onvisibilitychange={...} />
```

Avoid using `onMount` or `$effect` for this.

## Snippets

[Snippets](snippet) are a way to define reusable chunks of markup that can be instantiated with the [`{@render ...}`](@render) tag, or passed to components as props. They must be declared within the template.

```
{#snippet greeting(name)}
  <p>hello {name}!</p>
{/snippet}

{@render greeting('world')}
```

> \[!NOTE\] Snippets declared at the top level of a component (i.e. not inside elements or blocks) can be referenced inside `<script>`. A snippet that doesn't reference component state is also available in a `<script module>`, in which case it can be exported for use by other components.

## Each blocks

Prefer to use [keyed each blocks](each#Keyed-each-blocks) — this improves performance by allowing Svelte to surgically insert or remove items rather than updating the DOM belonging to existing items.

> \[!NOTE\] The key _must_ uniquely identify the object. Do not use the index as a key.

Avoid destructuring if you need to mutate the item (with something like `bind:value={item.count}`, for example).

## Using JavaScript variables in CSS

If you have a JS variable that you want to use inside CSS you can set a CSS custom property with the `style:` directive.

```
<div style:--columns={columns}>...</div>
```

You can then reference `var(--columns)` inside the component's `<style>`.

## Styling child components

The CSS in a component's `<style>` is scoped to that component. If a parent component needs to control the child's styles, the preferred way is to use CSS custom properties:

```
<!-- Parent.svelte -->
<Child --color="red" />

<!-- Child.svelte -->
<h1>Hello</h1>

<style>
	h1 {
		color: var(--color);
	}
</style>
```

If this is impossible (for example, the child component comes from a library) you can use `:global` to override styles:

```
<div>
	<Child />
</div>

<style>
	div :global {
		h1 {
			color: red;
		}
	}
</style>
```

## Context

Consider using context instead of declaring state in a shared module. This will scope the state to the part of the app that needs it, and eliminate the possibility of it leaking between users when server-side rendering.

Use `createContext` rather than `setContext` and `getContext`, as it provides type safety.

## Async Svelte

If using version 5.36 or higher, you can use [await expressions](await-expressions) and [hydratable](hydratable) to use promises directly inside components. Note that these require the `experimental.async` option to be enabled in `svelte.config.js` as they are not yet considered fully stable.

## Avoid legacy features

Always use runes mode for new code, and avoid features that have more modern replacements:

-   use `$state` instead of implicit reactivity (e.g. `let count = 0; count += 1`)
-   use `$derived` and `$effect` instead of `$:` assignments and statements (but only use effects when there is no better solution)
-   use `$props` instead of `export let`, `$$props` and `$$restProps`
-   use `onclick={...}` instead of `on:click={...}`
-   use `{#snippet ...}` and `{@render ...}` instead of `<slot>` and `$$slots` and `<svelte:fragment>`
-   use `<DynamicComponent>` instead of `<svelte:component this={DynamicComponent}>`
-   use `import Self from './ThisComponent.svelte'` and `<Self>` instead of `<svelte:self>`
-   use classes with `$state` fields to share reactivity between components, instead of using stores
-   use `{@attach ...}` instead of `use:action`
-   use clsx-style arrays and objects in `class` attributes, instead of the `class:` directive

## [Testing](https://docharvest.github.io/docs/svelte/07-misc/02-testing/)

Contents

svelte

Testing

Svelte

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 component tests with 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/). You can use the Svelte CLI to [setup Vitest](/docs/cli/vitest) either during project creation or later on.

To setup Vitest manually, first install it:

```
npm install -D vitest
```

Then adjust your `vite.config.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:

```
/// file: multiplier.svelte.test.js
// @filename: multiplier.svelte.ts
export function multiplier(initial: number, k: number) {
	let count = $state(initial);

	return {
		get value() {
			return count * k;
		},
		set: (c: number) => {
			count = c;
		}
	};
}
// @filename: multiplier.svelte.test.js
// ---cut---
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);
});
```

```
/// 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`:

```
/// file: multiplier.svelte.test.js
// @filename: multiplier.svelte.ts
export function multiplier(getCount: () => number, k: number) {
	return {
		get value() {
			return getCount() * k;
		}
	};
}
// @filename: multiplier.svelte.test.js
// ---cut---
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);
});
```

```
/// 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`:

```
/// file: logger.svelte.test.js
// @filename: logger.svelte.ts
export function logger(fn: () => void) {}
// @filename: logger.svelte.test.js
// ---cut---
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).toEqual([0]);

		count = 1;
		flushSync();

		expect(log).toEqual([0, 1]);
	});

	cleanup();
});
```

```
/// file: logger.svelte.js
/**
 * @param {() => any} getValue
 */
export function logger(getValue) {
	/** @type {any[]} */
	let log = [];

	$effect(() => {
		log.push(getValue());
	});

	return log;
}
```

### Component testing

It is possible to test your components in isolation, which allows you to render them in a browser (real or simulated), simulate behavior, and make assertions, without spinning up your whole app.

> \[!NOTE\] 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):

```
npm install -D jsdom
```

Then adjust your `vite.config.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 set up 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:

```
/// 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:

```
// @errors: 2339
/// 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).

## Component tests with Storybook

[Storybook](https://storybook.js.org) is a tool for developing and documenting UI components, and it can also be used to test your components. They're run with Vitest's browser mode, which renders your components in a real browser for the most realistic testing environment.

To get started, first install Storybook ([using Svelte's CLI](/docs/cli/storybook)) in your project via `npx sv add storybook` and choose the recommended configuration that includes testing features. If you're already using Storybook, and for more information on Storybook's testing capabilities, follow the [Storybook testing docs](https://storybook.js.org/docs/writing-tests?renderer=svelte) to get started.

You can create stories for component variations and test interactions with the [play function](https://storybook.js.org/docs/writing-tests/interaction-testing?renderer=svelte#writing-interaction-tests), which allows you to simulate behavior and make assertions using the Testing Library and Vitest APIs. Here's an example of two stories that can be tested, one that renders an empty LoginForm component and one that simulates a user filling out the form:

```
/// file: LoginForm.stories.svelte
<script module>
	import { defineMeta } from '@storybook/addon-svelte-csf';
	import { expect, fn } from 'storybook/test';

	import LoginForm from './LoginForm.svelte';

	const { Story } = defineMeta({
		component: LoginForm,
		args: {
			// Pass a mock function to the `onSubmit` prop
			onSubmit: fn(),
		}
	});
</script>

<Story name="Empty Form" />

<Story
	name="Filled Form"
	play={async ({ args, canvas, userEvent }) => {
		// Simulate a user filling out the form
		await userEvent.type(canvas.getByTestId('email'), 'email@provider.com');
		await userEvent.type(canvas.getByTestId('password'), 'a-random-password');
		await userEvent.click(canvas.getByRole('button'));

		// Run assertions
		await expect(args.onSubmit).toHaveBeenCalledTimes(1);
		await expect(canvas.getByText('You’re in!')).toBeInTheDocument();
	}}
/>
```

## End-to-end tests with 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/).

You can use the Svelte CLI to [setup Playwright](/docs/cli/playwright) either during project creation or later on. You can also [set it up with `npm init playwright`](https://playwright.dev/docs/intro). Additionally, you may also want to install an IDE plugin such as [the VS Code extension](https://playwright.dev/docs/getting-started-vscode) to be able to execute tests from inside your IDE.

If you've run `npm init playwright` or are not using Vite, you may need to adjust the Playwright config to tell Playwright what to do before running the tests — mainly starting your application at a certain port. For example:

```
/// 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.

```
// @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](https://docharvest.github.io/docs/svelte/07-misc/03-typescript/)

Contents

svelte

TypeScript

Svelte

TypeScript

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:

```
<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.

### Using Vite

If you're using SvelteKit, or Vite _without_ SvelteKit, you can use `vitePreprocess` from `@sveltejs/vite-plugin-svelte` in your config file:

```
/// 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 other build tools

If you're using tools like Rollup (via [rollup-plugin-svelte](https://github.com/sveltejs/rollup-plugin-svelte)) or Webpack (via [svelte-loader](https://github.com/sveltejs/svelte-loader)) instead, install `typescript` and `svelte-preprocess` and add the preprocessor to the plugin config. See the respective plugin READMEs for more info.

> \[!NOTE\] 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 `ES2015` so classes are not compiled to functions
-   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.

```
<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:

```
<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:

```
<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`:

```
<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.

```
let count: number = $state(0);
```

If you don't give `$state` an initial value, part of its types will be `undefined`.

```
// @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:

```
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:

```
<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`.

```
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:

```
<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 by augmenting the `svelte/elements` module like this:

```
/// file: additional-svelte-typings.d.ts
import { HTMLButtonAttributes } from 'svelte/elements';

declare module 'svelte/elements' {
	// add a new element
	export interface SvelteHTMLElements {
		'custom-button': HTMLButtonAttributes;
	}

	// add a new global attribute that is available on all html elements
	export interface HTMLAttributes<T> {
		globalattribute?: string;
	}

	// add a new attribute for button elements
	export interface HTMLButtonAttributes {
		veryexperimentalattribute?: string;
	}
}

export {}; // ensure this is not an ambient module, else types will be overridden instead of augmented
```

Then make sure that the `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.

## [Custom elements](https://docharvest.github.io/docs/svelte/07-misc/04-custom-elements/)

Contents

svelte

Custom elements

Svelte

Custom elements

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). Within the custom element you can access the host element via the [`$host`](https://svelte.dev/docs/svelte/$host) rune.

```
<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`.

```
// @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:

```
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).

```
// @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 to modify shadow root properties. It accepts the following values:
    -   `"none"`: No shadow root is created. Note that styles are then no longer encapsulated, and you can't use slots.
    -   `"open"`: Shadow root is created with the `mode: "open"` option.
    -   [`ShadowRootInit`](https://developer.mozilla.org/en-US/docs/Web/API/Element/attachShadow#options): You can pass a settings object that will be passed to `attachShadow()` when shadow root is created.
-   `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:options
	customElement={{
		tag: 'custom-element',
		shadow: {
			mode: import.meta.env.DEV ? 'open' : 'closed',
			clonable: true,
			// ...
		},
		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>

...
```

> \[!NOTE\] While Typescript is supported in the `extend` function, it is subject to limitations: you need to set `lang="ts"` on one of the scripts AND you can only use [erasable syntax](https://www.typescriptlang.org/tsconfig/#erasableSyntaxOnly) in it. They are not processed by script preprocessors.

## 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`)

## [Browser support](https://docharvest.github.io/docs/svelte/07-misc/05-browser-support/)

Contents

svelte

Browser support

Svelte

Browser support

The table below shows the minimum browser versions Svelte is expected to work in, derived from the browser APIs used by Svelte's internal code.

@include .generated/browser-support.md

This table only covers Svelte itself. It does not include [SvelteKit](/docs/kit), other Svelte libraries, or your own code.

## Exceptions

A few Svelte features require a higher minimum browser version. You'll only need to take the following table into consideration if you use these specific features.

@include .generated/browser-support-features.md

## [Svelte 4 migration guide](https://docharvest.github.io/docs/svelte/07-misc/06-v4-migration-guide/)

Contents

svelte

Svelte 4 migration guide

Svelte

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))

```
// @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))

```
// @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))

```
// @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: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`.

```
---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>`.

```
<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`:

```
{#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:

```
<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.

```
// @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.

```
// @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](https://docharvest.github.io/docs/svelte/07-misc/07-v5-migration-guide/)

Contents

svelte

Svelte 5 migration guide

Svelte

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`:

```
<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:

```
<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:

```
<script>
	let count = $state(0);

	---$:---+++$effect(() =>+++ {
		if (count > 5) {
			alert('Count is too high!');
		}
	}+++);+++
</script>
```

Note that [when `$effect` runs is different]($effect#Understanding-dependencies) than when `$:` runs.

> \[!DETAILS\] 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:

```
<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();`

```
<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):

```
<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...

```
<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:

```
<!--- 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}
```

```
<!--- 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:

```
<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:

```
<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:

```
<button on:click|once|preventDefault={handler}>...</button>
```

Modifiers are specific to `on:` and so 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:

```
<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:

```
<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:

```
<button on:click={one} on:click={two}>...</button>
```

Duplicate attributes/properties on elements — which now includes event handlers — are not allowed. Instead, do this:

```
<button
	onclick={(e) => {
		one(e);
		two(e);
	}}
>
	...
</button>
```

When spreading props, local event handlers must go _after_ the spread, or they risk being overwritten:

```
<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 usable 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 so slots are deprecated in Svelte 5.

They continue to work, however, and you can pass snippets to a component that uses slots:

```
<!--- file: Child.svelte --->
<slot />
<hr />
<slot name="foo" message="hello" />
```

```
<!--- 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()}`:

```
<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:

```
<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:

```
<!--- 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>
```

```
<!--- 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).

```
<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()`.

```
<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:

```
+++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.

```
+++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`.

```
+++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.

```
+++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.

```
+++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`.

```
/// 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`:

```
+++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:

```
import type { Component } from 'svelte';
export declare const MyComponent: Component<{
	foo: string;
}>;
```

To declare that a component of a certain type is required:

```
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:

```
<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:

```
{#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
    
    This new behavior is slightly different from native HTML rendering. For example, `<p>foo<span> - bar</span></p>` will render:
    
    -   `foo - bar` in HTML
    -   `foo- bar` in Svelte 5
    
    You can reintroduce the missing space by moving it outside the `<span>`...
    
    ```
    <p>foo <span>- bar</span></p>
    ```
    
    ...or, if necessary for styling reasons, including it as an expression:
    
    ```
    <p>foo<span>{' '}- bar</span></p>
    ```
    
-   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.

```
<svelte:options accessors={true} />

<script>
	// available via componentInstance.name
	export let name;
</script>
```

In runes mode, properties are never accessible on the component instance. You can use component exports instead if you need to expose them.

```
<script>
	let { name } = $props();
	// available via componentInstance.getName()
	export const getName = () => name;
</script>
```

Alternatively, if the place where they are instantiated is under your control, you can also make use of runes inside `.js/.ts` files by adjusting their ending to include `.svelte`, i.e. `.svelte.js` or `.svelte.ts`, and then use `$state`:

```
+++import { mount } from 'svelte';+++
import App from './App.svelte'

---const app = new App({ target: document.getElementById("app"), props: { foo: 'bar' } });
app.foo = 'baz'---
+++const props = $state({ foo: 'bar' });
const app = mount(App, { target: document.getElementById("app"), props });
props.foo = 'baz';+++
```

### `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:

```
<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 events are passive

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) 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:

```
<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:

```
<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...

```
<table>
	<tr>
		<td>hi</td>
	</tr>
</table>
```

... and the browser would auto-insert a `<tbody>` element:

```
<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(...), :has(...), and :where(...) are scoped

Previously, Svelte did not analyse selectors inside `:is(...)`, `:has(...)`, and `:where(...)`, effectively treating them as global. Svelte 5 analyses them in the context of the current component. Some selectors may now therefore be treated as unused if they were relying on this treatment. To fix this, use `:global(...)` inside the `:is(...)/:has(...)/:where(...)` selectors.

When using Tailwind's `@apply` directive, add a `:global` selector to preserve rules that use Tailwind-generated `:is(...)` selectors:

```
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:

```
// @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:

```
<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: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:

```
<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. You therefore should not 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.

## [Frequently asked questions](https://docharvest.github.io/docs/svelte/07-misc/99-faq/)

Contents

svelte

Frequently asked questions

Svelte

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/collection/a-list-of-books-and-courses-ac01dd10363184fa).

## 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.

```
<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:
  ```svelte
  <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](/packages#component-libraries) as well as standalone components listed on [the packages page](/packages).

## 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 docs on testing](/docs/svelte/testing)
-   [Setup Vitest using the Svelte CLI](/docs/cli/vitest)
-   [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 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 and Nuxt.js for Vue. SvelteKit also supports hash-based routing for client-side applications.

However, you can use any router library. A sampling of available routers are highlighted [on the packages page](/packages#routing).

## 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.

Some work has been completed towards [custom renderer support in Svelte 5](https://github.com/sveltejs/svelte/issues/15470), but this feature is not yet available. The custom rendering API would support additional mobile frameworks like Lynx JS and Svelte Native. Svelte Native was an option available for Svelte 4, but 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).

## [Reference](https://docharvest.github.io/docs/svelte/98-reference/)

Contents

svelte

Reference

Svelte

Reference

## [svelte](https://docharvest.github.io/docs/svelte/98-reference/20-svelte/)

Contents

svelte

svelte

Svelte

svelte

> MODULE: svelte

## [svelte/action](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-action/)

Contents

svelte

svelte/action

Svelte

svelte/action

This module provides types for [actions](use), which have been superseded by [attachments](@attach).

> MODULE: svelte/action

## [svelte/animate](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-animate/)

Contents

svelte

svelte/animate

Svelte

svelte/animate

> MODULE: svelte/animate

## [svelte/attachments](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-attachments/)

Contents

svelte

svelte/attachments

Svelte

svelte/attachments

> MODULE: svelte/attachments

## [svelte/compiler](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-compiler/)

Contents

svelte

svelte/compiler

Svelte

svelte/compiler

> MODULE: svelte/compiler

## [svelte/easing](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-easing/)

Contents

svelte

svelte/easing

Svelte

svelte/easing

> MODULE: svelte/easing

## [svelte/events](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-events/)

Contents

svelte

svelte/events

Svelte

svelte/events

> MODULE: svelte/events

## [svelte/legacy](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-legacy/)

Contents

svelte

svelte/legacy

Svelte

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.

> MODULE: svelte/legacy

## [svelte/motion](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-motion/)

Contents

svelte

svelte/motion

Svelte

svelte/motion

> MODULE: svelte/motion

## [svelte/reactivity](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-reactivity/)

Contents

svelte

svelte/reactivity

Svelte

svelte/reactivity

Svelte provides reactive versions of various built-ins like [`Map`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Map), [`Set`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Set) and [`URL`](https://developer.mozilla.org/en-US/docs/Web/API/URL) that can be used just like their native counterparts, as well as a handful of additional utilities for handling reactivity.

> MODULE: svelte/reactivity

## [svelte/reactivity/window](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-reactivity-window/)

Contents

svelte

svelte/reactivity/window

Svelte

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.

```
<script>
	import { innerWidth, innerHeight } from 'svelte/reactivity/window';
</script>

<p>{innerWidth.current}x{innerHeight.current}</p>
```

> MODULE: svelte/reactivity/window

## [svelte/server](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-server/)

Contents

svelte

svelte/server

Svelte

svelte/server

> MODULE: svelte/server

## [svelte/store](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-store/)

Contents

svelte

svelte/store

Svelte

svelte/store

> MODULE: svelte/store

## [svelte/transition](https://docharvest.github.io/docs/svelte/98-reference/21-svelte-transition/)

Contents

svelte

svelte/transition

Svelte

svelte/transition

> MODULE: svelte/transition

## [Compiler errors](https://docharvest.github.io/docs/svelte/98-reference/30-compiler-errors/)

Contents

svelte

Compiler errors

Svelte

Compiler errors

@include .generated/compile-errors.md

## [Compiler warnings](https://docharvest.github.io/docs/svelte/98-reference/30-compiler-warnings/)

Contents

svelte

Compiler warnings

Svelte

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-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-ignore a11y_click_events_have_key_events, a11y_no_static_element_interactions (because of reasons) -->
<div onclick>...</div>
```

@include .generated/compile-warnings.md

## [Runtime errors](https://docharvest.github.io/docs/svelte/98-reference/30-runtime-errors/)

Contents

svelte

Runtime errors

Svelte

Runtime errors

## Client errors

@include .generated/client-errors.md

## Server errors

@include .generated/server-errors.md

## Shared errors

@include .generated/shared-errors.md

## [Runtime warnings](https://docharvest.github.io/docs/svelte/98-reference/30-runtime-warnings/)

Contents

svelte

Runtime warnings

Svelte

Runtime warnings

## Client warnings

@include .generated/client-warnings.md

## Shared warnings

@include .generated/shared-warnings.md

## [Legacy APIs](https://docharvest.github.io/docs/svelte/99-legacy/)

Contents

svelte

Legacy APIs

Svelte

Legacy APIs

## [Overview](https://docharvest.github.io/docs/svelte/99-legacy/00-legacy-overview/)

Contents

svelte

Overview

Svelte

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](https://docharvest.github.io/docs/svelte/99-legacy/01-legacy-let/)

Contents

svelte

Reactive let/var declarations

Svelte

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.

```
<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:

```
<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](https://docharvest.github.io/docs/svelte/99-legacy/02-legacy-reactive-assignments/)

Contents

svelte

Reactive $: statements

Svelte

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.

```
<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:

```
// @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:

```
// @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:

```
// @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:

```
<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:

```
// @noErrors
$: if (browser) {
	document.title = title;
}
```

## [export let](https://docharvest.github.io/docs/svelte/99-legacy/03-legacy-export-let/)

Contents

svelte

export let

Svelte

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:

```
<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:

```
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:

```
<!--- file: Greeter.svelte--->
<script>
	export function greet(name) {
		alert(`hello ${name}!`);
	}
</script>
```

```
<!--- 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:

```
<!--- 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](https://docharvest.github.io/docs/svelte/99-legacy/04-legacy-$$props-and-$$restProps/)

Contents

svelte

$$props and $$restProps

Svelte

$$props and $$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:

```
<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:](https://docharvest.github.io/docs/svelte/99-legacy/10-legacy-on/)

Contents

svelte

on:

Svelte

on:

In runes mode, event handlers are just like any other attribute or prop.

In legacy mode, we use the `on:` directive:

```
<!--- 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:

```
<button on:click={() => (count += 1)}>
	count: {count}
</button>
```

Add _modifiers_ to element event handlers with the `|` character.

```
<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.

```
<button on:click>
	The component itself will emit the click event
</button>
```

It's possible to have multiple event listeners for the same event:

```
<!--- 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:

```
<!--- 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:

```
<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:
> 
> ```
> <!--- file: Stepper.svelte --->
> <script>
> 	export let decrement;
> 	export let increment;
> </script>
> 
> <button on:click={decrement}>decrement</button>
> <button on:click={increment}>increment</button>
> ```

## [<slot> · docharvest](https://docharvest.github.io/docs/svelte/99-legacy/20-legacy-slots/)



## [$$slots](https://docharvest.github.io/docs/svelte/99-legacy/21-legacy-$$slots/)

Contents

svelte

$$slots

Svelte

$$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.

```
<!--- 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>
```

```
<!--- 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](https://docharvest.github.io/docs/svelte/99-legacy/22-legacy-svelte-fragment/)

Contents

svelte

<svelte:fragment>

Svelte

<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.

```
<!--- file: Widget.svelte --->
<div>
	<slot name="header">No header was provided</slot>
	<p>Some content between header and footer</p>
	<slot name="footer" />
</div>
```

```
<!--- 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](https://docharvest.github.io/docs/svelte/99-legacy/30-legacy-svelte-component/)

Contents

svelte

<svelte:component>

Svelte

<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:component this={MyComponent} />
```

If `this` is falsy, no component is rendered.

## [svelte:self](https://docharvest.github.io/docs/svelte/99-legacy/31-legacy-svelte-self/)

Contents

svelte

<svelte:self>

Svelte

<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.

```
<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:
> 
> ```
> <!--- 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](https://docharvest.github.io/docs/svelte/99-legacy/40-legacy-component-api/)

Contents

svelte

Imperative component API

Svelte

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

```
// @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.

```
// @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.

```
/// 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`

```
// @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.

```
// @noErrors
component.$set({ answer: 42 });
```

> \[!NOTE\] In Svelte 5+, use `$state` instead to create a component props and update that
> 
> ```
> // @noErrors
> let props = $state({ answer: 42 });
> const component = mount(Component, { props });
> // ...
> props.answer = 24;
> ```

## `$on`

```
// @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.

```
// @noErrors
const off = component.$on('selected', (event) => {
	console.log(event.detail.selection);
});

off();
```

> \[!NOTE\] In Svelte 5+, pass callback props instead

## `$destroy`

```
// @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

```
// @noErrors
component.prop;
```

```
// @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.

```
// @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

```
// @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`.

```
// @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

```
// @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