svelte.dev/docs/svelte

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## Page: https://svelte.dev/docs/svelte

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...

App

    <script>
    	function greet() {
    		alert('Welcome to Svelte!');
    	}
    </script>
    
    <button onclick={greet}>click me</button>
    
    <style>
    	button {
    		font-size: 2em;
    	}
    </style>

    <script lang="ts">
    	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) and everything in between.

These pages serve as reference documentation. If you’re new to Svelte, we recommend starting with the interactive tutorial and coming back here when you have questions.

You can also try Svelte online in the playground or, if you need a more fully-featured environment, on StackBlitz.

---

## Page: https://svelte.dev/docs/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...

App

    <script>
    	function greet() {
    		alert('Welcome to Svelte!');
    	}
    </script>
    
    <button onclick={greet}>click me</button>
    
    <style>
    	button {
    		font-size: 2em;
    	}
    </style>

    <script lang="ts">
    	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) and everything in between.

These pages serve as reference documentation. If you’re new to Svelte, we recommend starting with the interactive tutorial and coming back here when you have questions.

You can also try Svelte online in the playground or, if you need a more fully-featured environment, on StackBlitz.

---

## Page: https://svelte.dev/docs/svelte/getting-started

We recommend using SvelteKit, which lets you build almost anything. It’s the official application framework from the Svelte team and powered by Vite. 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 by running `npm create vite@latest` and selecting the `svelte` option. With this, `npm run build` will generate HTML, JS, and CSS files inside the `dist` directory using vite-plugin-svelte. In most cases, you will probably need to choose a routing library as well.

> Vite is often used in standalone mode to build single page apps (SPAs), which you can also build with SvelteKit.

There are also plugins for Rollup, Webpack and a few others, but we recommend Vite.

## Editor tooling

The Svelte team maintains a VS Code extension, and there are integrations with various other editors and tools as well.

You can also check your code from the command line using sv check.

## Getting help

Don’t be shy about asking for help in the Discord chatroom! You can also find answers on Stack Overflow.

---

## Page: https://svelte.dev/docs/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.

MyComponent

    <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 module>
    	// module-level logic goes here
    	// (you will rarely use this)
    </script>
    
    <script lang="ts">
    	// 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 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.

> 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.

> Legacy mode
> 
> 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.

---

## Page: https://svelte.dev/docs/svelte/svelte-js-files

Besides `.svelte` files, Svelte also operates on `.svelte.js` and `.svelte.ts` files.

These behave like any other `.js` or `.ts` module, except that you can use runes. This is useful for creating reusable reactive logic, or sharing reactive state across your app.

> Legacy mode
> 
> This is a concept that didn’t exist prior to Svelte 5

---

## Page: https://svelte.dev/docs/svelte/what-are-runes

> **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 mode
> 
> Runes didn’t exist prior to Svelte 5.

---

## Page: https://svelte.dev/docs/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 allow Svelte to run code when you read or write properties, including via methods like `array.push(...)`, triggering granular updates.

> Classes like `Set` and `Map` will not be proxied, but Svelte provides reactive implementations for various built-ins like these that can be imported from `svelte/reactivity`.

State is proxified recursively until Svelte finds something other than an array or simple object. 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:

    todos[0].done = !todos[0].done;

If you push a new object to the array, it will also be proxified:

    todos.push({
    	done: false,
    	text: 'eat lunch'
    });

> When you update properties of proxies, the original object is _not_ mutated.

Note that if you destructure a reactive value, the references are not reactive — as in normal JavaScript, they are evaluated at the point of destructuring:

    let { done, text } = todos[0];
    
    // this will not affect the value of `done`
    todos[0].done = !todos[0].done;

### Classes

You can also use `$state` in class fields (whether public or private):

    class Todo {
    	done = $state(false);
    	text = $state();
    
    	constructor(text) {
    		this.text = text;
    	}
    
    	reset() {
    		this.text = '';
    		this.done = false;
    	}
    }

> 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` 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:

    class Todo {
    	done = $state(false);
    	text = $state();
    
    	constructor(text) {
    		this.text = text;
    	}
    
    	reset = () => {
    		this.text = '';
    		this.done = false;
    	}
    }

## $state.raw

In cases where you don’t want objects and arrays to be deeply reactive you can use `$state.raw`.

State declared with `$state.raw` cannot be mutated; it can only be _reassigned_. In other words, rather than assigning to a property of an object, or using an array method like `push`, replace the object or array altogether if you’d like to update it:

    let person = $state.raw({
    	name: 'Heraclitus',
    	age: 49
    });
    
    // this will have no effect
    person.age += 1;
    
    // this will work, because we're creating a new person
    person = {
    	name: 'Heraclitus',
    	age: 50
    };

This can improve performance with large arrays and objects that you weren’t planning to mutate anyway, since it avoids the cost of making them reactive. Note that raw state can _contain_ reactive state (for example, a raw array of reactive objects).

## $state.snapshot

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

    <script>
    	let counter = $state({ count: 0 });
    
    	function onclick() {
    		// Will log `{ count: ... }` rather than `Proxy { ... }`
    		console.log($state.snapshot(counter));
    	}
    </script>

This is handy when you want to pass some state to an external library or API that doesn’t expect a proxy, such as `structuredClone`.

## Passing state into functions

JavaScript is a _pass-by-value_ language — when you call a function, the arguments are the _values_ rather than the _variables_. In other words:

index

    /**
     * @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!

    function add(a: number, b: number) {
    	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:

index

    /**
     * @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

    function add(getA: () => number, getB: () => number) {
    	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`/`set` properties...

index

    /**
     * @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

    function add(input: { a: number, b: number }) {
    	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 instead.

---

## Page: https://svelte.dev/docs/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`.

> 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.

To exempt a piece of state from being treated as a dependency, use `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>

> Prior to Svelte 5.25, deriveds were read-only.

## Deriveds and reactivity

Unlike `$state`, which converts objects and arrays to deeply reactive proxies, `$derived` values are left as-is. For example, in a case like this...

    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.

## 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>

---

## Page: https://svelte.dev/docs/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` to learn about alternative approaches.

You can create an effect with the `$effect` rune (demo):

    <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" />

When Svelte runs an effect function, it tracks which pieces of state (and derived state) are accessed (unless accessed inside `untrack`), and re-runs the function when that state later changes.

> If you’re having difficulty understanding why your `$effect` is rerunning or is not running see understanding dependencies. Effects are triggered differently than the `$:` 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 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.

> 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 (demo).

    <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), 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):

    $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`.)

    <script>
    	let state = $state({ value: 0 });
    	let derived = $derived({ value: state.value * 2 });
    
    	// this will run once, because `state` is never reassigned (only mutated)
    	$effect(() => {
    		state;
    	});
    
    	// this will run whenever `state.value` changes...
    	$effect(() => {
    		state.value;
    	});
    
    	// ...and so will this, because `derived` is a new object each time
    	$effect(() => {
    		derived;
    	});
    </script>
    
    <button onclick={() => (state.value += 1)}>
    	{state.value}
    </button>
    
    <p>{state.value} doubled is {derived.value}</p>

An effect only depends on the values that it read the last time it ran. This has interesting implications for effects that have conditional code.

For instance, if `a` is `true` in the code snippet below, the code inside the `if` block will run and `b` will be evaluated. As such, changes to either `a` or `b` will cause the effect to re-run.

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

    $effect(() => {
    	console.log('running');
    
    	if (a) {
    		console.log('b:', b);
    	}
    });

## $effect.pre

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

    <script>
    	import { tick } from 'svelte';
    
    	let div = $state();
    	let messages = $state([]);
    
    	// ...
    
    	$effect.pre(() => {
    		if (!div) return; // not yet mounted
    
    		// reference `messages` array length so that this code re-runs whenever it changes
    		messages.length;
    
    		// autoscroll when new messages are added
    		if (div.offsetHeight + div.scrollTop > div.scrollHeight - 20) {
    			tick().then(() => {
    				div.scrollTo(0, div.scrollHeight);
    			});
    		}
    	});
    </script>
    
    <div bind:this={div}>
    	{#each messages as message}
    		<p>{message}</p>
    	{/each}
    </div>

Apart from the timing, `$effect.pre` works exactly like `$effect`.

## $effect.tracking

The `$effect.tracking` rune is an advanced feature that tells you whether or not the code is running inside a tracking context, such as an effect or inside your template (demo):

    <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`, which will create listeners to update reactive values but _only_ if those values are being tracked (rather than, for example, read inside an event handler).

## $effect.root

The `$effect.root` rune is an advanced feature that creates a non-tracked scope that doesn’t auto-cleanup. This is useful for nested effects that you want to manually control. This rune also allows for the creation of effects outside of the component initialisation phase.

    <script>
    	let count = $state(0);
    
    	const cleanup = $effect.root(() => {
    		$effect(() => {
    			console.log(count);
    		});
    
    		return () => {
    			console.log('effect root cleanup');
    		};
    	});
    </script>

## When not to use $effect

In general, `$effect` is best considered something of an escape hatch — useful for things like analytics and direct DOM manipulation — rather than a tool you should use frequently. In particular, avoid using it to synchronise state. Instead of this...

    <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>

> 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 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. Don’t use effects for this (demo):

    <script>
    	let total = 100;
    	let spent = $state(0);
    	let left = $state(total);
    
    	$effect(() => {
    		left = total - spent;
    	});
    
    	$effect(() => {
    		spent = total - left;
    	});
    </script>
    
    <label>
    	<input type="range" bind:value={spent} max={total} />
    	{spent}/{total} spent
    </label>
    
    <label>
    	<input type="range" bind:value={left} max={total} />
    	{left}/{total} left
    </label>

Instead, use `oninput` callbacks or — better still — function bindings where possible (demo):

    <script>
    	let total = 100;
    	let spent = $state(0);
    	let left = $state(total);
    
    	function updateSpent(value) {
    		spent = value;
    		left = total - spent;
    	}
    
    	function updateLeft(value) {
    		left = value;
    		spent = total - left;
    	}
    </script>
    
    <label>
    	<input type="range" bind:value={() => spent, updateSpent} max={total} />
    	{spent}/{total} spent
    </label>
    
    <label>
    	<input type="range" bind:value={() => left, updateLeft} max={total} />
    	{left}/{total} left
    </label>

If you absolutely have to update `$state` within an effect and run into an infinite loop because you read and write to the same `$state`, use untrack.

---

## Page: https://svelte.dev/docs/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:

App

    <script>
    	import MyComponent from './MyComponent.svelte';
    </script>
    
    <MyComponent adjective="cool" />

    <script lang="ts">
    	import MyComponent from './MyComponent.svelte';
    </script>
    
    <MyComponent adjective="cool" />

On the other side, inside `MyComponent.svelte`, we can receive props with the `$props` rune...

MyComponent

    <script>
    	let props = $props();
    </script>
    
    <p>this component is {props.adjective}</p>

    <script lang="ts">
    	let props = $props();
    </script>
    
    <p>this component is {props.adjective}</p>

...though more commonly, you’ll _destructure_ your props:

MyComponent

    <script>
    	let { adjective } = $props();
    </script>
    
    <p>this component is {adjective}</p>

    <script lang="ts">
    	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:

    let { adjective = 'happy' } = $props();

> Fallback values are not turned into reactive state proxies (see 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 (demo):

App

    <script>
    	import Child from './Child.svelte';
    
    	let count = $state(0);
    </script>
    
    <button onclick={() => (count += 1)}>
    	clicks (parent): {count}
    </button>
    
    <Child {count} />

    <script lang="ts">
    	import Child from './Child.svelte';
    
    	let count = $state(0);
    </script>
    
    <button onclick={() => (count += 1)}>
    	clicks (parent): {count}
    </button>
    
    <Child {count} />

Child

    <script>
    	let { count } = $props();
    </script>
    
    <button onclick={() => (count += 1)}>
    	clicks (child): {count}
    </button>

    <script lang="ts">
    	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.

If the prop is a regular object, the mutation will have no effect (demo):

App

    <script>
    	import Child from './Child.svelte';
    </script>
    
    <Child object={{ count: 0 }} />

    <script lang="ts">
    	import Child from './Child.svelte';
    </script>
    
    <Child object={{ count: 0 }} />

Child

    <script>
    	let { object } = $props();
    </script>
    
    <button onclick={() => {
    	// has no effect
    	object.count += 1
    }}>
    	clicks: {object.count}
    </button>

    <script lang="ts">
    	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` warning, because the component is mutating state that does not ‘belong’ to it (demo):

App

    <script>
    	import Child from './Child.svelte';
    
    	let object = $state({count: 0});
    </script>
    
    <Child {object} />

    <script lang="ts">
    	import Child from './Child.svelte';
    
    	let object = $state({count: 0});
    </script>
    
    <Child {object} />

Child

    <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>

    <script lang="ts">
    	let { object } = $props();
    </script>
    
    <button onclick={() => {
    	// will cause the count below to update,
    	// but with a warning. Don't mutate
    	// objects you don't own!
    	object.count += 1
    }}>
    	clicks: {object.count}
    </button>

The fallback value of a prop not declared with `$bindable` is left untouched — it is not turned into a reactive state proxy — meaning mutations will not cause updates (demo)

Child

    <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>

    <script lang="ts">
    	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` 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>

> Interfaces for native DOM elements are provided in the `svelte/elements` module (see Typing wrapper components)

Adding types is recommended, as it ensures that people using your component can easily discover which props they should provide.

## $props.id()

This rune, added in version 5.20.0, generates an ID that is unique to the current component instance. When hydrating a server-rendered component, the value will be consistent between server and client.

This is useful for linking elements via attributes like `for` and `aria-labelledby`.

    <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>

---

## Page: https://svelte.dev/docs/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, but it can simplify your code if used sparingly and carefully.

It also means that a state proxy can be _mutated_ in the child.

> 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:

FancyInput

    <script>
    	let { value = $bindable(), ...props } = $props();
    </script>
    
    <input bind:value={value} {...props} />
    
    <style>
    	input {
    		font-family: 'Comic Sans MS';
    		color: deeppink;
    	}
    </style>

    <script lang="ts">
    	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:` directive (demo):

App

    <script>
    	import FancyInput from './FancyInput.svelte';
    
    	let message = $state('hello');
    </script>
    
    <FancyInput bind:value={message} />
    <p>{message}</p>

    <script lang="ts">
    	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:

FancyInput

    let { value = $bindable('fallback'), ...props } = $props();

---

## Page: https://svelte.dev/docs/svelte/$inspect

> `$inspect` only works during development. In a production build it becomes a noop.

The `$inspect` rune is roughly equivalent to `console.log`, with the exception that it will re-run whenever its argument changes. `$inspect` tracks reactive state deeply, meaning that updating something inside an object or array using fine-grained reactivity will cause it to re-fire (demo):

    <script>
    	let count = $state(0);
    	let message = $state('hello');
    
    	$inspect(count, message); // will console.log when `count` or `message` change
    </script>
    
    <button onclick={() => count++}>Increment</button>
    <input bind:value={message} />

## $inspect(...).with

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

    <script>
    	let count = $state(0);
    
    	$inspect(count).with((type, count) => {
    		if (type === 'update') {
    			debugger; // or `console.trace`, or whatever you want
    		}
    	});
    </script>
    
    <button onclick={() => count++}>Increment</button>

A convenient way to find the origin of some change is to pass `console.trace` to `with`:

    $inspect(stuff).with(console.trace);
    

## $inspect.trace(...)

This rune, added in 5.14, causes the surrounding function to be _traced_ in development. Any time the function re-runs as part of an effect or a 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();
    		doSomeWork();
    	});
    </script>

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

---

## Page: https://svelte.dev/docs/svelte/$host

When compiling a component as a custom element, the `$host` rune provides access to the host element, allowing you to (for example) dispatch custom events (demo):

Stepper

    <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>

    <svelte:options customElement="my-stepper" />
    
    <script lang="ts">
    	function dispatch(type) {
    		$host().dispatchEvent(new CustomEvent(type));
    	}
    </script>
    
    <button onclick={() => dispatch('decrement')}>decrement</button>
    <button onclick={() => dispatch('increment')}>increment</button>

App

    <script>
    	import './Stepper.svelte';
    
    	let count = $state(0);
    </script>
    
    <my-stepper
    	ondecrement={() => count -= 1}
    	onincrement={() => count += 1}
    ></my-stepper>
    
    <p>count: {count}</p>

    <script lang="ts">
    	import './Stepper.svelte';
    
    	let count = $state(0);
    </script>
    
    <my-stepper
    	ondecrement={() => count -= 1}
    	onincrement={() => count += 1}
    ></my-stepper>
    
    <p>count: {count}</p>

---

## Page: https://svelte.dev/docs/svelte/basic-markup

Markup inside a Svelte component can be thought of as HTML++.

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 and excluded if it’s falsy.

All other attributes are included unless their value is nullish (`null` or `undefined`).

    <input required={false} placeholder="This input field is not required" />
    <div title={null}>This div has no title attribute</div>

> 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_ 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.

    <Widget {...things} />

## Events

Listening to DOM events is possible by adding attributes to the element that start with `on`. For example, to listen to the `click` event, add the `onclick` attribute to a button:

    <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 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` 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}

Curly braces can be included in a Svelte template by using their HTML entity strings: `&lbrace;`, `&lcub;`, or `&#123;` for `{` and `&rbrace;`, `&rcub;`, or `&#125;` for `}`.

If you’re using a regular expression (`RegExp`) literal notation, 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}

> 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

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>

---

## Page: https://svelte.dev/docs/svelte/if

    {#if expression}...{/if}

    {#if expression}...{:else if expression}...{/if}

    {#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.)

---

## Page: https://svelte.dev/docs/svelte/each

    {#each expression as name}...{/each}

    {#each expression as name, index}...{/each}

Iterating over values can be done with an each block. The values in question can be arrays, array-like objects (i.e. anything with a `length` property), or iterables like `Map` and `Set` — in other words, anything that can be used with `Array.from`.

    <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

    {#each expression as name (key)}...{/each}

    {#each expression as name, index (key)}...{/each}

If a _key_ expression is provided — which must uniquely identify each list item — Svelte will use it to diff the list when data changes, rather than adding or removing items at the end. The key can be any object, but strings and numbers are recommended since they allow identity to persist when the objects themselves change.

    {#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

    {#each expression}...{/each}

    {#each expression, index}...{/each}

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

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

## Else blocks

    {#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}

---

## Page: https://svelte.dev/docs/svelte/key

    {#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}

---

## Page: https://svelte.dev/docs/svelte/await

    {#await expression}...{:then name}...{:catch name}...{/await}

    {#await expression}...{:then name}...{/await}

    {#await expression then name}...{/await}

    {#await expression catch name}...{/await}

Await blocks allow you to branch on the three possible states of a `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}

> 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}

> You can use `#await` with `import(...)` to render components lazily:
> 
>     {#await import('./Component.svelte') then { default: Component }}
>     	<Component />
>     {/await}

---

## Page: https://svelte.dev/docs/svelte/snippet

    {#snippet name()}...{/snippet}

    {#snippet name(param1, param2, paramN)}...{/snippet}

Snippets, and render tags, are a way to create reusable chunks of markup inside your components. Instead of writing duplicative code like this...

    {#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:

    {#snippet figure(image)}
    	<figure>
    		<img src={image.src} alt={image.caption} width={image.width} height={image.height} />
    		<figcaption>{image.caption}</figcaption>
    	</figure>
    {/snippet}
    
    {#each images as image}
    	{#if image.href}
    		<a href={image.href}>
    			{@render figure(image)}
    		</a>
    	{:else}
    		{@render figure(image)}
    	{/if}
    {/each}

Like function declarations, snippets can have an arbitrary number of parameters, which can have default values, and you can destructure each parameter. You cannot use rest parameters, however.

## Snippet scope

Snippets can be declared anywhere inside your component. They can reference values declared outside themselves, for example in the `<script>` tag or in `{#each ...}` blocks (demo)...

    <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 (demo):

    {#snippet blastoff()}
    	<span>🚀</span>
    {/snippet}
    
    {#snippet countdown(n)}
    	{#if n > 0}
    		<span>{n}...</span>
    		{@render countdown(n - 1)}
    	{:else}
    		{@render blastoff()}
    	{/if}
    {/snippet}
    
    {@render countdown(10)}

## Passing snippets to components

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

    <script>
    	import Table from './Table.svelte';
    
    	const fruits = [
    		{ name: 'apples', qty: 5, price: 2 },
    		{ name: 'bananas', qty: 10, price: 1 },
    		{ name: 'cherries', qty: 20, price: 0.5 }
    	];
    </script>
    
    {#snippet header()}
    	<th>fruit</th>
    	<th>qty</th>
    	<th>price</th>
    	<th>total</th>
    {/snippet}
    
    {#snippet row(d)}
    	<td>{d.name}</td>
    	<td>{d.qty}</td>
    	<td>{d.price}</td>
    	<td>{d.qty * d.price}</td>
    {/snippet}
    
    <Table data={fruits} {header} {row} />

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

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

    <!-- this is semantically the same as the above -->
    <Table data={fruits}>
    	{#snippet header()}
    		<th>fruit</th>
    		<th>qty</th>
    		<th>price</th>
    		<th>total</th>
    	{/snippet}
    
    	{#snippet row(d)}
    		<td>{d.name}</td>
    		<td>{d.qty}</td>
    		<td>{d.price}</td>
    		<td>{d.qty * d.price}</td>
    	{/snippet}
    </Table>

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

App

    <Button>click me</Button>

Button

    <script>
    	let { children } = $props();
    </script>
    
    <!-- result will be <button>click me</button> -->
    <button>{@render children()}</button>

    <script lang="ts">
    	let { children } = $props();
    </script>
    
    <!-- result will be <button>click me</button> -->
    <button>{@render children()}</button>

> Note that you cannot have a prop called `children` if you also have content inside the component — for this reason, you should avoid having props with that name

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

    <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) (demo):

    <script module>
    	export { add };
    </script>
    
    {#snippet add(a, b)}
    	{a} + {b} = {a + b}
    {/snippet}

> This requires Svelte 5.5.0 or newer

## Programmatic snippets

Snippets can be created programmatically with the `createRawSnippet` API. This is intended for advanced use cases.

## Snippets and slots

In Svelte 4, content can be passed to components using slots. Snippets are more powerful and flexible, and as such slots are deprecated in Svelte 5.

---

## Page: https://svelte.dev/docs/svelte/@render

To render a 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 ...}` block with an `:else` clause to render fallback content:

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

---

## Page: https://svelte.dev/docs/svelte/@html

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

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

> 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. 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 — 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>

---

## Page: https://svelte.dev/docs/svelte/@const

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>`.

---

## Page: https://svelte.dev/docs/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.

---

## Page: https://svelte.dev/docs/svelte/bind

Data ordinarily flows down, from parent to child. The `bind:` directive allows data to flow the other way, from child to parent.

The general syntax is `bind:property={expression}`, where `expression` is an _lvalue_ (i.e. a variable or an object property). When the expression is an identifier with the same name as the property, we can omit the expression — in other words these are equivalent:

    <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, the `get` value should be `null`:

    <div
    	bind:clientWidth={null, redraw}
    	bind:clientHeight={null, redraw}
    >...</div>

> 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 (demo):

    <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>

> Use reset buttons sparingly, and ensure that users won’t accidentally click them while trying to submit the form.

## <input bind:checked>

Checkbox and radio inputs can be bound with `bind:checked`:

    <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>

## <input bind:group>

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

    <script>
    	let tortilla = $state('Plain');
    
    	/** @type {Array<string>} */
    	let fillings = $state([]);
    </script>
    
    <!-- grouped radio inputs are mutually exclusive -->
    <input type="radio" bind:group={tortilla} value="Plain" />
    <input type="radio" bind:group={tortilla} value="Whole wheat" />
    <input type="radio" bind:group={tortilla} value="Spinach" />
    
    <!-- grouped checkbox inputs populate an array -->
    <input type="checkbox" bind:group={fillings} value="Rice" />
    <input type="checkbox" bind:group={fillings} value="Beans" />
    <input type="checkbox" bind:group={fillings} value="Cheese" />
    <input type="checkbox" bind:group={fillings} value="Guac (extra)" />

> `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. 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` 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.

> `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`
*   `playbackRate`
*   `paused`
*   `volume`
*   `muted`

...and six readonly ones:

*   `duration`
*   `buffered`
*   `seekable`
*   `seeking`
*   `ended`
*   `readyState`

    <audio src={clip} bind:duration bind:currentTime bind:paused></audio>

## <video>

`<video>` elements have all the same bindings as `<audio>` elements, plus readonly `videoWidth` and `videoHeight` bindings.

## <img>

`<img>` elements have two readonly bindings:

*   `naturalWidth`
*   `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>

## Contenteditable bindings

Elements with the `contenteditable` attribute support the following bindings:

*   `innerHTML`
*   `innerText`
*   `textContent`

> There are subtle differences between `innerText` and `textContent`.

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

## Dimensions

All visible elements have the following readonly bindings, measured with a `ResizeObserver`:

*   `clientWidth`
*   `clientHeight`
*   `offsetWidth`
*   `offsetHeight`

    <div bind:offsetWidth={width} bind:offsetHeight={height}>
    	<Chart {width} {height} />
    </div>

> `display: inline` elements do not have a width or height (except for elements with ‘intrinsic’ dimensions, like `<img>` and `<canvas>`), and cannot be observed with a `ResizeObserver`. You will need to change the `display` style of these elements to something else, such as `inline-block`.

## bind:this

    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.

App

    <ShoppingCart bind:this={cart} />
    
    <button onclick={() => cart.empty()}> Empty shopping cart </button>

ShoppingCart

    <script>
    	// All instance exports are available on the instance object
    	export function empty() {
    		// ...
    	}
    </script>

    <script lang="ts">
    	// All instance exports are available on the instance object
    	export function empty() {
    		// ...
    	}
    </script>

## 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` 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.

---

## Page: https://svelte.dev/docs/svelte/use

Actions are functions that are called when an element is mounted. They are added with the `use:` directive, and will typically use an `$effect` so that they can reset any state when the element is unmounted:

App

    <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>

    <script lang="ts">
    	import type { Action } from 'svelte/action';
    
    	const myaction: Action = (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:

App

    <script>
    	/** @type {import('svelte/action').Action} */
    	function myaction(node, data) {
    		// ...
    	}
    </script>
    
    <div use:myaction={data}>...</div>

    <script lang="ts">
    	import type { Action } from 'svelte/action';
    
    	const myaction: Action = (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 mode
> 
> 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:

App

    <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>

    <script lang="ts">
    	import type { Action } from 'svelte/action';
    
    	const gestures: Action<
    		HTMLDivElement,
    		undefined,
    		{
    			onswiperight: (e: CustomEvent) => void;
    			onswipeleft: (e: CustomEvent) => void;
    			// ...
    		}
    	> = (node) => {
    		$effect(() => {
    			// ...
    			node.dispatchEvent(new CustomEvent('swipeleft'));
    
    			// ...
    			node.dispatchEvent(new CustomEvent('swiperight'));
    		});
    	};
    </script>
    
    <div
    	use:gestures
    	onswipeleft={next}
    	onswiperight={prev}
    >...</div>

---

## Page: https://svelte.dev/docs/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}

## Built-in transitions

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

## Local vs global

Transitions are local by default. Local transitions only play when the block they belong to is created or destroyed, _not_ when parent blocks are created or destroyed.

    {#if x}
    	{#if y}
    		<p transition:fade>fades in and out only when y changes</p>
    
    		<p transition:fade|global>fades in and out when x or y change</p>
    	{/if}
    {/if}

## Transition parameters

Transitions can have parameters.

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

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

## Custom transition functions

    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.

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.

App

    <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}

    <script lang="ts">
    	import { elasticOut } from 'svelte/easing';
    	export let visible: boolean;
    	function whoosh(node: HTMLElement, params: { delay?: number, duration?: number, easing?: (t: number) => number }) {
    		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.

> 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.

App

    <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}

    <script lang="ts">
    	export let visible = false;
    	function typewriter(node: HTMLElement, { speed = 1 }: { speed?: number }) {
    		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 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}

---

## Page: https://svelte.dev/docs/svelte/in-and-out

The `in:` and `out:` directives are identical to `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}

---

## Page: https://svelte.dev/docs/svelte/animate

An animation is triggered when the contents of a keyed each block 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 or 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

    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 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 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.

App

    <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}

    <script lang="ts">
    	import { cubicOut } from 'svelte/easing';
    	function whizz(node: HTMLElement, { from, to }: { from: DOMRect; to: DOMRect }, params: any) {
    		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.

> 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.

App

    <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}

    <script lang="ts">
    	import { cubicOut } from 'svelte/easing';
    	function whizz(node: HTMLElement, { from, to }: { from: DOMRect; to: DOMRect }, params: any) {
    		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}

---

## Page: https://svelte.dev/docs/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:

    <div style="color: blue;" style:color="red">This will be red</div>

---

## Page: https://svelte.dev/docs/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>

> 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.

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:

Button

    <script>
    	let props = $props();
    </script>
    
    <button {...props} class={['cool-button', props.class]}>
    	{@render props.children?.()}
    </button>

    <script lang="ts">
    	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:

App

    <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>

    <script lang="ts">
    	import Button from './Button.svelte';
    	let useTailwind = $state(false);
    </script>
    
    <Button
    	onclick={() => useTailwind = true}
    	class={{ 'bg-blue-700 sm:w-1/2': useTailwind }}
    >
    	Accept the inevitability of Tailwind
    </Button>

Svelte also exposes the `ClassValue` type, which is the type of value that the `class` attribute on elements accept. This is useful if you want to use a type-safe class name in component props:

    <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>

> Unless you’re using an older version of Svelte, consider avoiding `class:`, since the attribute is more powerful and composable.

---

## Page: https://svelte.dev/docs/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 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>

---

## Page: https://svelte.dev/docs/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>

> 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.

---

## Page: https://svelte.dev/docs/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(...)`:

    <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.

> 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.

---

## Page: https://svelte.dev/docs/svelte/nested-style-elements

There can only be one top-level `<style>` tag per component.

However, it is possible to have a `<style>` tag nested inside other elements or logic blocks.

In that case, the `<style>` tag will be inserted as-is into the DOM; no scoping or processing will be done on the `<style>` tag.

    <div>
    	<style>
    		/* this style tag will be inserted as-is */
    		div {
    			/* this will apply to all `<div>` elements in the DOM */
    			color: red;
    		}
    	</style>
    </div>

---

## Page: https://svelte.dev/docs/svelte/svelte-boundary

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

> This feature was added in 5.3.0

Boundaries allow you to guard against errors in part of your app from breaking the app as a whole, and to recover from those errors.

If an error occurs while rendering or updating the children of a `<svelte:boundary>`, or running any `$effect` functions contained therein, the contents will be removed.

Errors occurring outside the rendering process (for example, in event handlers or after a `setTimeout` or async work) are _not_ caught by error boundaries.

## Properties

For the boundary to do anything, one or both of `failed` and `onerror` must be provided.

### failed

If a `failed` snippet is provided, it will be rendered with the error that was thrown, and a `reset` function that recreates the contents (demo):

    <svelte:boundary>
    	<FlakyComponent />
    
    	{#snippet failed(error, reset)}
    		<button onclick={reset}>oops! try again</button>
    	{/snippet}
    </svelte:boundary>

> As with snippets passed 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.

---

## Page: https://svelte.dev/docs/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 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`.

---

## Page: https://svelte.dev/docs/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 actions 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} use:someAction />

You can also bind to the following properties:

*   `activeElement`
*   `fullscreenElement`
*   `pointerLockElement`
*   `visibilityState`

All are readonly.

---

## Page: https://svelte.dev/docs/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 on the `<body>` element.

As with `<svelte:window>` and `<svelte:document>`, this element may only appear the top level of your component and must never be inside a block or element.

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

---

## Page: https://svelte.dev/docs/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>

---

## Page: https://svelte.dev/docs/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 (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.

---

## Page: https://svelte.dev/docs/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. The possible options are:

*   `runes={true}` — forces a component into _runes mode_ (see the Legacy APIs 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 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 mode
> 
> 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" />

Edit this page on GitHub

previous next

---

## Page: https://svelte.dev/docs/svelte/stores

A _store_ is an object that allows reactive access to a value via a simple _store contract_. The `svelte/store` module 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

state.svelte

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

App

    <script>
    	import { userState } from './state.svelte.js';
    </script>
    
    <p>User name: {userState.name}</p>
    <button onclick={() => {
    	userState.name = 'new name';
    }}>
    	change name
    </button>

    <script lang="ts">
    	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.

store

    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.

store

    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.

    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`.

    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.

    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.

    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
    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.

> This works by creating a subscription, reading the value, then unsubscribing. It’s therefore not recommended in hot code paths.

    import { get } from 'svelte/store';
    
    const value = get(store);

## Store contract

    store = { subscribe: (subscription: (value: any) => void) => (() => void), set?: (value: any) => void }

You can create your own stores without relying on `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.

---

## Page: https://svelte.dev/docs/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’). The parent component sets context with `setContext(key, value)`...

Parent

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

    <script lang="ts">
    	import { setContext } from 'svelte';
    
    	setContext('my-context', 'hello from Parent.svelte');
    </script>

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

Child

    <script>
    	import { getContext } from 'svelte';
    
    	const message = getContext('my-context');
    </script>
    
    <h1>{message}, inside Child.svelte</h1>

    <script lang="ts">
    	import { getContext } from 'svelte';
    
    	const message = getContext('my-context');
    </script>
    
    <h1>{message}, inside Child.svelte</h1>

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

    <Parent>
    	<Child />
    </Parent>

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

In addition to `setContext` and `getContext`, Svelte exposes `hasContext` and `getAllContexts` functions.

## Using context with state

You can store reactive state in context (demo)...

    <script>
    	import { setContext } from 'svelte';
    	import Child from './Child.svelte';
    
    	let counter = $state({
    		count: 0
    	});
    
    	setContext('counter', counter);
    </script>
    
    <button onclick={() => counter.count += 1}>
    	increment
    </button>
    
    <Child />
    <Child />
    <Child />

...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.

## Type-safe context

A useful pattern is to wrap the calls to `setContext` and `getContext` inside helper functions that let you preserve type safety:

context

    import { getContext, setContext } from 'svelte';
    
    let key = {};
    
    /** @param {User} user */
    export function setUserContext(user) {
    	setContext(key, user);
    }
    
    export function getUserContext() {
    	return /** @type {User} */ (getContext(key));
    }

    import { getContext, setContext } from 'svelte';
    
    let key = {};
    export function setUserContext(user: User) {
    	setContext(key, user);
    }
    
    export function getUserContext() {
    	return getContext(key) as User;
    }

## 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:

state.svelte

    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!)...

App

    <script>
    	import { myGlobalState } from 'svelte';
    
    	let { data } = $props();
    
    	if (data.user) {
    		myGlobalState.user = data.user;
    	}
    </script>

    <script lang="ts">
    	import { myGlobalState } from 'svelte';
    
    	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.

---

## Page: https://svelte.dev/docs/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>

> This behaviour will only work when the function passed to `onMount` _synchronously_ returns a value. `async` functions always return a `Promise`, and as such cannot _synchronously_ return a function.

## onDestroy

Schedules a callback to run immediately before the component is unmounted.

Out of `onMount`, `beforeUpdate`, `afterUpdate` and `onDestroy`, this is the only one that runs inside a server-side component.

    <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
*   After

    <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>

---

## Page: https://svelte.dev/docs/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:

    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` or `hydrate`.

If `options.outro` is `true`, transitions 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:

    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` function) inside the target and make it interactive:

    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.

---

## Page: https://svelte.dev/docs/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, Jasmine, Cypress and Playwright.

## Unit and integration testing using Vitest

Unit tests allow you to test small isolated parts of your code. Integration tests allow you to test parts of your application to see if they work together. If you’re using Vite (including via SvelteKit), we recommend using Vitest.

To get started, install Vitest:

    npm install -D vitest

Then adjust your `vite.config.js`:

vite.config

    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
    });

> 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

You can now write unit tests for code inside your `.js/.ts` files:

multiplier.svelte.test

    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);
    });

multiplier.svelte

    /**
     * @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;
    		}
    	};
    }

    export function multiplier(initial: number, k: number) {
    	let count = $state(initial);
    
    	return {
    		get value() {
    			return count * k;
    		},
    		set: (c: number) => {
    			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`:

multiplier.svelte.test

    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);
    });

multiplier.svelte

    /**
     * @param {() => number} getCount
     * @param {number} k
     */
    export function multiplier(getCount, k) {
    	return {
    		get value() {
    			return getCount() * k;
    		}
    	};
    }

    export function multiplier(getCount: () => number, k: number) {
    	return {
    		get value() {
    			return getCount() * k;
    		}
    	};
    }

If the code being tested uses effects, you need to wrap the test inside `$effect.root`:

logger.svelte.test

    import { flushSync } from 'svelte';
    import { expect, test } from 'vitest';
    import { logger } from './logger.svelte.js';
    
    test('Effect', () => {
    	const cleanup = $effect.root(() => {
    		let count = $state(0);
    
    		// logger uses an $effect to log updates of its input
    		let log = logger(() => count);
    
    		// effects normally run after a microtask,
    		// use flushSync to execute all pending effects synchronously
    		flushSync();
    		expect(log.value).toEqual([0]);
    
    		count = 1;
    		flushSync();
    
    		expect(log.value).toEqual([0, 1]);
    	});
    
    	cleanup();
    });

logger.svelte

    /**
     * @param {() => any} getValue
     */
    export function logger(getValue) {
    	/** @type {any[]} */
    	let log = $state([]);
    
    	$effect(() => {
    		log.push(getValue());
    	});
    
    	return {
    		get value() {
    			return log;
    		}
    	};
    }

    export function logger(getValue: () => any) {
    	let log: any[] = $state([]);
    
    	$effect(() => {
    		log.push(getValue());
    	});
    
    	return {
    		get value() {
    			return log;
    		}
    	};
    }

### Component testing

It is possible to test your components in isolation using Vitest.

> 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`:

vite.config

    import { defineConfig } from 'vitest/config';
    
    export default defineConfig({
    	plugins: [
    		/* ... */
    	],
    	test: {
    		// If you are testing components client-side, you need to setup a DOM environment.
    		// If not all your files should have this environment, you can use a
    		// `// @vitest-environment jsdom` comment at the top of the test files instead.
    		environment: 'jsdom'
    	},
    	// Tell Vitest to use the `browser` entry points in `package.json` files, even though it's running in Node
    	resolve: process.env.VITEST
    		? {
    				conditions: ['browser']
    			}
    		: undefined
    });

After that, you can create a test file in which you import the component to test, interact with it programmatically and write expectations about the results:

component.test

    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 can help streamline your tests. The above test could be rewritten like this:

component.test

    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.

## E2E tests using Playwright

E2E (short for ‘end to end’) tests allow you to test your full application through the eyes of the user. This section uses Playwright as an example, but you can also use other solutions like Cypress or NightwatchJS.

To get started with Playwright, either install it via the VS Code extension, or install it from the command line using `npm init playwright`. It is also part of the setup CLI when you run `npx sv create`.

After you’ve done that, you should have a `tests` folder and a Playwright config. You may need to adjust that config to tell Playwright what to do before running the tests - mainly starting your application at a certain port:

playwright.config

    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.

tests/hello-world.spec

    import { expect, test } from '@playwright/test';
    
    test('home page has expected h1', async ({ page }) => {
    	await page.goto('/');
    	await expect(page.locator('h1')).toBeVisible();
    });

---

## Page: https://svelte.dev/docs/svelte/typescript

You can use TypeScript within Svelte components. IDE extensions like the Svelte VS Code extension will help you catch errors right in your editor, and `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.

svelte.config

    import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
    
    const config = {
    	// Note the additional `{ script: true }`
    	preprocess: vitePreprocess({ script: true })
    };
    
    export default config;

### Using SvelteKit or Vite

The easiest way to get started is scaffolding a new SvelteKit project by typing `npx sv create`, following the prompts and choosing the TypeScript option.

svelte.config

    import { vitePreprocess } from '@sveltejs/vite-plugin-svelte';
    
    const config = {
    	preprocess: vitePreprocess()
    };
    
    export default config;

If you don’t need or want all the features SvelteKit has to offer, you can scaffold a Svelte-flavoured Vite project instead by typing `npm create vite@latest` and selecting the `svelte-ts` option.

In both cases, a `svelte.config.js` with `vitePreprocess` will be added. Vite/SvelteKit will read from this config file.

### Other build tools

If you’re using tools like Rollup or Webpack instead, install their respective Svelte plugins. For Rollup that’s rollup-plugin-svelte and for Webpack that’s svelte-loader. For both, you need to install `typescript` and `svelte-preprocess` and add the preprocessor to the plugin config (see the respective READMEs for more info). If you’re starting a new project, you can also use the rollup or webpack template to scaffold the setup from a script.

> 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` of at least `ES2022`, or a `target` of at least `ES2015` alongside `useDefineForClassFields`. This ensures that rune declarations on class fields are not messed with, which would break the Svelte compiler
*   Set `verbatimModuleSyntax` to `true` so that imports are left as-is
*   Set `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`.

    // 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 mode
> 
> 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. In that case, you are welcome to open an issue and/or a PR fixing it.

In case this is a custom or experimental attribute/event, you can enhance the typings like this:

additional-svelte-typings.d

    declare namespace svelteHTML {
    	// enhance elements
    	interface IntrinsicElements {
    		'my-custom-element': { someattribute: string; 'on:event': (e: CustomEvent<any>) => void };
    	}
    	// enhance attributes
    	interface HTMLAttributes<T> {
    		// If you want to use the beforeinstallprompt event
    		onbeforeinstallprompt?: (event: any) => any;
    		// If you want to use myCustomAttribute={..} (note: all lowercase)
    		mycustomattribute?: any; // You can replace any with something more specific if you like
    	}
    }

Then make sure that `d.ts` file is referenced in your `tsconfig.json`. If it reads something like `"include": ["src/**/*"]` and your `d.ts` file is inside `src`, it should work. You may need to reload for the changes to take effect.

You can also declare the typings by augmenting the `svelte/elements` module like this:

additional-svelte-typings.d

    import { HTMLButtonAttributes } from 'svelte/elements';
    
    declare module 'svelte/elements' {
    	export interface SvelteHTMLElements {
    		'custom-button': HTMLButtonAttributes;
    	}
    
    	// allows for more granular control over what element to add the typings to
    	export interface HTMLButtonAttributes {
    		veryexperimentalattribute?: string;
    	}
    }
    
    export {}; // ensure this is not an ambient module, else types will be overridden instead of augmented

---

## Page: https://svelte.dev/docs/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 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`.

    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 are exposed as properties of the DOM element (as well as being readable/writable as attributes, where possible).

    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 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.

When a custom element written with Svelte is created or updated, the shadow DOM will reflect the value in the next tick, not immediately. This way updates can be batched, and DOM moves which temporarily (but synchronously) detach the element from the DOM don’t lead to unmounting the inner component.

The inner Svelte component is destroyed in the next tick after the `disconnectedCallback` is invoked.

## Component options

When constructing a custom element, you can tailor several aspects by defining `customElement` as an object within `<svelte:options>` since Svelte 4. This object may contain the following properties:

*   `tag: string`: an optional `tag` property for the custom element’s name. If set, a custom element with this tag name will be defined with the document’s `customElements` registry upon importing this component.
*   `shadow`: an optional property that can be set to `"none"` to forgo shadow root creation. Note that styles are then no longer encapsulated, and you can’t use slots
*   `props`: an optional property to modify certain details and behaviors of your component’s properties. It offers the following settings:
    *   `attribute: string`: To update a custom element’s prop, you have two alternatives: either set the property on the custom element’s reference as illustrated above or use an HTML attribute. For the latter, the default attribute name is the lowercase property name. Modify this by assigning `attribute: "<desired name>"`.
    *   `reflect: boolean`: By default, updated prop values do not reflect back to the DOM. To enable this behavior, set `reflect: true`.
    *   `type: 'String' | 'Boolean' | 'Number' | 'Array' | 'Object'`: While converting an attribute value to a prop value and reflecting it back, the prop value is assumed to be a `String` by default. This may not always be accurate. For instance, for a number type, define it using `type: "Number"` You don’t need to list all properties, those not listed will use the default settings.
*   `extend`: an optional property which expects a function as its argument. It is passed the custom element class generated by Svelte and expects you to return a custom element class. This comes in handy if you have very specific requirements to the life cycle of the custom element or want to enhance the class to for example use ElementInternals for better HTML form integration.

    <svelte:options
    	customElement={{
    		tag: 'custom-element',
    		shadow: 'none',
    		props: {
    			name: { reflect: true, type: 'Number', attribute: 'element-index' }
    		},
    		extend: (customElementConstructor) => {
    			// Extend the class so we can let it participate in HTML forms
    			return class extends customElementConstructor {
    				static formAssociated = true;
    
    				constructor() {
    					super();
    					this.attachedInternals = this.attachInternals();
    				}
    
    				// Add the function here, not below in the component so that
    				// it's always available, not just when the inner Svelte component
    				// is mounted
    				randomIndex() {
    					this.elementIndex = Math.random();
    				}
    			};
    		}
    	}}
    />
    
    <script>
    	let { elementIndex, attachedInternals } = $props();
    	// ...
    	function check() {
    		attachedInternals.checkValidity();
    	}
    </script>
    
    ...

## Caveats and limitations

Custom elements can be a useful way to package components for consumption in a non-Svelte app, as they will work with vanilla HTML and JavaScript as well as most frameworks. 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`)

---

## Page: https://svelte.dev/docs/svelte/v4-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)
*   If you are using SvelteKit, upgrade to 1.20.4 or newer (sveltejs/kit#10172)
*   If you are using Vite without SvelteKit, upgrade to `vite-plugin-svelte` 2.4.1 or newer (#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, 198dbcf)
*   If you are using Rollup, upgrade to `rollup-plugin-svelte` 7.1.5 or higher (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)

## 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` 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` documentation for more details

(#8516)

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)

## 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)

    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)

    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)

    // 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 for advanced use cases. The migration script will adjust your code automatically. The update timing of properties has changed slightly as well. (#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)

## 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)

## 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)

## 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.

    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.

    preprocess: [
    	vitePreprocess(),
    	mdsvex(mdsvexConfig)
    	mdsvex(mdsvexConfig),
    	vitePreprocess()
    ]

Each preprocessor must also have a name. (#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. See this Github post 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)
*   the runtime now uses `classList.toggle(name, boolean)` which may not work in very old browsers. Consider using a polyfill if you need to support these browsers. (#8629)
*   the runtime now uses the `CustomEvent` constructor which may not work in very old browsers. Consider using a polyfill if you need to support these browsers. (#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)
*   `derived` will now throw an error on falsy values instead of stores passed to it. (#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)
*   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

---

## Page: https://svelte.dev/docs/svelte/v5-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 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()`.

> 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 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 `$effect` runs is different than when `$:` runs.

> 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} {...$$restPropsrest}>click me</button>

> 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:

App

    <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}

    <script lang="ts">
    	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}

Pump

    <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>

    <script lang="ts">
    	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 as such do not work with modern event handlers. Adding things like `event.preventDefault()` inside the handler itself is preferable, since all the logic lives in one place rather than being split between handler and modifiers.

Since event handlers are just functions, you can create your own wrappers as necessary:

    <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` 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>

> Why we did this
> 
> `createEventDispatcher` was always a bit boilerplate-y:
> 
> *   import the function
> *   call the function to get a dispatch function
> *   call said dispatch function with a string and possibly a payload
> *   retrieve said payload on the other end through a `.detail` property, because the event itself was always a `CustomEvent`
> 
> It was always possible to use component callback props, but because you had to listen to DOM events using `on:`, it made sense to use `createEventDispatcher` for component events due to syntactical consistency. Now that we have event attributes (`onclick`), it’s the other way around: Callback props are now the more sensible thing to do.
> 
> The removal of event modifiers is arguably one of the changes that seems like a step back for those who’ve liked the shorthand syntax of event modifiers. Given that they are not used that frequently, we traded a smaller surface area for more explicitness. Modifiers also were inconsistent, because most of them were only useable on DOM elements.
> 
> Multiple listeners for the same event are also no longer possible, but it was something of an anti-pattern anyway, since it impedes readability: if there are many attributes, it becomes harder to spot that there are two handlers unless they are right next to each other. It also implies that the two handlers are independent, when in fact something like `event.stopImmediatePropagation()` inside `one` would prevent `two` from being called.
> 
> By deprecating `createEventDispatcher` and the `on:` directive in favour of callback props and normal element properties, we:
> 
> *   reduce Svelte’s learning curve
> *   remove boilerplate, particularly around `createEventDispatcher`
> *   remove the overhead of creating `CustomEvent` objects for events that may not even have listeners
> *   add the ability to spread event handlers
> *   add the ability to know which event handlers were provided to a component
> *   add the ability to express whether a given event handler is required or optional
> *   increase type safety (previously, it was effectively impossible for Svelte to guarantee that a component didn’t emit a particular event)

## Snippets instead of slots

In Svelte 4, content can be passed to components using slots. Svelte 5 replaces them with snippets which are more powerful and flexible, and as such slots are deprecated in Svelte 5.

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

Child

    <slot />
    <hr />
    <slot name="foo" message="hello" />

Parent

    <script>
    	import Child from './Child.svelte';
    </script>
    
    <Child>
    	default child content
    
    	{#snippet foo({ message })}
    		message from child: {message}
    	{/snippet}
    </Child>

    <script lang="ts">
    	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 ...}` 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:

App

    <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>

    <script lang="ts">
    	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>

List

    <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}

    <script lang="ts">
    	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}

> 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 that using `events` is discouraged — instead, use callbacks

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. 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
*   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`
*   elements with `clientWidth` / `clientHeight`/`offsetWidth`/`offsetHeight` bindings use a `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 and wheel events are passive

When using `onwheel`, `onmousewheel`, `ontouchstart` and `ontouchmove` event attributes, the handlers are passive 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` 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(...) and :where(...) are scoped

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

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

    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.

### 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.

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:

    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. As such, you shouldn’t remove comments from your HTML output if you intend to hydrate it, and if you manually authored HTML to be hydrated by a Svelte component, you need to adjust that HTML to include said comments at the correct positions.

### onevent attributes are delegated

Event attributes replace event directives: Instead of `on:click={handler}` you write `onclick={handler}`. For backwards compatibility the `on:event` syntax is still supported and behaves the same as in Svelte 4. Some of the `onevent` attributes however are delegated, which means you need to take care to not stop event propagation on those manually, as they then might never reach the listener for this event type at the root.

### \--style-props uses a different element

Svelte 5 uses an extra `<svelte-css-wrapper>` element instead of a `<div>` to wrap the component when using CSS custom properties.

---

## Page: https://svelte.dev/docs/svelte/faq

## I’m new to Svelte. Where should I start?

We think the best way to get started is playing through the interactive 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 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 or ask your own!

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 or the Reddit channel 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.

## How can I get VS Code to syntax-highlight my .svelte files?

There is an official VS Code extension for Svelte.

## Is there a tool to automatically format my .svelte files?

You can use prettier with the 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.

## Is there a UI component library?

There are several UI component libraries as well as standalone components. Find them under the design systems section of the components page on the Svelte Society website.

## How do I test Svelte apps?

How your application is structured and where logic is defined will determine the best way to ensure it is properly tested. It is important to note that not all logic belongs within a component - this includes concerns such as data transformation, cross-component state management, and logging, among others. Remember that the Svelte library has its own test suite, so you do not need to write tests to validate implementation details provided by Svelte.

A Svelte application will typically have three different types of tests: Unit, Component, and End-to-End (E2E).

_Unit Tests_: Focus on testing business logic in isolation. Often this is validating individual functions and edge cases. By minimizing the surface area of these tests they can be kept lean and fast, and by extracting as much logic as possible from your Svelte components more of your application can be covered using them. When creating a new SvelteKit project, you will be asked whether you would like to setup Vitest 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 to solutions that leverage an actual browser to provide a visual testing capability such as Playwright or Cypress.

_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 for end-to-end testing. There are many other E2E test libraries available for use as well.

Some resources for getting started with testing:

*   Svelte Testing Library
*   Svelte Component Testing in Cypress
*   Example using vitest
*   Example using uvu test runner with JSDOM
*   Test Svelte components using Vitest & Playwright
*   Component testing with WebdriverIO

## Is there a router?

The official routing library is SvelteKit. SvelteKit provides a filesystem router, server-side rendering (SSR), and hot module reloading (HMR) in one easy-to-use package. It shares similarities with Next.js for React.

However, you can use any router library. A lot of people use page.js. There’s also navaid, which is very similar. And universal-router, which is isomorphic with child routes, but without built-in history support.

If you prefer a declarative HTML approach, there’s the isomorphic svelte-routing library and a fork of it called svelte-navigator containing some additional functionality.

If you need hash-based routing on the client side, check out the hash option in SvelteKit, svelte-spa-router, or abstract-state-router.

Routify is another filesystem-based router, similar to SvelteKit’s router. Version 3 supports Svelte’s native SSR.

You can see a community-maintained list of routers on sveltesociety.dev.

## 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 into a mobile app with Tauri or Capacitor. Mobile features like the camera, geolocation, and push notifications are available via plugins for both platforms.

Svelte Native was an option available for Svelte 4, but note that Svelte 5 does not currently support it. Svelte Native lets you write NativeScript apps using Svelte components that contain NativeScript UI components 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.

## How do I do hot module reloading?

We recommend using SvelteKit, which supports HMR out of the box and is built on top of Vite and svelte-hmr. There are also community plugins for rollup and webpack.

---

## Page: https://svelte.dev/docs/svelte/svelte

    import {
    	SvelteComponent,
    	SvelteComponentTyped,
    	afterUpdate,
    	beforeUpdate,
    	createEventDispatcher,
    	createRawSnippet,
    	flushSync,
    	getAllContexts,
    	getContext,
    	hasContext,
    	hydrate,
    	mount,
    	onDestroy,
    	onMount,
    	setContext,
    	tick,
    	unmount,
    	untrack
    } from 'svelte';

## SvelteComponent

This was the base class for Svelte components in Svelte 4. Svelte 5+ components are completely different under the hood. For typing, use `Component` instead. To instantiate components, use `mount` instead. See migration guide for more info.

    class SvelteComponent<
    	Props extends Record<string, any> = Record<string, any>,
    	Events extends Record<string, any> = any,
    	Slots extends Record<string, any> = any
    > {…}

    static element?: typeof HTMLElement;

The custom element version of the component. Only present if compiled with the `customElement` compiler option

    [prop: string]: any;

    constructor(options: ComponentConstructorOptions<Properties<Props, Slots>>);

*   deprecated This constructor only exists when using the `asClassComponent` compatibility helper, which is a stop-gap solution. Migrate towards using `mount` instead. See migration guide for more info.

    $destroy(): void;

*   deprecated This method only exists when using one of the legacy compatibility helpers, which is a stop-gap solution. See migration guide for more info.

    $on<K extends Extract<keyof Events, string>>(
    	type: K,
    	callback: (e: Events[K]) => void
    ): () => void;

*   deprecated This method only exists when using one of the legacy compatibility helpers, which is a stop-gap solution. See migration guide for more info.

    $set(props: Partial<Props>): void;

*   deprecated This method only exists when using one of the legacy compatibility helpers, which is a stop-gap solution. See migration guide for more info.

## SvelteComponentTyped

> Use `Component` instead. See migration guide for more information.

    class SvelteComponentTyped<
    	Props extends Record<string, any> = Record<string, any>,
    	Events extends Record<string, any> = any,
    	Slots extends Record<string, any> = any
    > extends SvelteComponent<Props, Events, Slots> {}

## afterUpdate

> Use `$effect` instead

Schedules a callback to run immediately after the component has been updated.

The first time the callback runs will be after the initial `onMount`.

In runes mode use `$effect` instead.

    function afterUpdate(fn: () => void): void;

## beforeUpdate

> Use `$effect.pre` instead

Schedules a callback to run immediately before the component is updated after any state change.

The first time the callback runs will be before the initial `onMount`.

In runes mode use `$effect.pre` instead.

    function beforeUpdate(fn: () => void): void;

## createEventDispatcher

> Use callback props and/or the `$host()` rune instead — see migration guide

Creates an event dispatcher that can be used to dispatch component events. Event dispatchers are functions that can take two arguments: `name` and `detail`.

Component events created with `createEventDispatcher` create a CustomEvent. These events do not bubble. The `detail` argument corresponds to the CustomEvent.detail property and can contain any type of data.

The event dispatcher can be typed to narrow the allowed event names and the type of the `detail` argument:

    const dispatch = createEventDispatcher<{
     loaded: never; // does not take a detail argument
     change: string; // takes a detail argument of type string, which is required
     optional: number | null; // takes an optional detail argument of type number
    }>();

    function createEventDispatcher<
    	EventMap extends Record<string, any> = any
    >(): EventDispatcher<EventMap>;

## createRawSnippet

Create a snippet programmatically

    function createRawSnippet<Params extends unknown[]>(
    	fn: (...params: Getters<Params>) => {
    		render: () => string;
    		setup?: (element: Element) => void | (() => void);
    	}
    ): Snippet<Params>;

## flushSync

Synchronously flush any pending updates. Returns void if no callback is provided, otherwise returns the result of calling the callback.

    function flushSync<T = void>(fn?: (() => T) | undefined): T;

## getAllContexts

Retrieves the whole context map that belongs to the closest parent component. Must be called during component initialisation. Useful, for example, if you programmatically create a component and want to pass the existing context to it.

    function getAllContexts<
    	T extends Map<any, any> = Map<any, any>
    >(): T;

## getContext

Retrieves the context that belongs to the closest parent component with the specified `key`. Must be called during component initialisation.

    function getContext<T>(key: any): T;

## hasContext

Checks whether a given `key` has been set in the context of a parent component. Must be called during component initialisation.

    function hasContext(key: any): boolean;

## hydrate

Hydrates a component on the given target and returns the exports and potentially the props (if compiled with `accessors: true`) of the component

    function hydrate<
    	Props extends Record<string, any>,
    	Exports extends Record<string, any>
    >(
    	component:
    		| ComponentType<SvelteComponent<Props>>
    		| Component<Props, Exports, any>,
    	options: {} extends Props
    		? {
    				target: Document | Element | ShadowRoot;
    				props?: Props;
    				events?: Record<string, (e: any) => any>;
    				context?: Map<any, any>;
    				intro?: boolean;
    				recover?: boolean;
    			}
    		: {
    				target: Document | Element | ShadowRoot;
    				props: Props;
    				events?: Record<string, (e: any) => any>;
    				context?: Map<any, any>;
    				intro?: boolean;
    				recover?: boolean;
    			}
    ): Exports;

## mount

Mounts a component to the given target and returns the exports and potentially the props (if compiled with `accessors: true`) of the component. Transitions will play during the initial render unless the `intro` option is set to `false`.

    function mount<
    	Props extends Record<string, any>,
    	Exports extends Record<string, any>
    >(
    	component:
    		| ComponentType<SvelteComponent<Props>>
    		| Component<Props, Exports, any>,
    	options: MountOptions<Props>
    ): Exports;

## 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.

    function onDestroy(fn: () => any): void;

## onMount

`onMount`, like `$effect`, schedules a function to run as soon as the component has been mounted to the DOM. Unlike `$effect`, the provided function only runs once.

It must be called during the component’s initialisation (but doesn’t need to live _inside_ the component; it can be called from an external module). If a function is returned _synchronously_ from `onMount`, it will be called when the component is unmounted.

`onMount` functions do not run during server-side rendering.

    function onMount<T>(
    	fn: () =>
    		| NotFunction<T>
    		| Promise<NotFunction<T>>
    		| (() => any)
    ): void;

## setContext

Associates an arbitrary `context` object with the current component and the specified `key` and returns that object. The context is then available to children of the component (including slotted content) with `getContext`.

Like lifecycle functions, this must be called during component initialisation.

    function setContext<T>(key: any, context: T): T;

## tick

Returns a promise that resolves once any pending state changes have been applied.

    function tick(): Promise<void>;

## unmount

Unmounts a component that was previously mounted using `mount` or `hydrate`.

Since 5.13.0, if `options.outro` is `true`, transitions will play before the component is removed from the DOM.

Returns a `Promise` that resolves after transitions have completed if `options.outro` is true, or immediately otherwise (prior to 5.13.0, returns `void`).

    import { mount, unmount } from 'svelte';
    import App from './App.svelte';
    
    const app = mount(App, { target: document.body });
    
    // later...
    unmount(app, { outro: true });

    function unmount(
    	component: Record<string, any>,
    	options?:
    		| {
    				outro?: boolean;
    		  }
    		| undefined
    ): Promise<void>;

## untrack

When used inside a `$derived` or `$effect`, any state read inside `fn` will not be treated as a dependency.

    $effect(() => {
    	// this will run when `data` changes, but not when `time` changes
    	save(data, {
    		timestamp: untrack(() => time)
    	});
    });

    function untrack<T>(fn: () => T): T;

## Component

Can be used to create strongly typed Svelte components.

#### Example:

You have component library on npm called `component-library`, from which you export a component called `MyComponent`. For Svelte+TypeScript users, you want to provide typings. Therefore you create a `index.d.ts`:

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

Typing this makes it possible for IDEs like VS Code with the Svelte extension to provide intellisense and to use the component like this in a Svelte file with TypeScript:

    <script lang="ts">
    	import { MyComponent } from "component-library";
    </script>
    <MyComponent foo={'bar'} />

    interface Component<
    	Props extends Record<string, any> = {},
    	Exports extends Record<string, any> = {},
    	Bindings extends keyof Props | '' = string
    > {…}

    (
    	this: void,
    	internals: ComponentInternals,
    	props: Props
    ): {
    	/**
    	 * @deprecated This method only exists when using one of the legacy compatibility helpers, which
    	 * is a stop-gap solution. See [migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
    	 * for more info.
    	 */
    	$on?(type: string, callback: (e: any) => void): () => void;
    	/**
    	 * @deprecated This method only exists when using one of the legacy compatibility helpers, which
    	 * is a stop-gap solution. See [migration guide](https://svelte.dev/docs/svelte/v5-migration-guide#Components-are-no-longer-classes)
    	 * for more info.
    	 */
    	$set?(props: Partial<Props>): void;
    } & Exports;

*   `internal` An internal object used by Svelte. Do not use or modify.
*   `props` The props passed to the component.

    element?: typeof HTMLElement;

The custom element version of the component. Only present if compiled with the `customElement` compiler option

## ComponentConstructorOptions

> In Svelte 4, components are classes. In Svelte 5, they are functions. Use `mount` instead to instantiate components. See migration guide for more info.

    interface ComponentConstructorOptions<
    	Props extends Record<string, any> = Record<string, any>
    > {…}

    target: Element | Document | ShadowRoot;

    anchor?: Element;

    props?: Props;

    context?: Map<any, any>;

    hydrate?: boolean;

    intro?: boolean;

    recover?: boolean;

    sync?: boolean;

    idPrefix?: string;

    $$inline?: boolean;

## ComponentEvents

> The new `Component` type does not have a dedicated Events type. Use `ComponentProps` instead.

    type ComponentEvents<Comp extends SvelteComponent> =
    	Comp extends SvelteComponent<any, infer Events>
    		? Events
    		: never;

## ComponentInternals

Internal implementation details that vary between environments

    type ComponentInternals = Branded<{}, 'ComponentInternals'>;

## ComponentProps

Convenience type to get the props the given component expects.

Example: Ensure a variable contains the props expected by `MyComponent`:

    import type { ComponentProps } from 'svelte';
    import MyComponent from './MyComponent.svelte';
    
    // Errors if these aren't the correct props expected by MyComponent.
    const props: ComponentProps<typeof MyComponent> = { foo: 'bar' };

> In Svelte 4, you would do `ComponentProps<MyComponent>` because `MyComponent` was a class.

Example: A generic function that accepts some component and infers the type of its props:

    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' });

    type ComponentProps<
    	Comp extends SvelteComponent | Component<any, any>
    > =
    	Comp extends SvelteComponent<infer Props>
    		? Props
    		: Comp extends Component<infer Props, any>
    			? Props
    			: never;

## ComponentType

> This type is obsolete when working with the new `Component` type.

    type ComponentType<
    	Comp extends SvelteComponent = SvelteComponent
    > = (new (
    	options: ComponentConstructorOptions<
    		Comp extends SvelteComponent<infer Props>
    			? Props
    			: Record<string, any>
    	>
    ) => Comp) & {
    	/** The custom element version of the component. Only present if compiled with the `customElement` compiler option */
    	element?: typeof HTMLElement;
    };

## EventDispatcher

    interface EventDispatcher<
    	EventMap extends Record<string, any>
    > {…}

    <Type extends keyof EventMap>(
    	...args: null extends EventMap[Type]
    		? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
    		: undefined extends EventMap[Type]
    			? [type: Type, parameter?: EventMap[Type] | null | undefined, options?: DispatchOptions]
    			: [type: Type, parameter: EventMap[Type], options?: DispatchOptions]
    ): boolean;

## MountOptions

Defines the options accepted by the `mount()` function.

    type MountOptions<
    	Props extends Record<string, any> = Record<string, any>
    > = {
    	/**
    	 * Target element where the component will be mounted.
    	 */
    	target: Document | Element | ShadowRoot;
    	/**
    	 * Optional node inside `target`. When specified, it is used to render the component immediately before it.
    	 */
    	anchor?: Node;
    	/**
    	 * Allows the specification of events.
    	 * @deprecated Use callback props instead.
    	 */
    	events?: Record<string, (e: any) => any>;
    	/**
    	 * Can be accessed via `getContext()` at the component level.
    	 */
    	context?: Map<any, any>;
    	/**
    	 * Whether or not to play transitions on initial render.
    	 * @default true
    	 */
    	intro?: boolean;
    } & ({} extends Props
    	? {
    			/**
    			 * Component properties.
    			 */
    			props?: Props;
    		}
    	: {
    			/**
    			 * Component properties.
    			 */
    			props: Props;
    		});

## Snippet

The type of a `#snippet` block. You can use it to (for example) express that your component expects a snippet of a certain type:

    let { banner }: { banner: Snippet<[{ text: string }]> } = $props();

You can only call a snippet through the `{@render ...}` tag.

See the snippet documentation for more info.

    interface Snippet<Parameters extends unknown[] = []> {…}

    (
    	this: void,
    	// this conditional allows tuples but not arrays. Arrays would indicate a
    	// rest parameter type, which is not supported. If rest parameters are added
    	// in the future, the condition can be removed.
    	...args: number extends Parameters['length'] ? never : Parameters
    ): {
    	'{@render ...} must be called with a Snippet': "import type { Snippet } from 'svelte'";
    } & typeof SnippetReturn;

---

## Page: https://svelte.dev/docs/svelte/svelte-action

## Action

Actions are functions that are called when an element is created. You can use this interface to type such actions. The following example defines an action that only works on `<div>` elements and optionally accepts a parameter which it has a default value for:

    export const myAction: Action<HTMLDivElement, { someProperty: boolean } | undefined> = (node, param = { someProperty: true }) => {
    	// ...
    }

`Action<HTMLDivElement>` and `Action<HTMLDivElement, undefined>` both signal that the action accepts no parameters.

You can return an object with methods `update` and `destroy` from the function and type which additional attributes and events it has. See interface `ActionReturn` for more details.

    interface Action<
    	Element = HTMLElement,
    	Parameter = undefined,
    	Attributes extends Record<string, any> = Record<
    		never,
    		any
    	>
    > {…}

    <Node extends Element>(
    	...args: undefined extends Parameter
    		? [node: Node, parameter?: Parameter]
    		: [node: Node, parameter: Parameter]
    ): void | ActionReturn<Parameter, Attributes>;

## ActionReturn

Actions can return an object containing the two properties defined in this interface. Both are optional.

*   update: An action can have a parameter. This method will be called whenever that parameter changes, immediately after Svelte has applied updates to the markup. `ActionReturn` and `ActionReturn<undefined>` both mean that the action accepts no parameters.
*   destroy: Method that is called after the element is unmounted

Additionally, you can specify which additional attributes and events the action enables on the applied element. This applies to TypeScript typings only and has no effect at runtime.

Example usage:

    interface Attributes {
    	newprop?: string;
    	'on:event': (e: CustomEvent<boolean>) => void;
    }
    
    export function myAction(node: HTMLElement, parameter: Parameter): ActionReturn<Parameter, Attributes> {
    	// ...
    	return {
    		update: (updatedParameter) => {...},
    		destroy: () => {...}
    	};
    }

    interface ActionReturn<
    	Parameter = undefined,
    	Attributes extends Record<string, any> = Record<
    		never,
    		any
    	>
    > {…}

    update?: (parameter: Parameter) => void;

    destroy?: () => void;

---

## Page: https://svelte.dev/docs/svelte/svelte-animate

    import { flip } from 'svelte/animate';

## flip

The flip function calculates the start and end position of an element and animates between them, translating the x and y values. `flip` stands for First, Last, Invert, Play.

    function flip(
    	node: Element,
    	{
    		from,
    		to
    	}: {
    		from: DOMRect;
    		to: DOMRect;
    	},
    	params?: FlipParams
    ): AnimationConfig;

## AnimationConfig

    interface AnimationConfig {…}

    delay?: number;

    duration?: number;

    easing?: (t: number) => number;

    css?: (t: number, u: number) => string;

    tick?: (t: number, u: number) => void;

## FlipParams

    interface FlipParams {…}

    delay?: number;

    duration?: number | ((len: number) => number);

    easing?: (t: number) => number;

---

## Page: https://svelte.dev/docs/svelte/svelte-compiler

    import {
    	VERSION,
    	compile,
    	compileModule,
    	migrate,
    	parse,
    	preprocess,
    	walk
    } from 'svelte/compiler';

## VERSION

The current version, as set in package.json.

    const VERSION: string;

## compile

`compile` converts your `.svelte` source code into a JavaScript module that exports a component

    function compile(
    	source: string,
    	options: CompileOptions
    ): CompileResult;

## compileModule

`compileModule` takes your JavaScript source code containing runes, and turns it into a JavaScript module.

    function compileModule(
    	source: string,
    	options: ModuleCompileOptions
    ): CompileResult;

## migrate

Does a best-effort migration of Svelte code towards using runes, event attributes and render tags. May throw an error if the code is too complex to migrate automatically.

    function migrate(
    	source: string,
    	{
    		filename,
    		use_ts
    	}?:
    		| {
    				filename?: string;
    				use_ts?: boolean;
    		  }
    		| undefined
    ): {
    	code: string;
    };

## parse

The parse function parses a component, returning only its abstract syntax tree.

The `modern` option (`false` by default in Svelte 5) makes the parser return a modern AST instead of the legacy AST. `modern` will become `true` by default in Svelte 6, and the option will be removed in Svelte 7.

    function parse(
    	source: string,
    	options: {
    		filename?: string;
    		modern: true;
    		loose?: boolean;
    	}
    ): AST.Root;

    function parse(
    	source: string,
    	options?:
    		| {
    				filename?: string;
    				modern?: false;
    				loose?: boolean;
    		  }
    		| undefined
    ): Record<string, any>;

## preprocess

The preprocess function provides convenient hooks for arbitrarily transforming component source code. For example, it can be used to convert a `<style lang="sass">` block into vanilla CSS.

    function preprocess(
    	source: string,
    	preprocessor: PreprocessorGroup | PreprocessorGroup[],
    	options?:
    		| {
    				filename?: string;
    		  }
    		| undefined
    ): Promise<Processed>;

## walk

> Replace this with `import { walk } from 'estree-walker'`

    function walk(): never;

## AST

    namespace AST {
    	export interface BaseNode {
    		type: string;
    		start: number;
    		end: number;
    	}
    
    	export interface Fragment {
    		type: 'Fragment';
    		nodes: Array<
    			Text | Tag | ElementLike | Block | Comment
    		>;
    	}
    
    	export interface Root extends BaseNode {
    		type: 'Root';
    		/**
    		 * Inline options provided by `<svelte:options>` — these override options passed to `compile(...)`
    		 */
    		options: SvelteOptions | null;
    		fragment: Fragment;
    		/** The parsed `<style>` element, if exists */
    		css: AST.CSS.StyleSheet | null;
    		/** The parsed `<script>` element, if exists */
    		instance: Script | null;
    		/** The parsed `<script module>` element, if exists */
    		module: Script | null;
    	}
    
    	export interface SvelteOptions {
    		// start/end info (needed for warnings and for our Prettier plugin)
    		start: number;
    		end: number;
    		// options
    		runes?: boolean;
    		immutable?: boolean;
    		accessors?: boolean;
    		preserveWhitespace?: boolean;
    		namespace?: Namespace;
    		css?: 'injected';
    		customElement?: {
    			tag?: string;
    			shadow?: 'open' | 'none';
    			props?: Record<
    				string,
    				{
    					attribute?: string;
    					reflect?: boolean;
    					type?:
    						| 'Array'
    						| 'Boolean'
    						| 'Number'
    						| 'Object'
    						| 'String';
    				}
    			>;
    			/**
    			 * Is of type
    			 * ```ts
    			 * (ceClass: new () => HTMLElement) => new () => HTMLElement
    			 * ```
    			 */
    			extend?: ArrowFunctionExpression | Identifier;
    		};
    		attributes: Attribute[];
    	}
    
    	/** Static text */
    	export interface Text extends BaseNode {
    		type: 'Text';
    		/** Text with decoded HTML entities */
    		data: string;
    		/** The original text, with undecoded HTML entities */
    		raw: string;
    	}
    
    	/** A (possibly reactive) template expression — `{...}` */
    	export interface ExpressionTag extends BaseNode {
    		type: 'ExpressionTag';
    		expression: Expression;
    	}
    
    	/** A (possibly reactive) HTML template expression — `{@html ...}` */
    	export interface HtmlTag extends BaseNode {
    		type: 'HtmlTag';
    		expression: Expression;
    	}
    
    	/** An HTML comment */
    	// TODO rename to disambiguate
    	export interface Comment extends BaseNode {
    		type: 'Comment';
    		/** the contents of the comment */
    		data: string;
    	}
    
    	/** A `{@const ...}` tag */
    	export interface ConstTag extends BaseNode {
    		type: 'ConstTag';
    		declaration: VariableDeclaration & {
    			declarations: [
    				VariableDeclarator & {
    					id: Pattern;
    					init: Expression;
    				}
    			];
    		};
    	}
    
    	/** A `{@debug ...}` tag */
    	export interface DebugTag extends BaseNode {
    		type: 'DebugTag';
    		identifiers: Identifier[];
    	}
    
    	/** A `{@render foo(...)} tag */
    	export interface RenderTag extends BaseNode {
    		type: 'RenderTag';
    		expression:
    			| SimpleCallExpression
    			| (ChainExpression & {
    					expression: SimpleCallExpression;
    			  });
    	}
    
    	/** An `animate:` directive */
    	export interface AnimateDirective extends BaseNode {
    		type: 'AnimateDirective';
    		/** The 'x' in `animate:x` */
    		name: string;
    		/** The y in `animate:x={y}` */
    		expression: null | Expression;
    	}
    
    	/** A `bind:` directive */
    	export interface BindDirective extends BaseNode {
    		type: 'BindDirective';
    		/** The 'x' in `bind:x` */
    		name: string;
    		/** The y in `bind:x={y}` */
    		expression:
    			| Identifier
    			| MemberExpression
    			| SequenceExpression;
    	}
    
    	/** A `class:` directive */
    	export interface ClassDirective extends BaseNode {
    		type: 'ClassDirective';
    		/** The 'x' in `class:x` */
    		name: 'class';
    		/** The 'y' in `class:x={y}`, or the `x` in `class:x` */
    		expression: Expression;
    	}
    
    	/** A `let:` directive */
    	export interface LetDirective extends BaseNode {
    		type: 'LetDirective';
    		/** The 'x' in `let:x` */
    		name: string;
    		/** The 'y' in `let:x={y}` */
    		expression:
    			| null
    			| Identifier
    			| ArrayExpression
    			| ObjectExpression;
    	}
    
    	/** An `on:` directive */
    	export interface OnDirective extends BaseNode {
    		type: 'OnDirective';
    		/** The 'x' in `on:x` */
    		name: string;
    		/** The 'y' in `on:x={y}` */
    		expression: null | Expression;
    		modifiers: string[];
    	}
    
    	/** A `style:` directive */
    	export interface StyleDirective extends BaseNode {
    		type: 'StyleDirective';
    		/** The 'x' in `style:x` */
    		name: string;
    		/** The 'y' in `style:x={y}` */
    		value:
    			| true
    			| ExpressionTag
    			| Array<ExpressionTag | Text>;
    		modifiers: Array<'important'>;
    	}
    
    	// TODO have separate in/out/transition directives
    	/** A `transition:`, `in:` or `out:` directive */
    	export interface TransitionDirective extends BaseNode {
    		type: 'TransitionDirective';
    		/** The 'x' in `transition:x` */
    		name: string;
    		/** The 'y' in `transition:x={y}` */
    		expression: null | Expression;
    		modifiers: Array<'local' | 'global'>;
    		/** True if this is a `transition:` or `in:` directive */
    		intro: boolean;
    		/** True if this is a `transition:` or `out:` directive */
    		outro: boolean;
    	}
    
    	/** A `use:` directive */
    	export interface UseDirective extends BaseNode {
    		type: 'UseDirective';
    		/** The 'x' in `use:x` */
    		name: string;
    		/** The 'y' in `use:x={y}` */
    		expression: null | Expression;
    	}
    
    	interface BaseElement extends BaseNode {
    		name: string;
    		attributes: Array<
    			Attribute | SpreadAttribute | Directive
    		>;
    		fragment: Fragment;
    	}
    
    	export interface Component extends BaseElement {
    		type: 'Component';
    	}
    
    	export interface TitleElement extends BaseElement {
    		type: 'TitleElement';
    		name: 'title';
    	}
    
    	export interface SlotElement extends BaseElement {
    		type: 'SlotElement';
    		name: 'slot';
    	}
    
    	export interface RegularElement extends BaseElement {
    		type: 'RegularElement';
    	}
    
    	export interface SvelteBody extends BaseElement {
    		type: 'SvelteBody';
    		name: 'svelte:body';
    	}
    
    	export interface SvelteComponent extends BaseElement {
    		type: 'SvelteComponent';
    		name: 'svelte:component';
    		expression: Expression;
    	}
    
    	export interface SvelteDocument extends BaseElement {
    		type: 'SvelteDocument';
    		name: 'svelte:document';
    	}
    
    	export interface SvelteElement extends BaseElement {
    		type: 'SvelteElement';
    		name: 'svelte:element';
    		tag: Expression;
    	}
    
    	export interface SvelteFragment extends BaseElement {
    		type: 'SvelteFragment';
    		name: 'svelte:fragment';
    	}
    
    	export interface SvelteBoundary extends BaseElement {
    		type: 'SvelteBoundary';
    		name: 'svelte:boundary';
    	}
    
    	export interface SvelteHead extends BaseElement {
    		type: 'SvelteHead';
    		name: 'svelte:head';
    	}
    
    	/** This is only an intermediate representation while parsing, it doesn't exist in the final AST */
    	export interface SvelteOptionsRaw extends BaseElement {
    		type: 'SvelteOptions';
    		name: 'svelte:options';
    	}
    
    	export interface SvelteSelf extends BaseElement {
    		type: 'SvelteSelf';
    		name: 'svelte:self';
    	}
    
    	export interface SvelteWindow extends BaseElement {
    		type: 'SvelteWindow';
    		name: 'svelte:window';
    	}
    
    	/** An `{#each ...}` block */
    	export interface EachBlock extends BaseNode {
    		type: 'EachBlock';
    		expression: Expression;
    		/** The `entry` in `{#each item as entry}`. `null` if `as` part is omitted */
    		context: Pattern | null;
    		body: Fragment;
    		fallback?: Fragment;
    		index?: string;
    		key?: Expression;
    	}
    
    	/** An `{#if ...}` block */
    	export interface IfBlock extends BaseNode {
    		type: 'IfBlock';
    		elseif: boolean;
    		test: Expression;
    		consequent: Fragment;
    		alternate: Fragment | null;
    	}
    
    	/** An `{#await ...}` block */
    	export interface AwaitBlock extends BaseNode {
    		type: 'AwaitBlock';
    		expression: Expression;
    		// TODO can/should we move these inside the ThenBlock and CatchBlock?
    		/** The resolved value inside the `then` block */
    		value: Pattern | null;
    		/** The rejection reason inside the `catch` block */
    		error: Pattern | null;
    		pending: Fragment | null;
    		then: Fragment | null;
    		catch: Fragment | null;
    	}
    
    	export interface KeyBlock extends BaseNode {
    		type: 'KeyBlock';
    		expression: Expression;
    		fragment: Fragment;
    	}
    
    	export interface SnippetBlock extends BaseNode {
    		type: 'SnippetBlock';
    		expression: Identifier;
    		parameters: Pattern[];
    		body: Fragment;
    	}
    
    	export interface Attribute extends BaseNode {
    		type: 'Attribute';
    		name: string;
    		/**
    		 * Quoted/string values are represented by an array, even if they contain a single expression like `"{x}"`
    		 */
    		value:
    			| true
    			| ExpressionTag
    			| Array<Text | ExpressionTag>;
    	}
    
    	export interface SpreadAttribute extends BaseNode {
    		type: 'SpreadAttribute';
    		expression: Expression;
    	}
    
    	export interface Script extends BaseNode {
    		type: 'Script';
    		context: 'default' | 'module';
    		content: Program;
    		attributes: Attribute[];
    	}
    
    	export type AttributeLike =
    		| Attribute
    		| SpreadAttribute
    		| Directive;
    
    	export type Directive =
    		| AST.AnimateDirective
    		| AST.BindDirective
    		| AST.ClassDirective
    		| AST.LetDirective
    		| AST.OnDirective
    		| AST.StyleDirective
    		| AST.TransitionDirective
    		| AST.UseDirective;
    
    	export type Block =
    		| AST.EachBlock
    		| AST.IfBlock
    		| AST.AwaitBlock
    		| AST.KeyBlock
    		| AST.SnippetBlock;
    
    	export type ElementLike =
    		| AST.Component
    		| AST.TitleElement
    		| AST.SlotElement
    		| AST.RegularElement
    		| AST.SvelteBody
    		| AST.SvelteBoundary
    		| AST.SvelteComponent
    		| AST.SvelteDocument
    		| AST.SvelteElement
    		| AST.SvelteFragment
    		| AST.SvelteHead
    		| AST.SvelteOptionsRaw
    		| AST.SvelteSelf
    		| AST.SvelteWindow
    		| AST.SvelteBoundary;
    
    	export type Tag =
    		| AST.ExpressionTag
    		| AST.HtmlTag
    		| AST.ConstTag
    		| AST.DebugTag
    		| AST.RenderTag;
    
    	export type TemplateNode =
    		| AST.Root
    		| AST.Text
    		| Tag
    		| ElementLike
    		| AST.Attribute
    		| AST.SpreadAttribute
    		| Directive
    		| AST.Comment
    		| Block;
    
    	export type SvelteNode =
    		| Node
    		| TemplateNode
    		| AST.Fragment
    		| _CSS.Node;
    
    	export type { _CSS as CSS };
    }

## CompileError

    interface CompileError extends ICompileDiagnostic {}

## CompileOptions

    interface CompileOptions extends ModuleCompileOptions {…}

    name?: string;

Sets the name of the resulting JavaScript class (though the compiler will rename it if it would otherwise conflict with other variables in scope). If unspecified, will be inferred from `filename`

    customElement?: boolean;

*   default `false`

If `true`, tells the compiler to generate a custom element constructor instead of a regular Svelte component.

    accessors?: boolean;

*   default `false`
*   deprecated This will have no effect in runes mode

If `true`, getters and setters will be created for the component’s props. If `false`, they will only be created for readonly exported values (i.e. those declared with `const`, `class` and `function`). If compiling with `customElement: true` this option defaults to `true`.

    namespace?: Namespace;

*   default `'html'`

The namespace of the element; e.g., `"html"`, `"svg"`, `"mathml"`.

    immutable?: boolean;

*   default `false`
*   deprecated This will have no effect in runes mode

If `true`, tells the compiler that you promise not to mutate any objects. This allows it to be less conservative about checking whether values have changed.

    css?: 'injected' | 'external';

*   `'injected'`: styles will be included in the `head` when using `render(...)`, and injected into the document (if not already present) when the component mounts. For components compiled as custom elements, styles are injected to the shadow root.
*   `'external'`: the CSS will only be returned in the `css` field of the compilation result. Most Svelte bundler plugins will set this to `'external'` and use the CSS that is statically generated for better performance, as it will result in smaller JavaScript bundles and the output can be served as cacheable `.css` files. This is always `'injected'` when compiling with `customElement` mode.

    cssHash?: CssHashGetter;

*   default `undefined`

A function that takes a `{ hash, css, name, filename }` argument and returns the string that is used as a classname for scoped CSS. It defaults to returning `svelte-${hash(css)}`.

    preserveComments?: boolean;

*   default `false`

If `true`, your HTML comments will be preserved in the output. By default, they are stripped out.

    preserveWhitespace?: boolean;

*   default `false`

If `true`, whitespace inside and between elements is kept as you typed it, rather than removed or collapsed to a single space where possible.

    runes?: boolean | undefined;

*   default `undefined`

Set to `true` to force the compiler into runes mode, even if there are no indications of runes usage. Set to `false` to force the compiler into ignoring runes, even if there are indications of runes usage. Set to `undefined` (the default) to infer runes mode from the component code. Is always `true` for JS/TS modules compiled with Svelte. Will be `true` by default in Svelte 6. Note that setting this to `true` in your `svelte.config.js` will force runes mode for your entire project, including components in `node_modules`, which is likely not what you want. If you’re using Vite, consider using dynamicCompileOptions instead.

    discloseVersion?: boolean;

*   default `true`

If `true`, exposes the Svelte major version in the browser by adding it to a `Set` stored in the global `window.__svelte.v`.

    compatibility?: {…}

*   deprecated Use these only as a temporary solution before migrating your code

    componentApi?: 4 | 5;

*   default `5`

Applies a transformation so that the default export of Svelte files can still be instantiated the same way as in Svelte 4 — as a class when compiling for the browser (as though using `createClassComponent(MyComponent, {...})` from `svelte/legacy`) or as an object with a `.render(...)` method when compiling for the server

    sourcemap?: object | string;

*   default `null`

An initial sourcemap that will be merged into the final output sourcemap. This is usually the preprocessor sourcemap.

    outputFilename?: string;

*   default `null`

Used for your JavaScript sourcemap.

    cssOutputFilename?: string;

*   default `null`

Used for your CSS sourcemap.

    hmr?: boolean;

*   default `false`

If `true`, compiles components with hot reloading support.

    modernAst?: boolean;

*   default `false`

If `true`, returns the modern version of the AST. Will become `true` by default in Svelte 6, and the option will be removed in Svelte 7.

## CompileResult

The return value of `compile` from `svelte/compiler`

    interface CompileResult {…}

    js: {…}

The compiled JavaScript

    code: string;

The generated code

    map: SourceMap;

A source map

    css: null | {
    	/** The generated code */
    	code: string;
    	/** A source map */
    	map: SourceMap;
    };

The compiled CSS

    warnings: Warning[];

An array of warning objects that were generated during compilation. Each warning has several properties:

*   `code` is a string identifying the category of warning
*   `message` describes the issue in human-readable terms
*   `start` and `end`, if the warning relates to a specific location, are objects with `line`, `column` and `character` properties

    metadata: {…}

Metadata about the compiled component

    runes: boolean;

Whether the file was compiled in runes mode, either because of an explicit option or inferred from usage. For `compileModule`, this is always `true`

    ast: any;

The AST

## MarkupPreprocessor

A markup preprocessor that takes a string of code and returns a processed version.

    type MarkupPreprocessor = (options: {
    	/**
    	 * The whole Svelte file content
    	 */
    	content: string;
    	/**
    	 * The filename of the Svelte file
    	 */
    	filename?: string;
    }) => Processed | void | Promise<Processed | void>;

## ModuleCompileOptions

    interface ModuleCompileOptions {…}

    dev?: boolean;

*   default `false`

If `true`, causes extra code to be added that will perform runtime checks and provide debugging information during development.

    generate?: 'client' | 'server' | false;

*   default `'client'`

If `"client"`, Svelte emits code designed to run in the browser. If `"server"`, Svelte emits code suitable for server-side rendering. If `false`, nothing is generated. Useful for tooling that is only interested in warnings.

    filename?: string;

Used for debugging hints and sourcemaps. Your bundler plugin will set it automatically.

    rootDir?: string;

*   default `process.cwd() on node-like environments, undefined elsewhere`

Used for ensuring filenames don’t leak filesystem information. Your bundler plugin will set it automatically.

    warningFilter?: (warning: Warning) => boolean;

A function that gets a `Warning` as an argument and returns a boolean. Use this to filter out warnings. Return `true` to keep the warning, `false` to discard it.

## Preprocessor

A script/style preprocessor that takes a string of code and returns a processed version.

    type Preprocessor = (options: {
    	/**
    	 * The script/style tag content
    	 */
    	content: string;
    	/**
    	 * The attributes on the script/style tag
    	 */
    	attributes: Record<string, string | boolean>;
    	/**
    	 * The whole Svelte file content
    	 */
    	markup: string;
    	/**
    	 * The filename of the Svelte file
    	 */
    	filename?: string;
    }) => Processed | void | Promise<Processed | void>;

## PreprocessorGroup

A preprocessor group is a set of preprocessors that are applied to a Svelte file.

    interface PreprocessorGroup {…}

    name?: string;

Name of the preprocessor. Will be a required option in the next major version

    markup?: MarkupPreprocessor;

    style?: Preprocessor;

    script?: Preprocessor;

## Processed

The result of a preprocessor run. If the preprocessor does not return a result, it is assumed that the code is unchanged.

    interface Processed {…}

    code: string;

The new code

    map?: string | object;

A source map mapping back to the original code

    dependencies?: string[];

A list of additional files to watch for changes

    attributes?: Record<string, string | boolean>;

Only for script/style preprocessors: The updated attributes to set on the tag. If undefined, attributes stay unchanged.

    toString?: () => string;

## Warning

    interface Warning extends ICompileDiagnostic {}

---

## Page: https://svelte.dev/docs/svelte/svelte-easing

    import {
    	backIn,
    	backInOut,
    	backOut,
    	bounceIn,
    	bounceInOut,
    	bounceOut,
    	circIn,
    	circInOut,
    	circOut,
    	cubicIn,
    	cubicInOut,
    	cubicOut,
    	elasticIn,
    	elasticInOut,
    	elasticOut,
    	expoIn,
    	expoInOut,
    	expoOut,
    	linear,
    	quadIn,
    	quadInOut,
    	quadOut,
    	quartIn,
    	quartInOut,
    	quartOut,
    	quintIn,
    	quintInOut,
    	quintOut,
    	sineIn,
    	sineInOut,
    	sineOut
    } from 'svelte/easing';

## backIn

    function backIn(t: number): number;

## backInOut

    function backInOut(t: number): number;

## backOut

    function backOut(t: number): number;

## bounceIn

    function bounceIn(t: number): number;

## bounceInOut

    function bounceInOut(t: number): number;

## bounceOut

    function bounceOut(t: number): number;

## circIn

    function circIn(t: number): number;

## circInOut

    function circInOut(t: number): number;

## circOut

    function circOut(t: number): number;

## cubicIn

    function cubicIn(t: number): number;

## cubicInOut

    function cubicInOut(t: number): number;

## cubicOut

    function cubicOut(t: number): number;

## elasticIn

    function elasticIn(t: number): number;

## elasticInOut

    function elasticInOut(t: number): number;

## elasticOut

    function elasticOut(t: number): number;

## expoIn

    function expoIn(t: number): number;

## expoInOut

    function expoInOut(t: number): number;

## expoOut

    function expoOut(t: number): number;

## linear

    function linear(t: number): number;

## quadIn

    function quadIn(t: number): number;

## quadInOut

    function quadInOut(t: number): number;

## quadOut

    function quadOut(t: number): number;

## quartIn

    function quartIn(t: number): number;

## quartInOut

    function quartInOut(t: number): number;

## quartOut

    function quartOut(t: number): number;

## quintIn

    function quintIn(t: number): number;

## quintInOut

    function quintInOut(t: number): number;

## quintOut

    function quintOut(t: number): number;

## sineIn

    function sineIn(t: number): number;

## sineInOut

    function sineInOut(t: number): number;

## sineOut

    function sineOut(t: number): number;

---

## Page: https://svelte.dev/docs/svelte/svelte-events

    import { on } from 'svelte/events';

## on

Attaches an event handler to the window and returns a function that removes the handler. Using this rather than `addEventListener` will preserve the correct order relative to handlers added declaratively (with attributes like `onclick`), which use event delegation for performance reasons

    function on<Type extends keyof WindowEventMap>(
    	window: Window,
    	type: Type,
    	handler: (
    		this: Window,
    		event: WindowEventMap[Type]
    	) => any,
    	options?: AddEventListenerOptions | undefined
    ): () => void;

    function on<Type extends keyof DocumentEventMap>(
    	document: Document,
    	type: Type,
    	handler: (
    		this: Document,
    		event: DocumentEventMap[Type]
    	) => any,
    	options?: AddEventListenerOptions | undefined
    ): () => void;

    function on<
    	Element extends HTMLElement,
    	Type extends keyof HTMLElementEventMap
    >(
    	element: Element,
    	type: Type,
    	handler: (
    		this: Element,
    		event: HTMLElementEventMap[Type]
    	) => any,
    	options?: AddEventListenerOptions | undefined
    ): () => void;

    function on<
    	Element extends MediaQueryList,
    	Type extends keyof MediaQueryListEventMap
    >(
    	element: Element,
    	type: Type,
    	handler: (
    		this: Element,
    		event: MediaQueryListEventMap[Type]
    	) => any,
    	options?: AddEventListenerOptions | undefined
    ): () => void;

    function on(
    	element: EventTarget,
    	type: string,
    	handler: EventListener,
    	options?: AddEventListenerOptions | undefined
    ): () => void;

---

## Page: https://svelte.dev/docs/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.

    import {
    	asClassComponent,
    	createBubbler,
    	createClassComponent,
    	run,
    	self,
    	stopImmediatePropagation,
    	stopPropagation,
    	trusted
    } from 'svelte/legacy';

## asClassComponent

> Use this only as a temporary solution to migrate your imperative component code to Svelte 5.

Takes the component function and returns a Svelte 4 compatible component constructor.

    function asClassComponent<
    	Props extends Record<string, any>,
    	Exports extends Record<string, any>,
    	Events extends Record<string, any>,
    	Slots extends Record<string, any>
    >(
    	component:
    		| SvelteComponent<Props, Events, Slots>
    		| Component<Props>
    ): ComponentType<
    	SvelteComponent<Props, Events, Slots> & Exports
    >;

## createBubbler

> Use this only as a temporary solution to migrate your automatically delegated events in Svelte 5.

Function to create a `bubble` function that mimic the behavior of `on:click` without handler available in svelte 4.

    function createBubbler(): (
    	type: string
    ) => (event: Event) => boolean;

## createClassComponent

> Use this only as a temporary solution to migrate your imperative component code to Svelte 5.

Takes the same options as a Svelte 4 component and the component function and returns a Svelte 4 compatible component.

    function createClassComponent<
    	Props extends Record<string, any>,
    	Exports extends Record<string, any>,
    	Events extends Record<string, any>,
    	Slots extends Record<string, any>
    >(
    	options: ComponentConstructorOptions<Props> & {
    		component:
    			| ComponentType<SvelteComponent<Props, Events, Slots>>
    			| Component<Props>;
    	}
    ): SvelteComponent<Props, Events, Slots> & Exports;

## handlers

Function to mimic the multiple listeners available in svelte 4

    function handlers(
    	...handlers: EventListener[]
    ): EventListener;

## nonpassive

Substitute for the `nonpassive` event modifier, implemented as an action

    function nonpassive(
    	node: HTMLElement,
    	[event, handler]: [
    		event: string,
    		handler: () => EventListener
    	]
    ): void;

## once

Substitute for the `once` event modifier

    function once(
    	fn: (event: Event, ...args: Array<unknown>) => void
    ): (event: Event, ...args: unknown[]) => void;

## passive

Substitute for the `passive` event modifier, implemented as an action

    function passive(
    	node: HTMLElement,
    	[event, handler]: [
    		event: string,
    		handler: () => EventListener
    	]
    ): void;

## preventDefault

Substitute for the `preventDefault` event modifier

    function preventDefault(
    	fn: (event: Event, ...args: Array<unknown>) => void
    ): (event: Event, ...args: unknown[]) => void;

## run

> Use this only as a temporary solution to migrate your component code to Svelte 5.

Runs the given function once immediately on the server, and works like `$effect.pre` on the client.

    function run(fn: () => void | (() => void)): void;

## self

Substitute for the `self` event modifier

    function self(
    	fn: (event: Event, ...args: Array<unknown>) => void
    ): (event: Event, ...args: unknown[]) => void;

Substitute for the `stopImmediatePropagation` event modifier

    function stopImmediatePropagation(
    	fn: (event: Event, ...args: Array<unknown>) => void
    ): (event: Event, ...args: unknown[]) => void;

## stopPropagation

Substitute for the `stopPropagation` event modifier

    function stopPropagation(
    	fn: (event: Event, ...args: Array<unknown>) => void
    ): (event: Event, ...args: unknown[]) => void;

## trusted

Substitute for the `trusted` event modifier

    function trusted(
    	fn: (event: Event, ...args: Array<unknown>) => void
    ): (event: Event, ...args: unknown[]) => void;

## LegacyComponentType

Support using the component as both a class and function during the transition period

    type LegacyComponentType = {
    	new (o: ComponentConstructorOptions): SvelteComponent;
    	(
    		...args: Parameters<Component<Record<string, any>>>
    	): ReturnType<
    		Component<Record<string, any>, Record<string, any>>
    	>;
    };

---

## Page: https://svelte.dev/docs/svelte/svelte-motion

    import {
    	Spring,
    	Tween,
    	prefersReducedMotion,
    	spring,
    	tweened
    } from 'svelte/motion';

## Spring

> Available since 5.8.0

A wrapper for a value that behaves in a spring-like fashion. Changes to `spring.target` will cause `spring.current` to move towards it over time, taking account of the `spring.stiffness` and `spring.damping` parameters.

    <script>
    	import { Spring } from 'svelte/motion';
    
    	const spring = new Spring(0);
    </script>
    
    <input type="range" bind:value={spring.target} />
    <input type="range" bind:value={spring.current} disabled />

    class Spring<T> {…}

    constructor(value: T, options?: SpringOpts);

    static of<U>(fn: () => U, options?: SpringOpts): Spring<U>;

Create a spring whose value is bound to the return value of `fn`. This must be called inside an effect root (for example, during component initialisation).

    <script>
    	import { Spring } from 'svelte/motion';
    
    	let { number } = $props();
    
    	const spring = Spring.of(() => number);
    </script>

    set(value: T, options?: SpringUpdateOpts): Promise<void>;

Sets `spring.target` to `value` and returns a `Promise` that resolves if and when `spring.current` catches up to it.

If `options.instant` is `true`, `spring.current` immediately matches `spring.target`.

If `options.preserveMomentum` is provided, the spring will continue on its current trajectory for the specified number of milliseconds. This is useful for things like ‘fling’ gestures.

    damping: number;

    precision: number;

    stiffness: number;

    target: T;

The end value of the spring. This property only exists on the `Spring` class, not the legacy `spring` store.

    get current(): T;

The current value of the spring. This property only exists on the `Spring` class, not the legacy `spring` store.

## Tween

> Available since 5.8.0

A wrapper for a value that tweens smoothly to its target value. Changes to `tween.target` will cause `tween.current` to move towards it over time, taking account of the `delay`, `duration` and `easing` options.

    <script>
    	import { Tween } from 'svelte/motion';
    
    	const tween = new Tween(0);
    </script>
    
    <input type="range" bind:value={tween.target} />
    <input type="range" bind:value={tween.current} disabled />

    class Tween<T> {…}

    static of<U>(fn: () => U, options?: TweenedOptions<U> | undefined): Tween<U>;

Create a tween whose value is bound to the return value of `fn`. This must be called inside an effect root (for example, during component initialisation).

    <script>
    	import { Tween } from 'svelte/motion';
    
    	let { number } = $props();
    
    	const tween = Tween.of(() => number);
    </script>

    constructor(value: T, options?: TweenedOptions<T>);

    set(value: T, options?: TweenedOptions<T> | undefined): Promise<void>;

Sets `tween.target` to `value` and returns a `Promise` that resolves if and when `tween.current` catches up to it.

If `options` are provided, they will override the tween’s defaults.

    get current(): T;

    set target(v: T);

    get target(): T;

## prefersReducedMotion

> Available since 5.7.0

A media query that matches if the user prefers reduced motion.

    <script>
    	import { prefersReducedMotion } from 'svelte/motion';
    	import { fly } from 'svelte/transition';
    
    	let visible = $state(false);
    </script>
    
    <button onclick={() => visible = !visible}>
    	toggle
    </button>
    
    {#if visible}
    	<p transition:fly={{ y: prefersReducedMotion.current ? 0 : 200 }}>
    		flies in, unless the user prefers reduced motion
    	</p>
    {/if}

    const prefersReducedMotion: MediaQuery;

## spring

> Use `Spring` instead

The spring function in Svelte creates a store whose value is animated, with a motion that simulates the behavior of a spring. This means when the value changes, instead of transitioning at a steady rate, it “bounces” like a spring would, depending on the physics parameters provided. This adds a level of realism to the transitions and can enhance the user experience.

    function spring<T = any>(
    	value?: T | undefined,
    	opts?: SpringOpts | undefined
    ): Spring<T>;

## tweened

> Use `Tween` instead

A tweened store in Svelte is a special type of store that provides smooth transitions between state values over time.

    function tweened<T>(
    	value?: T | undefined,
    	defaults?: TweenedOptions<T> | undefined
    ): Tweened<T>;

## Spring

    interface Spring<T> extends Readable<T> {…}

    set(new_value: T, opts?: SpringUpdateOpts): Promise<void>;

    update: (fn: Updater<T>, opts?: SpringUpdateOpts) => Promise<void>;

*   deprecated Only exists on the legacy `spring` store, not the `Spring` class

    subscribe(fn: (value: T) => void): Unsubscriber;

*   deprecated Only exists on the legacy `spring` store, not the `Spring` class

    precision: number;

    damping: number;

    stiffness: number;

## Tweened

    interface Tweened<T> extends Readable<T> {…}

    set(value: T, opts?: TweenedOptions<T>): Promise<void>;

    update(updater: Updater<T>, opts?: TweenedOptions<T>): Promise<void>;

---

## Page: https://svelte.dev/docs/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 and effects) without using `<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>

    import {
    	devicePixelRatio,
    	innerHeight,
    	innerWidth,
    	online,
    	outerHeight,
    	outerWidth,
    	screenLeft,
    	screenTop,
    	scrollX,
    	scrollY
    } from 'svelte/reactivity/window';

## devicePixelRatio

> Available since 5.11.0

`devicePixelRatio.current` is a reactive view of `window.devicePixelRatio`. On the server it is `undefined`. Note that behaviour differs between browsers — on Chrome it will respond to the current zoom level, on Firefox and Safari it won’t.

    const devicePixelRatio: {
    	get current(): number | undefined;
    };

## innerHeight

> Available since 5.11.0

`innerHeight.current` is a reactive view of `window.innerHeight`. On the server it is `undefined`.

    const innerHeight: ReactiveValue<number | undefined>;

## innerWidth

> Available since 5.11.0

`innerWidth.current` is a reactive view of `window.innerWidth`. On the server it is `undefined`.

    const innerWidth: ReactiveValue<number | undefined>;

## online

> Available since 5.11.0

`online.current` is a reactive view of `navigator.onLine`. On the server it is `undefined`.

    const online: ReactiveValue<boolean | undefined>;

## outerHeight

> Available since 5.11.0

`outerHeight.current` is a reactive view of `window.outerHeight`. On the server it is `undefined`.

    const outerHeight: ReactiveValue<number | undefined>;

## outerWidth

> Available since 5.11.0

`outerWidth.current` is a reactive view of `window.outerWidth`. On the server it is `undefined`.

    const outerWidth: ReactiveValue<number | undefined>;

## screenLeft

> Available since 5.11.0

`screenLeft.current` is a reactive view of `window.screenLeft`. It is updated inside a `requestAnimationFrame` callback. On the server it is `undefined`.

    const screenLeft: ReactiveValue<number | undefined>;

## screenTop

> Available since 5.11.0

`screenTop.current` is a reactive view of `window.screenTop`. It is updated inside a `requestAnimationFrame` callback. On the server it is `undefined`.

    const screenTop: ReactiveValue<number | undefined>;

> Available since 5.11.0

`scrollX.current` is a reactive view of `window.scrollX`. On the server it is `undefined`.

    const scrollX: ReactiveValue<number | undefined>;

> Available since 5.11.0

`scrollY.current` is a reactive view of `window.scrollY`. On the server it is `undefined`.

    const scrollY: ReactiveValue<number | undefined>;

---

## Page: https://svelte.dev/docs/svelte/svelte-reactivity

Svelte provides reactive versions of various built-ins like `SvelteMap`, `SvelteSet` and `SvelteURL`. These can be imported from `svelte/reactivity` and used just like their native counterparts.

    <script>
    	import { SvelteURL } from 'svelte/reactivity';
    
    	const url = new SvelteURL('https://example.com/path');
    </script>
    
    <!-- changes to these... -->
    <input bind:value={url.protocol} />
    <input bind:value={url.hostname} />
    <input bind:value={url.pathname} />
    
    <hr />
    
    <!-- will update `href` and vice versa -->
    <input bind:value={url.href} />

    import {
    	MediaQuery,
    	SvelteDate,
    	SvelteMap,
    	SvelteSet,
    	SvelteURL,
    	SvelteURLSearchParams,
    	createSubscriber
    } from 'svelte/reactivity';

> Available since 5.7.0

Creates a media query and provides a `current` property that reflects whether or not it matches.

Use it carefully — during server-side rendering, there is no way to know what the correct value should be, potentially causing content to change upon hydration. If you can use the media query in CSS to achieve the same effect, do that.

    <script>
    	import { MediaQuery } from 'svelte/reactivity';
    
    	const large = new MediaQuery('min-width: 800px');
    </script>
    
    <h1>{large.current ? 'large screen' : 'small screen'}</h1>

    class MediaQuery extends ReactiveValue<boolean> {…}

    constructor(query: string, fallback?: boolean | undefined);

*   `query` A media query string
*   `fallback` Fallback value for the server

## SvelteDate

    class SvelteDate extends Date {…}

    constructor(...params: any[]);

## SvelteMap

    class SvelteMap<K, V> extends Map<K, V> {…}

    constructor(value?: Iterable<readonly [K, V]> | null | undefined);

    set(key: K, value: V): this;

## SvelteSet

    class SvelteSet<T> extends Set<T> {…}

    constructor(value?: Iterable<T> | null | undefined);

    add(value: T): this;

## SvelteURL

    class SvelteURL extends URL {…}

    get searchParams(): SvelteURLSearchParams;

## SvelteURLSearchParams

    class SvelteURLSearchParams extends URLSearchParams {…}

    [REPLACE](params: URLSearchParams): void;

## createSubscriber

> Available since 5.7.0

Returns a `subscribe` function that, if called in an effect (including expressions in the template), calls its `start` callback with an `update` function. Whenever `update` is called, the effect re-runs.

If `start` returns a function, it will be called when the effect is destroyed.

If `subscribe` is called in multiple effects, `start` will only be called once as long as the effects are active, and the returned teardown function will only be called when all effects are destroyed.

It’s best understood with an example. Here’s an implementation of `MediaQuery`:

    import { createSubscriber } from 'svelte/reactivity';
    import { on } from 'svelte/events';
    
    export class MediaQuery {
    	#query;
    	#subscribe;
    
    	constructor(query) {
    		this.#query = window.matchMedia(`(${query})`);
    
    		this.#subscribe = createSubscriber((update) => {
    			// when the `change` event occurs, re-run any effects that read `this.current`
    			const off = on(this.#query, 'change', update);
    
    			// stop listening when all the effects are destroyed
    			return () => off();
    		});
    	}
    
    	get current() {
    		this.#subscribe();
    
    		// Return the current state of the query, whether or not we're in an effect
    		return this.#query.matches;
    	}
    }

    function createSubscriber(
    	start: (update: () => void) => (() => void) | void
    ): () => void;

---

## Page: https://svelte.dev/docs/svelte/svelte-server

    import { render } from 'svelte/server';

## render

Only available on the server and when compiling with the `server` option. Takes a component and returns an object with `body` and `head` properties on it, which you can use to populate the HTML when server-rendering your app.

    function render<
    	Comp extends SvelteComponent<any> | Component<any>,
    	Props extends ComponentProps<Comp> = ComponentProps<Comp>
    >(
    	...args: {} extends Props
    		? [
    				component: Comp extends SvelteComponent<any>
    					? ComponentType<Comp>
    					: Comp,
    				options?: {
    					props?: Omit<Props, '$$slots' | '$$events'>;
    					context?: Map<any, any>;
    					idPrefix?: string;
    				}
    			]
    		: [
    				component: Comp extends SvelteComponent<any>
    					? ComponentType<Comp>
    					: Comp,
    				options: {
    					props: Omit<Props, '$$slots' | '$$events'>;
    					context?: Map<any, any>;
    					idPrefix?: string;
    				}
    			]
    ): RenderOutput;

---

## Page: https://svelte.dev/docs/svelte/svelte-store

    import {
    	derived,
    	fromStore,
    	get,
    	readable,
    	readonly,
    	toStore,
    	writable
    } from 'svelte/store';

## derived

Derived value store by synchronizing one or more readable stores and applying an aggregation function over its input values.

    function derived<S extends Stores, T>(
    	stores: S,
    	fn: (
    		values: StoresValues<S>,
    		set: (value: T) => void,
    		update: (fn: Updater<T>) => void
    	) => Unsubscriber | void,
    	initial_value?: T | undefined
    ): Readable<T>;

    function derived<S extends Stores, T>(
    	stores: S,
    	fn: (values: StoresValues<S>) => T,
    	initial_value?: T | undefined
    ): Readable<T>;

## fromStore

    function fromStore<V>(store: Writable<V>): {
    	current: V;
    };

    function fromStore<V>(store: Readable<V>): {
    	readonly current: V;
    };

## get

Get the current value from a store by subscribing and immediately unsubscribing.

    function get<T>(store: Readable<T>): T;

## readable

Creates a `Readable` store that allows reading by subscription.

    function readable<T>(
    	value?: T | undefined,
    	start?: StartStopNotifier<T> | undefined
    ): Readable<T>;

## readonly

Takes a store and returns a new one derived from the old one that is readable.

    function readonly<T>(store: Readable<T>): Readable<T>;

## toStore

    function toStore<V>(
    	get: () => V,
    	set: (v: V) => void
    ): Writable<V>;

    function toStore<V>(get: () => V): Readable<V>;

## writable

Create a `Writable` store that allows both updating and reading by subscription.

    function writable<T>(
    	value?: T | undefined,
    	start?: StartStopNotifier<T> | undefined
    ): Writable<T>;

## Readable

Readable interface for subscribing.

    interface Readable<T> {…}

    subscribe(this: void, run: Subscriber<T>, invalidate?: () => void): Unsubscriber;

*   `run` subscription callback
*   `invalidate` cleanup callback

Subscribe on value changes.

## StartStopNotifier

Start and stop notification callbacks. This function is called when the first subscriber subscribes.

    type StartStopNotifier<T> = (
    	set: (value: T) => void,
    	update: (fn: Updater<T>) => void
    ) => void | (() => void);

## Subscriber

Callback to inform of a value updates.

    type Subscriber<T> = (value: T) => void;

## Unsubscriber

Unsubscribes from value updates.

    type Unsubscriber = () => void;

## Updater

Callback to update a value.

    type Updater<T> = (value: T) => T;

## Writable

Writable interface for both updating and subscribing.

    interface Writable<T> extends Readable<T> {…}

    set(this: void, value: T): void;

*   `value` to set

Set value and inform subscribers.

    update(this: void, updater: Updater<T>): void;

*   `updater` callback

Update value using callback and inform subscribers.

---

## Page: https://svelte.dev/docs/svelte/svelte-transition

    import {
    	blur,
    	crossfade,
    	draw,
    	fade,
    	fly,
    	scale,
    	slide
    } from 'svelte/transition';

## blur

Animates a `blur` filter alongside an element’s opacity.

    function blur(
    	node: Element,
    	{
    		delay,
    		duration,
    		easing,
    		amount,
    		opacity
    	}?: BlurParams | undefined
    ): TransitionConfig;

## crossfade

The `crossfade` function creates a pair of transitions called `send` and `receive`. When an element is ‘sent’, it looks for a corresponding element being ‘received’, and generates a transition that transforms the element to its counterpart’s position and fades it out. When an element is ‘received’, the reverse happens. If there is no counterpart, the `fallback` transition is used.

    function crossfade({
    	fallback,
    	...defaults
    }: CrossfadeParams & {
    	fallback?: (
    		node: Element,
    		params: CrossfadeParams,
    		intro: boolean
    	) => TransitionConfig;
    }): [
    	(
    		node: any,
    		params: CrossfadeParams & {
    			key: any;
    		}
    	) => () => TransitionConfig,
    	(
    		node: any,
    		params: CrossfadeParams & {
    			key: any;
    		}
    	) => () => TransitionConfig
    ];

## draw

Animates the stroke of an SVG element, like a snake in a tube. `in` transitions begin with the path invisible and draw the path to the screen over time. `out` transitions start in a visible state and gradually erase the path. `draw` only works with elements that have a `getTotalLength` method, like `<path>` and `<polyline>`.

    function draw(
    	node: SVGElement & {
    		getTotalLength(): number;
    	},
    	{
    		delay,
    		speed,
    		duration,
    		easing
    	}?: DrawParams | undefined
    ): TransitionConfig;

## fade

Animates the opacity of an element from 0 to the current opacity for `in` transitions and from the current opacity to 0 for `out` transitions.

    function fade(
    	node: Element,
    	{ delay, duration, easing }?: FadeParams | undefined
    ): TransitionConfig;

## fly

Animates the x and y positions and the opacity of an element. `in` transitions animate from the provided values, passed as parameters to the element’s default values. `out` transitions animate from the element’s default values to the provided values.

    function fly(
    	node: Element,
    	{
    		delay,
    		duration,
    		easing,
    		x,
    		y,
    		opacity
    	}?: FlyParams | undefined
    ): TransitionConfig;

## scale

Animates the opacity and scale of an element. `in` transitions animate from the provided values, passed as parameters, to an element’s current (default) values. `out` transitions animate from an element’s default values to the provided values.

    function scale(
    	node: Element,
    	{
    		delay,
    		duration,
    		easing,
    		start,
    		opacity
    	}?: ScaleParams | undefined
    ): TransitionConfig;

## slide

Slides an element in and out.

    function slide(
    	node: Element,
    	{
    		delay,
    		duration,
    		easing,
    		axis
    	}?: SlideParams | undefined
    ): TransitionConfig;

## BlurParams

    interface BlurParams {…}

    delay?: number;

    duration?: number;

    easing?: EasingFunction;

    amount?: number | string;

    opacity?: number;

## CrossfadeParams

    interface CrossfadeParams {…}

    delay?: number;

    duration?: number | ((len: number) => number);

    easing?: EasingFunction;

## DrawParams

    interface DrawParams {…}

    delay?: number;

    speed?: number;

    duration?: number | ((len: number) => number);

    easing?: EasingFunction;

## EasingFunction

    type EasingFunction = (t: number) => number;

## FadeParams

    interface FadeParams {…}

    delay?: number;

    duration?: number;

    easing?: EasingFunction;

## FlyParams

    interface FlyParams {…}

    delay?: number;

    duration?: number;

    easing?: EasingFunction;

    x?: number | string;

    y?: number | string;

    opacity?: number;

## ScaleParams

    interface ScaleParams {…}

    delay?: number;

    duration?: number;

    easing?: EasingFunction;

    start?: number;

    opacity?: number;

## SlideParams

    interface SlideParams {…}

    delay?: number;

    duration?: number;

    easing?: EasingFunction;

    axis?: 'x' | 'y';

## TransitionConfig

    interface TransitionConfig {…}

    delay?: number;

    duration?: number;

    easing?: EasingFunction;

    css?: (t: number, u: number) => string;

    tick?: (t: number, u: number) => void;

---

## Page: https://svelte.dev/docs/svelte/compiler-errors

### animation\_duplicate

    An element can only have one 'animate' directive

### animation\_invalid\_placement

    An element that uses the `animate:` directive must be the only child of a keyed `{#each ...}` block

### animation\_missing\_key

    An element that uses the `animate:` directive must be the only child of a keyed `{#each ...}` block. Did you forget to add a key to your each block?

### attribute\_contenteditable\_dynamic

    'contenteditable' attribute cannot be dynamic if element uses two-way binding

### attribute\_contenteditable\_missing

    'contenteditable' attribute is required for textContent, innerHTML and innerText two-way bindings

### attribute\_duplicate

    Attributes need to be unique

### attribute\_empty\_shorthand

    Attribute shorthand cannot be empty

### attribute\_invalid\_event\_handler

    Event attribute must be a JavaScript expression, not a string

### attribute\_invalid\_multiple

    'multiple' attribute must be static if select uses two-way binding

### attribute\_invalid\_name

    '%name%' is not a valid attribute name

### attribute\_invalid\_sequence\_expression

    Sequence expressions are not allowed as attribute/directive values in runes mode, unless wrapped in parentheses

### attribute\_invalid\_type

    'type' attribute must be a static text value if input uses two-way binding

### attribute\_unquoted\_sequence

    Attribute values containing `{...}` must be enclosed in quote marks, unless the value only contains the expression

### bind\_group\_invalid\_expression

    `bind:group` can only bind to an Identifier or MemberExpression

### bind\_group\_invalid\_snippet\_parameter

    Cannot `bind:group` to a snippet parameter

### bind\_invalid\_expression

    Can only bind to an Identifier or MemberExpression or a `{get, set}` pair

### bind\_invalid\_name

    `bind:%name%` is not a valid binding

    `bind:%name%` is not a valid binding. %explanation%

### bind\_invalid\_parens

    `bind:%name%={get, set}` must not have surrounding parentheses

### bind\_invalid\_target

    `bind:%name%` can only be used with %elements%

### bind\_invalid\_value

    Can only bind to state or props

### bindable\_invalid\_location

    `$bindable()` can only be used inside a `$props()` declaration

### block\_duplicate\_clause

    %name% cannot appear more than once within a block

### block\_invalid\_continuation\_placement

    {:...} block is invalid at this position (did you forget to close the preceding element or block?)

### block\_invalid\_elseif

    'elseif' should be 'else if'

### block\_invalid\_placement

    {#%name% ...} block cannot be %location%

### block\_unclosed

    Block was left open

### block\_unexpected\_character

    Expected a `%character%` character immediately following the opening bracket

### block\_unexpected\_close

    Unexpected block closing tag

### component\_invalid\_directive

    This type of directive is not valid on components

### const\_tag\_cycle

    Cyclical dependency detected: %cycle%

### const\_tag\_invalid\_expression

    {@const ...} must consist of a single variable declaration

### const\_tag\_invalid\_placement

    `{@const}` must be the immediate child of `{#snippet}`, `{#if}`, `{:else if}`, `{:else}`, `{#each}`, `{:then}`, `{:catch}`, `<svelte:fragment>`, `<svelte:boundary` or `<Component>`

### constant\_assignment

    Cannot assign to %thing%

### constant\_binding

    Cannot bind to %thing%

### css\_empty\_declaration

    Declaration cannot be empty

### css\_expected\_identifier

    Expected a valid CSS identifier

### css\_global\_block\_invalid\_combinator

    A `:global` selector cannot follow a `%name%` combinator

### css\_global\_block\_invalid\_declaration

    A top-level `:global {...}` block can only contain rules, not declarations

### css\_global\_block\_invalid\_list

    A `:global` selector cannot be part of a selector list with more than one item

### css\_global\_block\_invalid\_modifier

    A `:global` selector cannot modify an existing selector

### css\_global\_block\_invalid\_modifier\_start

    A `:global` selector can only be modified if it is a descendant of other selectors

### css\_global\_invalid\_placement

    `:global(...)` can be at the start or end of a selector sequence, but not in the middle

### css\_global\_invalid\_selector

    `:global(...)` must contain exactly one selector

### css\_global\_invalid\_selector\_list

    `:global(...)` must not contain type or universal selectors when used in a compound selector

### css\_nesting\_selector\_invalid\_placement

    Nesting selectors can only be used inside a rule or as the first selector inside a lone `:global(...)`

### css\_selector\_invalid

    Invalid selector

### css\_type\_selector\_invalid\_placement

    `:global(...)` must not be followed by a type selector

### debug\_tag\_invalid\_arguments

    {@debug ...} arguments must be identifiers, not arbitrary expressions

### declaration\_duplicate

    `%name%` has already been declared

### declaration\_duplicate\_module\_import

    Cannot declare a variable with the same name as an import inside `<script module>`

### derived\_invalid\_export

    Cannot export derived state from a module. To expose the current derived value, export a function returning its value

### directive\_invalid\_value

    Directive value must be a JavaScript expression enclosed in curly braces

### directive\_missing\_name

    `%type%` name cannot be empty

### dollar\_binding\_invalid

    The $ name is reserved, and cannot be used for variables and imports

### dollar\_prefix\_invalid

    The $ prefix is reserved, and cannot be used for variables and imports

### each\_item\_invalid\_assignment

    Cannot reassign or bind to each block argument in runes mode. Use the array and index variables instead (e.g. `array[i] = value` instead of `entry = value`, or `bind:value={array[i]}` instead of `bind:value={entry}`)

In legacy mode, it was possible to reassign or bind to the each block argument itself:

    <script>
    	let array = [1, 2, 3];
    </script>
    
    {#each array as entry}
    	<!-- reassignment -->
    	<button on:click={() => entry = 4}>change</button>
    
    	<!-- binding -->
    	<input bind:value={entry}>
    {/each}

This turned out to be buggy and unpredictable, particularly when working with derived values (such as `array.map(...)`), and as such is forbidden in runes mode. You can achieve the same outcome by using the index instead:

    <script>
    	let array = $state([1, 2, 3]);
    </script>
    
    {#each array as entry, i}
    	<!-- reassignment -->
    	<button onclick={() => array[i] = 4}>change</button>
    
    	<!-- binding -->
    	<input bind:value={array[i]}>
    {/each}

### effect\_invalid\_placement

    `$effect()` can only be used as an expression statement

### element\_invalid\_closing\_tag

    `</%name%>` attempted to close an element that was not open

### element\_invalid\_closing\_tag\_autoclosed

    `</%name%>` attempted to close element that was already automatically closed by `<%reason%>` (cannot nest `<%reason%>` inside `<%name%>`)

### element\_unclosed

    `<%name%>` was left open

### event\_handler\_invalid\_component\_modifier

    Event modifiers other than 'once' can only be used on DOM elements

### event\_handler\_invalid\_modifier

    Valid event modifiers are %list%

### event\_handler\_invalid\_modifier\_combination

    The '%modifier1%' and '%modifier2%' modifiers cannot be used together

### expected\_attribute\_value

    Expected attribute value

### expected\_block\_type

    Expected 'if', 'each', 'await', 'key' or 'snippet'

### expected\_identifier

    Expected an identifier

### expected\_pattern

    Expected identifier or destructure pattern

### expected\_token

    Expected token %token%

### expected\_whitespace

    Expected whitespace

### export\_undefined

    `%name%` is not defined

### global\_reference\_invalid

    `%name%` is an illegal variable name. To reference a global variable called `%name%`, use `globalThis.%name%`

### host\_invalid\_placement

    `$host()` can only be used inside custom element component instances

### illegal\_element\_attribute

    `<%name%>` does not support non-event attributes or spread attributes

### import\_svelte\_internal\_forbidden

    Imports of `svelte/internal/*` are forbidden. It contains private runtime code which is subject to change without notice. If you're importing from `svelte/internal/*` to work around a limitation of Svelte, please open an issue at https://github.com/sveltejs/svelte and explain your use case

### inspect\_trace\_generator

    `$inspect.trace(...)` cannot be used inside a generator function

### inspect\_trace\_invalid\_placement

    `$inspect.trace(...)` must be the first statement of a function body

### invalid\_arguments\_usage

    The arguments keyword cannot be used within the template or at the top level of a component

### js\_parse\_error

    %message%

### legacy\_export\_invalid

    Cannot use `export let` in runes mode — use `$props()` instead

### legacy\_props\_invalid

    Cannot use `$$props` in runes mode

### legacy\_reactive\_statement\_invalid

    `$:` is not allowed in runes mode, use `$derived` or `$effect` instead

### legacy\_rest\_props\_invalid

    Cannot use `$$restProps` in runes mode

### let\_directive\_invalid\_placement

    `let:` directive at invalid position

### mixed\_event\_handler\_syntaxes

    Mixing old (on:%name%) and new syntaxes for event handling is not allowed. Use only the on%name% syntax

### module\_illegal\_default\_export

    A component cannot have a default export

### node\_invalid\_placement

    %message%. The browser will 'repair' the HTML (by moving, removing, or inserting elements) which breaks Svelte's assumptions about the structure of your components.

HTML restricts where certain elements can appear. In case of a violation the browser will ‘repair’ the HTML in a way that breaks Svelte’s assumptions about the structure of your components. Some examples:

*   `<p>hello <div>world</div></p>` will result in `<p>hello </p><div>world</div><p></p>` (the `<div>` autoclosed the `<p>` because `<p>` cannot contain block-level elements)
*   `<option><div>option a</div></option>` will result in `<option>option a</option>` (the `<div>` is removed)
*   `<table><tr><td>cell</td></tr></table>` will result in `<table><tbody><tr><td>cell</td></tr></tbody></table>` (a `<tbody>` is auto-inserted)

### options\_invalid\_value

    Invalid compiler option: %details%

### options\_removed

    Invalid compiler option: %details%

### options\_unrecognised

    Unrecognised compiler option %keypath%

### props\_duplicate

    Cannot use `%rune%()` more than once

### props\_id\_invalid\_placement

    `$props.id()` can only be used at the top level of components as a variable declaration initializer

### props\_illegal\_name

    Declaring or accessing a prop starting with `$$` is illegal (they are reserved for Svelte internals)

### props\_invalid\_identifier

    `$props()` can only be used with an object destructuring pattern

### props\_invalid\_pattern

    `$props()` assignment must not contain nested properties or computed keys

### props\_invalid\_placement

    `$props()` can only be used at the top level of components as a variable declaration initializer

### reactive\_declaration\_cycle

    Cyclical dependency detected: %cycle%

### render\_tag\_invalid\_call\_expression

    Calling a snippet function using apply, bind or call is not allowed

### render\_tag\_invalid\_expression

    `{@render ...}` tags can only contain call expressions

### render\_tag\_invalid\_spread\_argument

    cannot use spread arguments in `{@render ...}` tags

### rune\_invalid\_arguments

    `%rune%` cannot be called with arguments

### rune\_invalid\_arguments\_length

    `%rune%` must be called with %args%

### rune\_invalid\_computed\_property

    Cannot access a computed property of a rune

### rune\_invalid\_name

    `%name%` is not a valid rune

### rune\_invalid\_spread

    `%rune%` cannot be called with a spread argument

### rune\_invalid\_usage

    Cannot use `%rune%` rune in non-runes mode

### rune\_missing\_parentheses

    Cannot use rune without parentheses

### rune\_removed

    The `%name%` rune has been removed

### rune\_renamed

    `%name%` is now `%replacement%`

### runes\_mode\_invalid\_import

    %name% cannot be used in runes mode

### script\_duplicate

    A component can have a single top-level `<script>` element and/or a single top-level `<script module>` element

### script\_invalid\_attribute\_value

    If the `%name%` attribute is supplied, it must be a boolean attribute

### script\_invalid\_context

    If the context attribute is supplied, its value must be "module"

### script\_reserved\_attribute

    The `%name%` attribute is reserved and cannot be used

### slot\_attribute\_duplicate

    Duplicate slot name '%name%' in <%component%>

### slot\_attribute\_invalid

    slot attribute must be a static value

### slot\_attribute\_invalid\_placement

    Element with a slot='...' attribute must be a child of a component or a descendant of a custom element

### slot\_default\_duplicate

    Found default slot content alongside an explicit slot="default"

### slot\_element\_invalid\_attribute

    `<slot>` can only receive attributes and (optionally) let directives

### slot\_element\_invalid\_name

    slot attribute must be a static value

### slot\_element\_invalid\_name\_default

    `default` is a reserved word — it cannot be used as a slot name

### slot\_snippet\_conflict

    Cannot use `<slot>` syntax and `{@render ...}` tags in the same component. Migrate towards `{@render ...}` tags completely

### snippet\_conflict

    Cannot use explicit children snippet at the same time as implicit children content. Remove either the non-whitespace content or the children snippet block

### snippet\_invalid\_export

    An exported snippet can only reference things declared in a `<script module>`, or other exportable snippets

It’s possible to export a snippet from a `<script module>` block, but only if it doesn’t reference anything defined inside a non-module-level `<script>`. For example you can’t do this...

    <script module>
    	export { greeting };
    </script>
    
    <script>
    	let message = 'hello';
    </script>
    
    {#snippet greeting(name)}
    	<p>{message} {name}!</p>
    {/snippet}

...because `greeting` references `message`, which is defined in the second `<script>`.

### snippet\_invalid\_rest\_parameter

    Snippets do not support rest parameters; use an array instead

### snippet\_parameter\_assignment

    Cannot reassign or bind to snippet parameter

### snippet\_shadowing\_prop

    This snippet is shadowing the prop `%prop%` with the same name

### state\_invalid\_export

    Cannot export state from a module if it is reassigned. Either export a function returning the state value or only mutate the state value's properties

### state\_invalid\_placement

    `%rune%(...)` can only be used as a variable declaration initializer or a class field

### store\_invalid\_scoped\_subscription

    Cannot subscribe to stores that are not declared at the top level of the component

### store\_invalid\_subscription

    Cannot reference store value inside `<script module>`

### store\_invalid\_subscription\_module

    Cannot reference store value outside a `.svelte` file

Using a `$` prefix to refer to the value of a store is only possible inside `.svelte` files, where Svelte can automatically create subscriptions when a component is mounted and unsubscribe when the component is unmounted. Consider migrating to runes instead.

### style\_directive\_invalid\_modifier

    `style:` directive can only use the `important` modifier

### style\_duplicate

    A component can have a single top-level `<style>` element

### svelte\_body\_illegal\_attribute

    `<svelte:body>` does not support non-event attributes or spread attributes

### svelte\_boundary\_invalid\_attribute

    Valid attributes on `<svelte:boundary>` are `onerror` and `failed`

### svelte\_boundary\_invalid\_attribute\_value

    Attribute value must be a non-string expression

### svelte\_component\_invalid\_this

    Invalid component definition — must be an `{expression}`

### svelte\_component\_missing\_this

    `<svelte:component>` must have a 'this' attribute

### svelte\_element\_missing\_this

    `<svelte:element>` must have a 'this' attribute with a value

### svelte\_fragment\_invalid\_attribute

    `<svelte:fragment>` can only have a slot attribute and (optionally) a let: directive

### svelte\_fragment\_invalid\_placement

    `<svelte:fragment>` must be the direct child of a component

### svelte\_head\_illegal\_attribute

    `<svelte:head>` cannot have attributes nor directives

### svelte\_meta\_duplicate

    A component can only have one `<%name%>` element

### svelte\_meta\_invalid\_content

    <%name%> cannot have children

### svelte\_meta\_invalid\_placement

    `<%name%>` tags cannot be inside elements or blocks

### svelte\_meta\_invalid\_tag

    Valid `<svelte:...>` tag names are %list%

### svelte\_options\_deprecated\_tag

    "tag" option is deprecated — use "customElement" instead

### svelte\_options\_invalid\_attribute

    `<svelte:options>` can only receive static attributes

### svelte\_options\_invalid\_attribute\_value

    Value must be %list%, if specified

### svelte\_options\_invalid\_customelement

    "customElement" must be a string literal defining a valid custom element name or an object of the form { tag?: string; shadow?: "open" | "none"; props?: { [key: string]: { attribute?: string; reflect?: boolean; type: .. } } }

### svelte\_options\_invalid\_customelement\_props

    "props" must be a statically analyzable object literal of the form "{ [key: string]: { attribute?: string; reflect?: boolean; type?: "String" | "Boolean" | "Number" | "Array" | "Object" }"

### svelte\_options\_invalid\_customelement\_shadow

    "shadow" must be either "open" or "none"

### svelte\_options\_invalid\_tagname

    Tag name must be lowercase and hyphenated

See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names

### svelte\_options\_reserved\_tagname

    Tag name is reserved

See https://html.spec.whatwg.org/multipage/custom-elements.html#valid-custom-element-name for more information on valid tag names

### svelte\_options\_unknown\_attribute

    `<svelte:options>` unknown attribute '%name%'

### svelte\_self\_invalid\_placement

    `<svelte:self>` components can only exist inside `{#if}` blocks, `{#each}` blocks, `{#snippet}` blocks or slots passed to components

### tag\_invalid\_name

    Expected a valid element or component name. Components must have a valid variable name or dot notation expression

### tag\_invalid\_placement

    {@%name% ...} tag cannot be %location%

### textarea\_invalid\_content

    A `<textarea>` can have either a value attribute or (equivalently) child content, but not both

### title\_illegal\_attribute

    `<title>` cannot have attributes nor directives

### title\_invalid\_content

    `<title>` can only contain text and {tags}

### transition\_conflict

    Cannot use `%type%:` alongside existing `%existing%:` directive

### transition\_duplicate

    Cannot use multiple `%type%:` directives on a single element

### typescript\_invalid\_feature

    TypeScript language features like %feature% are not natively supported, and their use is generally discouraged. Outside of `<script>` tags, these features are not supported. For use within `<script>` tags, you will need to use a preprocessor to convert it to JavaScript before it gets passed to the Svelte compiler. If you are using `vitePreprocess`, make sure to specifically enable preprocessing script tags (`vitePreprocess({ script: true })`)

### unexpected\_eof

    Unexpected end of input

### unexpected\_reserved\_word

    '%word%' is a reserved word in JavaScript and cannot be used here

### unterminated\_string\_constant

    Unterminated string constant

### void\_element\_invalid\_content

    Void elements cannot have children or closing tags

---

## Page: https://svelte.dev/docs/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>

### a11y\_accesskey

    Avoid using accesskey

Enforce no `accesskey` on element. Access keys are HTML attributes that allow web developers to assign keyboard shortcuts to elements. Inconsistencies between keyboard shortcuts and keyboard commands used by screen reader and keyboard-only users create accessibility complications. To avoid complications, access keys should not be used.

    <!-- A11y: Avoid using accesskey -->
    <div accesskey="z"></div>

### a11y\_aria\_activedescendant\_has\_tabindex

    An element with an aria-activedescendant attribute should have a tabindex value

An element with `aria-activedescendant` must be tabbable, so it must either have an inherent `tabindex` or declare `tabindex` as an attribute.

    <!-- A11y: Elements with attribute aria-activedescendant should have tabindex value -->
    <div aria-activedescendant="some-id"></div>

### a11y\_aria\_attributes

    `<%name%>` should not have aria-* attributes

Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example `meta`, `html`, `script`, `style`. This rule enforces that these DOM elements do not contain the `aria-*` props.

    <!-- A11y: <meta> should not have aria-* attributes -->
    <meta aria-hidden="false" />

### a11y\_autocomplete\_valid

    '%value%' is an invalid value for 'autocomplete' on `<input type="%type%">`

### a11y\_autofocus

    Avoid using autofocus

Enforce that `autofocus` is not used on elements. Autofocusing elements can cause usability issues for sighted and non-sighted users alike.

    <!-- A11y: Avoid using autofocus -->
    <input autofocus />

### a11y\_click\_events\_have\_key\_events

    Visible, non-interactive elements with a click event must be accompanied by a keyboard event handler. Consider whether an interactive element such as `<button type="button">` or `<a>` might be more appropriate

Enforce that visible, non-interactive elements with an `onclick` event are accompanied by a keyboard event handler.

Users should first consider whether an interactive element might be more appropriate such as a `<button type="button">` element for actions or `<a>` element for navigations. These elements are more semantically meaningful and will have built-in key handling. E.g. `Space` and `Enter` will trigger a `<button>` and `Enter` will trigger an `<a>` element.

If a non-interactive element is required then `onclick` should be accompanied by an `onkeyup` or `onkeydown` handler that enables the user to perform equivalent actions via the keyboard. In order for the user to be able to trigger a key press, the element will also need to be focusable by adding a `tabindex`. While an `onkeypress` handler will also silence this warning, it should be noted that the `keypress` event is deprecated.

    <!-- A11y: visible, non-interactive elements with an onclick event must be accompanied by a keyboard event handler. -->
    <div onclick={() => {}}></div>

Coding for the keyboard is important for users with physical disabilities who cannot use a mouse, AT compatibility, and screenreader users.

### a11y\_consider\_explicit\_label

    Buttons and links should either contain text or have an `aria-label` or `aria-labelledby` attribute

### a11y\_distracting\_elements

    Avoid `<%name%>` elements

Enforces that no distracting elements are used. Elements that can be visually distracting can cause accessibility issues with visually impaired users. Such elements are most likely deprecated, and should be avoided.

The following elements are visually distracting: `<marquee>` and `<blink>`.

    <!-- A11y: Avoid <marquee> elements -->
    <marquee></marquee>

### a11y\_figcaption\_index

    `<figcaption>` must be first or last child of `<figure>`

### a11y\_figcaption\_parent

    `<figcaption>` must be an immediate child of `<figure>`

Enforce that certain DOM elements have the correct structure.

    <!-- A11y: <figcaption> must be an immediate child of <figure> -->
    <div>
    	<figcaption>Image caption</figcaption>
    </div>

### a11y\_hidden

    `<%name%>` element should not be hidden

Certain DOM elements are useful for screen reader navigation and should not be hidden.

    <!-- A11y: <h2> element should not be hidden -->
    <h2 aria-hidden="true">invisible header</h2>

### a11y\_img\_redundant\_alt

    Screenreaders already announce `<img>` elements as an image

Enforce img alt attribute does not contain the word image, picture, or photo. Screen readers already announce `img` elements as an image. There is no need to use words such as _image_, _photo_, and/or _picture_.

    <img src="foo" alt="Foo eating a sandwich." />
    
    <!-- aria-hidden, won't be announced by screen reader -->
    <img src="bar" aria-hidden="true" alt="Picture of me taking a photo of an image" />
    
    <!-- A11y: Screen readers already announce <img> elements as an image. -->
    <img src="foo" alt="Photo of foo being weird." />
    
    <!-- A11y: Screen readers already announce <img> elements as an image. -->
    <img src="bar" alt="Image of me at a bar!" />
    
    <!-- A11y: Screen readers already announce <img> elements as an image. -->
    <img src="foo" alt="Picture of baz fixing a bug." />

### a11y\_incorrect\_aria\_attribute\_type

    The value of '%attribute%' must be a %type%

Enforce that only the correct type of value is used for aria attributes. For example, `aria-hidden` should only receive a boolean.

    <!-- A11y: The value of 'aria-hidden' must be exactly one of true or false -->
    <div aria-hidden="yes"></div>

### a11y\_incorrect\_aria\_attribute\_type\_boolean

    The value of '%attribute%' must be either 'true' or 'false'. It cannot be empty

### a11y\_incorrect\_aria\_attribute\_type\_id

    The value of '%attribute%' must be a string that represents a DOM element ID

### a11y\_incorrect\_aria\_attribute\_type\_idlist

    The value of '%attribute%' must be a space-separated list of strings that represent DOM element IDs

### a11y\_incorrect\_aria\_attribute\_type\_integer

    The value of '%attribute%' must be an integer

### a11y\_incorrect\_aria\_attribute\_type\_token

    The value of '%attribute%' must be exactly one of %values%

### a11y\_incorrect\_aria\_attribute\_type\_tokenlist

    The value of '%attribute%' must be a space-separated list of one or more of %values%

### a11y\_incorrect\_aria\_attribute\_type\_tristate

    The value of '%attribute%' must be exactly one of true, false, or mixed

### a11y\_interactive\_supports\_focus

    Elements with the '%role%' interactive role must have a tabindex value

Enforce that elements with an interactive role and interactive handlers (mouse or key press) must be focusable or tabbable.

    <!-- A11y: Elements with the 'button' interactive role must have a tabindex value. -->
    <div role="button" onkeypress={() => {}} />

### a11y\_invalid\_attribute

    '%href_value%' is not a valid %href_attribute% attribute

Enforce that attributes important for accessibility have a valid value. For example, `href` should not be empty, `'#'`, or `javascript:`.

    <!-- A11y: '' is not a valid href attribute -->
    <a href="">invalid</a>

### a11y\_label\_has\_associated\_control

    A form label must be associated with a control

Enforce that a label tag has a text label and an associated control.

There are two supported ways to associate a label with a control:

*   Wrapping a control in a label tag.
*   Adding `for` to a label and assigning it the ID of an input on the page.

    <label for="id">B</label>
    
    <label>C <input type="text" /></label>
    
    <!-- A11y: A form label must be associated with a control. -->
    <label>A</label>

### a11y\_media\_has\_caption

    `<video>` elements must have a `<track kind="captions">`

Providing captions for media is essential for deaf users to follow along. Captions should be a transcription or translation of the dialogue, sound effects, relevant musical cues, and other relevant audio information. Not only is this important for accessibility, but can also be useful for all users in the case that the media is unavailable (similar to `alt` text on an image when an image is unable to load).

The captions should contain all important and relevant information to understand the corresponding media. This may mean that the captions are not a 1:1 mapping of the dialogue in the media content. However, captions are not necessary for video components with the `muted` attribute.

    <video><track kind="captions" /></video>
    
    <audio muted></audio>
    
    <!-- A11y: Media elements must have a <track kind=\"captions\"> -->
    <video></video>
    
    <!-- A11y: Media elements must have a <track kind=\"captions\"> -->
    <video><track /></video>

### a11y\_misplaced\_role

    `<%name%>` should not have role attribute

Certain reserved DOM elements do not support ARIA roles, states and properties. This is often because they are not visible, for example `meta`, `html`, `script`, `style`. This rule enforces that these DOM elements do not contain the `role` props.

    <!-- A11y: <meta> should not have role attribute -->
    <meta role="tooltip" />

### a11y\_misplaced\_scope

    The scope attribute should only be used with `<th>` elements

The scope attribute should only be used on `<th>` elements.

    <!-- A11y: The scope attribute should only be used with <th> elements -->
    <div scope="row" />

### a11y\_missing\_attribute

    `<%name%>` element should have %article% %sequence% attribute

Enforce that attributes required for accessibility are present on an element. This includes the following checks:

*   `<a>` should have an href (unless it’s a fragment-defining tag)
*   `<area>` should have alt, aria-label, or aria-labelledby
*   `<html>` should have lang
*   `<iframe>` should have title
*   `<img>` should have alt
*   `<object>` should have title, aria-label, or aria-labelledby
*   `<input type="image">` should have alt, aria-label, or aria-labelledby

    <!-- A11y: <input type=\"image\"> element should have an alt, aria-label or aria-labelledby attribute -->
    <input type="image" />
    
    <!-- A11y: <html> element should have a lang attribute -->
    <html></html>
    
    <!-- A11y: <a> element should have an href attribute -->
    <a>text</a>

### a11y\_missing\_content

    `<%name%>` element should contain text

Enforce that heading elements (`h1`, `h2`, etc.) and anchors have content and that the content is accessible to screen readers

    <!-- A11y: <a> element should have child content -->
    <a href="/foo"></a>
    
    <!-- A11y: <h1> element should have child content -->
    <h1></h1>

### a11y\_mouse\_events\_have\_key\_events

    '%event%' event must be accompanied by '%accompanied_by%' event

Enforce that `onmouseover` and `onmouseout` are accompanied by `onfocus` and `onblur`, respectively. This helps to ensure that any functionality triggered by these mouse events is also accessible to keyboard users.

    <!-- A11y: onmouseover must be accompanied by onfocus -->
    <div onmouseover={handleMouseover} />
    
    <!-- A11y: onmouseout must be accompanied by onblur -->
    <div onmouseout={handleMouseout} />

### a11y\_no\_abstract\_role

    Abstract role '%role%' is forbidden

### a11y\_no\_interactive\_element\_to\_noninteractive\_role

    `<%element%>` cannot have role '%role%'

WAI-ARIA roles should not be used to convert an interactive element to a non-interactive element. Non-interactive ARIA roles include `article`, `banner`, `complementary`, `img`, `listitem`, `main`, `region` and `tooltip`.

    <!-- A11y: <textarea> cannot have role 'listitem' -->
    <textarea role="listitem"></textarea>

### a11y\_no\_noninteractive\_element\_interactions

    Non-interactive element `<%element%>` should not be assigned mouse or keyboard event listeners

A non-interactive element does not support event handlers (mouse and key handlers). Non-interactive elements include `<main>`, `<area>`, `<h1>` (,`<h2>`, etc), `<p>`, `<img>`, `<li>`, `<ul>` and `<ol>`. Non-interactive WAI-ARIA roles include `article`, `banner`, `complementary`, `img`, `listitem`, `main`, `region` and `tooltip`.

    <!-- `A11y: Non-interactive element <li> should not be assigned mouse or keyboard event listeners.` -->
    <li onclick={() => {}}></li>
    
    <!-- `A11y: Non-interactive element <div> should not be assigned mouse or keyboard event listeners.` -->
    <div role="listitem" onclick={() => {}}></div>

### a11y\_no\_noninteractive\_element\_to\_interactive\_role

    Non-interactive element `<%element%>` cannot have interactive role '%role%'

WAI-ARIA roles should not be used to convert a non-interactive element to an interactive element. Interactive ARIA roles include `button`, `link`, `checkbox`, `menuitem`, `menuitemcheckbox`, `menuitemradio`, `option`, `radio`, `searchbox`, `switch` and `textbox`.

    <!-- A11y: Non-interactive element <h3> cannot have interactive role 'searchbox' -->
    <h3 role="searchbox">Button</h3>

### a11y\_no\_noninteractive\_tabindex

    noninteractive element cannot have nonnegative tabIndex value

Tab key navigation should be limited to elements on the page that can be interacted with.

    <!-- A11y: noninteractive element cannot have nonnegative tabIndex value -->
    <div tabindex="0"></div>

### a11y\_no\_redundant\_roles

    Redundant role '%role%'

Some HTML elements have default ARIA roles. Giving these elements an ARIA role that is already set by the browser has no effect and is redundant.

    <!-- A11y: Redundant role 'button' -->
    <button role="button">...</button>
    
    <!-- A11y: Redundant role 'img' -->
    <img role="img" src="foo.jpg" />

### a11y\_no\_static\_element\_interactions

    `<%element%>` with a %handler% handler must have an ARIA role

Elements like `<div>` with interactive handlers like `click` must have an ARIA role.

    <!-- A11y: <div> with click handler must have an ARIA role -->
    <div onclick={() => ''}></div>

### a11y\_positive\_tabindex

    Avoid tabindex values above zero

Avoid positive `tabindex` property values. This will move elements out of the expected tab order, creating a confusing experience for keyboard users.

    <!-- A11y: avoid tabindex values above zero -->
    <div tabindex="1"></div>

### a11y\_role\_has\_required\_aria\_props

    Elements with the ARIA role "%role%" must have the following attributes defined: %props%

Elements with ARIA roles must have all required attributes for that role.

    <!-- A11y: A11y: Elements with the ARIA role "checkbox" must have the following attributes defined: "aria-checked" -->
    <span role="checkbox" aria-labelledby="foo" tabindex="0"></span>

### a11y\_role\_supports\_aria\_props

    The attribute '%attribute%' is not supported by the role '%role%'

Elements with explicit or implicit roles defined contain only `aria-*` properties supported by that role.

    <!-- A11y: The attribute 'aria-multiline' is not supported by the role 'link'. -->
    <div role="link" aria-multiline></div>
    
    <!-- A11y: The attribute 'aria-required' is not supported by the role 'listitem'. This role is implicit on the element <li>. -->
    <li aria-required></li>

### a11y\_role\_supports\_aria\_props\_implicit

    The attribute '%attribute%' is not supported by the role '%role%'. This role is implicit on the element `<%name%>`

Elements with explicit or implicit roles defined contain only `aria-*` properties supported by that role.

    <!-- A11y: The attribute 'aria-multiline' is not supported by the role 'link'. -->
    <div role="link" aria-multiline></div>
    
    <!-- A11y: The attribute 'aria-required' is not supported by the role 'listitem'. This role is implicit on the element <li>. -->
    <li aria-required></li>

### a11y\_unknown\_aria\_attribute

    Unknown aria attribute 'aria-%attribute%'

    Unknown aria attribute 'aria-%attribute%'. Did you mean '%suggestion%'?

Enforce that only known ARIA attributes are used. This is based on the WAI-ARIA States and Properties spec.

    <!-- A11y: Unknown aria attribute 'aria-labeledby' (did you mean 'labelledby'?) -->
    <input type="image" aria-labeledby="foo" />

### a11y\_unknown\_role

    Unknown role '%role%'

    Unknown role '%role%'. Did you mean '%suggestion%'?

Elements with ARIA roles must use a valid, non-abstract ARIA role. A reference to role definitions can be found at WAI-ARIA site.

    <!-- A11y: Unknown role 'toooltip' (did you mean 'tooltip'?) -->
    <div role="toooltip"></div>

### attribute\_avoid\_is

    The "is" attribute is not supported cross-browser and should be avoided

### attribute\_global\_event\_reference

    You are referencing `globalThis.%name%`. Did you forget to declare a variable with that name?

### attribute\_illegal\_colon

    Attributes should not contain ':' characters to prevent ambiguity with Svelte directives

### attribute\_invalid\_property\_name

    '%wrong%' is not a valid HTML attribute. Did you mean '%right%'?

### attribute\_quoted

    Quoted attributes on components and custom elements will be stringified in a future version of Svelte. If this isn't what you want, remove the quotes

### bind\_invalid\_each\_rest

    The rest operator (...) will create a new object and binding '%name%' with the original object will not work

### block\_empty

    Empty block

### component\_name\_lowercase

    `<%name%>` will be treated as an HTML element unless it begins with a capital letter

### css\_unused\_selector

    Unused CSS selector "%name%"

Svelte traverses both the template and the `<style>` tag to find out which of the CSS selectors are not used within the template, so it can remove them.

In some situations a selector may target an element that is not ‘visible’ to the compiler, for example because it is part of an `{@html ...}` tag or you’re overriding styles in a child component. In these cases, use `:global` to preserve the selector as-is:

    <div class="post">{@html content}</div>
    
    <style>
      .post :global {
    	p {...}
      }
    </style>

### element\_invalid\_self\_closing\_tag

    Self-closing HTML tags for non-void elements are ambiguous — use `<%name% ...></%name%>` rather than `<%name% ... />`

In HTML, there’s no such thing as a self-closing tag. While this _looks_ like a self-contained element with some text next to it...

    <div>
    	<span class="icon" /> some text!
    </div>

...a spec-compliant HTML parser (such as a browser) will in fact parse it like this, with the text _inside_ the icon:

    <div>
    	<span class="icon"> some text! </span>
    </div>

Some templating languages (including Svelte) will ‘fix’ HTML by turning `<span />` into `<span></span>`. Others adhere to the spec. Both result in ambiguity and confusion when copy-pasting code between different contexts, and as such Svelte prompts you to resolve the ambiguity directly by having an explicit closing tag.

To automate this, run the dedicated migration:

    npx sv migrate self-closing-tags

In a future version of Svelte, self-closing tags may be upgraded from a warning to an error.

### event\_directive\_deprecated

    Using `on:%name%` to listen to the %name% event is deprecated. Use the event attribute `on%name%` instead

See the migration guide for more info.

### export\_let\_unused

    Component has unused export property '%name%'. If it is for external reference only, please consider using `export const %name%`

### legacy\_code

    `%code%` is no longer valid — please use `%suggestion%` instead

### legacy\_component\_creation

    Svelte 5 components are no longer classes. Instantiate them using `mount` or `hydrate` (imported from 'svelte') instead.

See the migration guide for more info.

### node\_invalid\_placement\_ssr

    %message%. When rendering this component on the server, the resulting HTML will be modified by the browser (by moving, removing, or inserting elements), likely resulting in a `hydration_mismatch` warning

HTML restricts where certain elements can appear. In case of a violation the browser will ‘repair’ the HTML in a way that breaks Svelte’s assumptions about the structure of your components. Some examples:

*   `<p>hello <div>world</div></p>` will result in `<p>hello </p><div>world</div><p></p>` (the `<div>` autoclosed the `<p>` because `<p>` cannot contain block-level elements)
*   `<option><div>option a</div></option>` will result in `<option>option a</option>` (the `<div>` is removed)
*   `<table><tr><td>cell</td></tr></table>` will result in `<table><tbody><tr><td>cell</td></tr></tbody></table>` (a `<tbody>` is auto-inserted)

This code will work when the component is rendered on the client (which is why this is a warning rather than an error), but if you use server rendering it will cause hydration to fail.

### non\_reactive\_update

    `%name%` is updated, but is not declared with `$state(...)`. Changing its value will not correctly trigger updates

This warning is thrown when the compiler detects the following:

*   a variable was declared without `$state` or `$state.raw`
*   the variable is reassigned
*   the variable is read in a reactive context

In this case, changing the value will not correctly trigger updates. Example:

    <script>
    	let reactive = $state('reactive');
    	let stale = 'stale';
    </script>
    
    <p>This value updates: {reactive}</p>
    <p>This value does not update: {stale}</p>
    
    <button onclick={() => {
    	stale = 'updated';
    	reactive = 'updated';
    }}>update</button>

To fix this, wrap your variable declaration with `$state`.

### options\_deprecated\_accessors

    The `accessors` option has been deprecated. It will have no effect in runes mode

### options\_deprecated\_immutable

    The `immutable` option has been deprecated. It will have no effect in runes mode

### options\_missing\_custom\_element

    The `customElement` option is used when generating a custom element. Did you forget the `customElement: true` compile option?

### options\_removed\_enable\_sourcemap

    The `enableSourcemap` option has been removed. Source maps are always generated now, and tooling can choose to ignore them

### options\_removed\_hydratable

    The `hydratable` option has been removed. Svelte components are always hydratable now

### options\_removed\_loop\_guard\_timeout

    The `loopGuardTimeout` option has been removed

### options\_renamed\_ssr\_dom

    `generate: "dom"` and `generate: "ssr"` options have been renamed to "client" and "server" respectively

### perf\_avoid\_inline\_class

    Avoid 'new class' — instead, declare the class at the top level scope

### perf\_avoid\_nested\_class

    Avoid declaring classes below the top level scope

### reactive\_declaration\_invalid\_placement

    Reactive declarations only exist at the top level of the instance script

### reactive\_declaration\_module\_script\_dependency

    Reassignments of module-level declarations will not cause reactive statements to update

### script\_context\_deprecated

    `context="module"` is deprecated, use the `module` attribute instead

    <script context="module" module>
    	let foo = 'bar';
    </script>

### script\_unknown\_attribute

    Unrecognized attribute — should be one of `generics`, `lang` or `module`. If this exists for a preprocessor, ensure that the preprocessor removes it

### slot\_element\_deprecated

    Using `<slot>` to render parent content is deprecated. Use `{@render ...}` tags instead

See the migration guide for more info.

### state\_referenced\_locally

    State referenced in its own scope will never update. Did you mean to reference it inside a closure?

This warning is thrown when the compiler detects the following:

*   A reactive variable is declared
*   the variable is reassigned
*   the variable is referenced inside the same scope it is declared and it is a non-reactive context

In this case, the state reassignment will not be noticed by whatever you passed it to. For example, if you pass the state to a function, that function will not notice the updates:

Parent

    <script>
    	import { setContext } from 'svelte';
    
    	let count = $state(0);
    
    	// warning: state_referenced_locally
    	setContext('count', count);
    </script>
    
    <button onclick={() => count++}>
    	increment
    </button>

    <script lang="ts">
    	import { setContext } from 'svelte';
    
    	let count = $state(0);
    
    	// warning: state_referenced_locally
    	setContext('count', count);
    </script>
    
    <button onclick={() => count++}>
    	increment
    </button>

Child

    <script>
    	import { getContext } from 'svelte';
    
    	const count = getContext('count');
    </script>
    
    <!-- This will never update -->
    <p>The count is {count}</p>

    <script lang="ts">
    	import { getContext } from 'svelte';
    
    	const count = getContext('count');
    </script>
    
    <!-- This will never update -->
    <p>The count is {count}</p>

To fix this, reference the variable such that it is lazily evaluated. For the above example, this can be achieved by wrapping `count` in a function:

Parent

    <script>
    	import { setContext } from 'svelte';
    
    	let count = $state(0);
    	setContext('count', () => count);
    </script>
    
    <button onclick={() => count++}>
    	increment
    </button>

    <script lang="ts">
    	import { setContext } from 'svelte';
    
    	let count = $state(0);
    	setContext('count', () => count);
    </script>
    
    <button onclick={() => count++}>
    	increment
    </button>

Child

    <script>
    	import { getContext } from 'svelte';
    
    	const count = getContext('count');
    </script>
    
    <!-- This will update -->
    <p>The count is {count()}</p>

    <script lang="ts">
    	import { getContext } from 'svelte';
    
    	const count = getContext('count');
    </script>
    
    <!-- This will update -->
    <p>The count is {count()}</p>

For more info, see Passing state into functions.

### store\_rune\_conflict

    It looks like you're using the `$%name%` rune, but there is a local binding called `%name%`. Referencing a local variable with a `$` prefix will create a store subscription. Please rename `%name%` to avoid the ambiguity

### svelte\_component\_deprecated

    `<svelte:component>` is deprecated in runes mode — components are dynamic by default

In previous versions of Svelte, the component constructor was fixed when the component was rendered. In other words, if you wanted `<X>` to re-render when `X` changed, you would either have to use `<svelte:component this={X}>` or put the component inside a `{#key X}...{/key}` block.

In Svelte 5 this is no longer true — if `X` changes, `<X>` re-renders.

In some cases `<object.property>` syntax can be used as a replacement; a lowercased variable with property access is recognized as a component in Svelte 5.

For complex component resolution logic, an intermediary, capitalized variable may be necessary. E.g. in places where `@const` can be used:

    {#each items as item}
    	<svelte:component this={item.condition ? Y : Z} />
    	{@const Component = item.condition ? Y : Z}
    	<Component />
    {/each}

A derived value may be used in other contexts:

    <script>
    	// ...
    	let condition = $state(false);
    	const Component = $derived(condition ? Y : Z);
    </script>
    
    <svelte:component this={condition ? Y : Z} />
    <Component />

### svelte\_element\_invalid\_this

    `this` should be an `{expression}`. Using a string attribute value will cause an error in future versions of Svelte

### svelte\_self\_deprecated

    `<svelte:self>` is deprecated — use self-imports (e.g. `import %name% from './%basename%'`) instead

See the note in the docs for more info.

### unknown\_code

    `%code%` is not a recognised code

    `%code%` is not a recognised code (did you mean `%suggestion%`?)

---

## Page: https://svelte.dev/docs/svelte/runtime-errors

## Client errors

### bind\_invalid\_checkbox\_value

    Using `bind:value` together with a checkbox input is not allowed. Use `bind:checked` instead

### bind\_invalid\_export

    Component %component% has an export named `%key%` that a consumer component is trying to access using `bind:%key%`, which is disallowed. Instead, use `bind:this` (e.g. `<%name% bind:this={component} />`) and then access the property on the bound component instance (e.g. `component.%key%`)

### bind\_not\_bindable

    A component is attempting to bind to a non-bindable property `%key%` belonging to %component% (i.e. `<%name% bind:%key%={...}>`). To mark a property as bindable: `let { %key% = $bindable() } = $props()`

### component\_api\_changed

    %parent% called `%method%` on an instance of %component%, which is no longer valid in Svelte 5

See the migration guide for more information.

### component\_api\_invalid\_new

    Attempted to instantiate %component% with `new %name%`, which is no longer valid in Svelte 5. If this component is not under your control, set the `compatibility.componentApi` compiler option to `4` to keep it working.

See the migration guide for more information.

### derived\_references\_self

    A derived value cannot reference itself recursively

### each\_key\_duplicate

    Keyed each block has duplicate key at indexes %a% and %b%

    Keyed each block has duplicate key `%value%` at indexes %a% and %b%

### effect\_in\_teardown

    `%rune%` cannot be used inside an effect cleanup function

### effect\_in\_unowned\_derived

    Effect cannot be created inside a `$derived` value that was not itself created inside an effect

### effect\_orphan

    `%rune%` can only be used inside an effect (e.g. during component initialisation)

### effect\_update\_depth\_exceeded

    Maximum update depth exceeded. This can happen when a reactive block or effect repeatedly sets a new value. Svelte limits the number of nested updates to prevent infinite loops

### hydration\_failed

    Failed to hydrate the application

### invalid\_snippet

    Could not `{@render}` snippet due to the expression being `null` or `undefined`. Consider using optional chaining `{@render snippet?.()}`

### lifecycle\_legacy\_only

    `%name%(...)` cannot be used in runes mode

### props\_invalid\_value

    Cannot do `bind:%key%={undefined}` when `%key%` has a fallback value

### props\_rest\_readonly

    Rest element properties of `$props()` such as `%property%` are readonly

### rune\_outside\_svelte

    The `%rune%` rune is only available inside `.svelte` and `.svelte.js/ts` files

### state\_descriptors\_fixed

    Property descriptors defined on `$state` objects must contain `value` and always be `enumerable`, `configurable` and `writable`.

### state\_prototype\_fixed

    Cannot set prototype of `$state` object

### state\_unsafe\_mutation

    Updating state inside a derived or a template expression is forbidden. If the value should not be reactive, declare it without `$state`

This error occurs when state is updated while evaluating a `$derived`. You might encounter it while trying to ‘derive’ two pieces of state in one go:

    <script>
    	let count = $state(0);
    
    	let even = $state(true);
    
    	let odd = $derived.by(() => {
    		even = count % 2 === 0;
    		return !even;
    	});
    </script>
    
    <button onclick={() => count++}>{count}</button>
    
    <p>{count} is even: {even}</p>
    <p>{count} is odd: {odd}</p>

This is forbidden because it introduces instability: if `<p>{count} is even: {even}</p>` is updated before `odd` is recalculated, `even` will be stale. In most cases the solution is to make everything derived:

    let even = $derived(count % 2 === 0);
    let odd = $derived(!even);

If side-effects are unavoidable, use `$effect` instead.

## Server errors

### lifecycle\_function\_unavailable

    `%name%(...)` is not available on the server

Certain methods such as `mount` cannot be invoked while running in a server context. Avoid calling them eagerly, i.e. not during render.

### invalid\_default\_snippet

    Cannot use `{@render children(...)}` if the parent component uses `let:` directives. Consider using a named snippet instead

This error would be thrown in a setup like this:

Parent

    <List {items} let:entry>
    	<span>{entry}</span>
    </List>

List

    <script>
    	let { items, children } = $props();
    </script>
    
    <ul>
    	{#each items as item}
    		<li>{@render children(item)}</li>
    	{/each}
    </ul>

    <script lang="ts">
    	let { items, children } = $props();
    </script>
    
    <ul>
    	{#each items as item}
    		<li>{@render children(item)}</li>
    	{/each}
    </ul>

Here, `List.svelte` is using `{@render children(item)` which means it expects `Parent.svelte` to use snippets. Instead, `Parent.svelte` uses the deprecated `let:` directive. This combination of APIs is incompatible, hence the error.

### lifecycle\_outside\_component

    `%name%(...)` can only be used during component initialisation

Certain lifecycle methods can only be used during component initialisation. To fix this, make sure you’re invoking the method inside the _top level of the instance script_ of your component.

    <script>
    	import { onMount } from 'svelte';
    
    	function handleClick() {
    		// This is wrong
    		onMount(() => {})
    	}
    
    	// This is correct
    	onMount(() => {})
    </script>
    
    <button onclick={handleClick}>click me</button>

### store\_invalid\_shape

    `%name%` is not a store with a `subscribe` method

### svelte\_element\_invalid\_this\_value

    The `this` prop on `<svelte:element>` must be a string, if defined

---

## Page: https://svelte.dev/docs/svelte/runtime-warnings

## Client warnings

### assignment\_value\_stale

    Assignment to `%property%` property (%location%) will evaluate to the right-hand side, not the value of `%property%` following the assignment. This may result in unexpected behaviour.

Given a case like this...

    <script>
    	let object = $state({ array: null });
    
    	function add() {
    		(object.array ??= []).push(object.array.length);
    	}
    </script>
    
    <button onclick={add}>add</button>
    <p>items: {JSON.stringify(object.items)}</p>

...the array being pushed to when the button is first clicked is the `[]` on the right-hand side of the assignment, but the resulting value of `object.array` is an empty state proxy. As a result, the pushed value will be discarded.

You can fix this by separating it into two statements:

    function add() {
    	object.array ??= [];
    	object.array.push(object.array.length);
    }

### binding\_property\_non\_reactive

    `%binding%` is binding to a non-reactive property

    `%binding%` (%location%) is binding to a non-reactive property

### console\_log\_state

    Your `console.%method%` contained `$state` proxies. Consider using `$inspect(...)` or `$state.snapshot(...)` instead

When logging a proxy, browser devtools will log the proxy itself rather than the value it represents. In the case of Svelte, the ‘target’ of a `$state` proxy might not resemble its current value, which can be confusing.

The easiest way to log a value as it changes over time is to use the `$inspect` rune. Alternatively, to log things on a one-off basis (for example, inside an event handler) you can use `$state.snapshot` to take a snapshot of the current value.

### event\_handler\_invalid

    %handler% should be a function. Did you mean to %suggestion%?

### hydration\_attribute\_changed

    The `%attribute%` attribute on `%html%` changed its value between server and client renders. The client value, `%value%`, will be ignored in favour of the server value

Certain attributes like `src` on an `<img>` element will not be repaired during hydration, i.e. the server value will be kept. That’s because updating these attributes can cause the image to be refetched (or in the case of an `<iframe>`, for the frame to be reloaded), even if they resolve to the same resource.

To fix this, either silence the warning with a `svelte-ignore` comment, or ensure that the value stays the same between server and client. If you really need the value to change on hydration, you can force an update like this:

    <script>
    	let { src } = $props();
    
    	if (typeof window !== 'undefined') {
    		// stash the value...
    		const initial = src;
    
    		// unset it...
    		src = undefined;
    
    		$effect(() => {
    			// ...and reset after we've mounted
    			src = initial;
    		});
    	}
    </script>
    
    <img {src} />

### hydration\_html\_changed

    The value of an `{@html ...}` block changed between server and client renders. The client value will be ignored in favour of the server value

    The value of an `{@html ...}` block %location% changed between server and client renders. The client value will be ignored in favour of the server value

If the `{@html ...}` value changes between the server and the client, it will not be repaired during hydration, i.e. the server value will be kept. That’s because change detection during hydration is expensive and usually unnecessary.

To fix this, either silence the warning with a `svelte-ignore` comment, or ensure that the value stays the same between server and client. If you really need the value to change on hydration, you can force an update like this:

    <script>
    	let { markup } = $props();
    
    	if (typeof window !== 'undefined') {
    		// stash the value...
    		const initial = markup;
    
    		// unset it...
    		markup = undefined;
    
    		$effect(() => {
    			// ...and reset after we've mounted
    			markup = initial;
    		});
    	}
    </script>
    
    {@html markup}

### hydration\_mismatch

    Hydration failed because the initial UI does not match what was rendered on the server

    Hydration failed because the initial UI does not match what was rendered on the server. The error occurred near %location%

This warning is thrown when Svelte encounters an error while hydrating the HTML from the server. During hydration, Svelte walks the DOM, expecting a certain structure. If that structure is different (for example because the HTML was repaired by the DOM because of invalid HTML), then Svelte will run into issues, resulting in this warning.

During development, this error is often preceeded by a `console.error` detailing the offending HTML, which needs fixing.

### invalid\_raw\_snippet\_render

    The `render` function passed to `createRawSnippet` should return HTML for a single element

### legacy\_recursive\_reactive\_block

    Detected a migrated `$:` reactive block in `%filename%` that both accesses and updates the same reactive value. This may cause recursive updates when converted to an `$effect`.

### lifecycle\_double\_unmount

    Tried to unmount a component that was not mounted

### ownership\_invalid\_binding

    %parent% passed a value to %child% with `bind:`, but the value is owned by %owner%. Consider creating a binding between %owner% and %parent%

Consider three components `GrandParent`, `Parent` and `Child`. If you do `<GrandParent bind:value>`, inside `GrandParent` pass on the variable via `<Parent {value} />` (note the missing `bind:`) and then do `<Child bind:value>` inside `Parent`, this warning is thrown.

To fix it, `bind:` to the value instead of just passing a property (i.e. in this example do `<Parent bind:value />`).

### ownership\_invalid\_mutation

    Mutating a value outside the component that created it is strongly discouraged. Consider passing values to child components with `bind:`, or use a callback instead

    %component% mutated a value owned by %owner%. This is strongly discouraged. Consider passing values to child components with `bind:`, or use a callback instead

Consider the following code:

App

    <script>
    	import Child from './Child.svelte';
    	let person = $state({ name: 'Florida', surname: 'Man' });
    </script>
    
    <Child {person} />

    <script lang="ts">
    	import Child from './Child.svelte';
    	let person = $state({ name: 'Florida', surname: 'Man' });
    </script>
    
    <Child {person} />

Child

    <script>
    	let { person } = $props();
    </script>
    
    <input bind:value={person.name}>
    <input bind:value={person.surname}>

    <script lang="ts">
    	let { person } = $props();
    </script>
    
    <input bind:value={person.name}>
    <input bind:value={person.surname}>

`Child` is mutating `person` which is owned by `App` without being explicitly “allowed” to do so. This is strongly discouraged since it can create code that is hard to reason about at scale (“who mutated this value?”), hence the warning.

To fix it, either create callback props to communicate changes, or mark `person` as `$bindable`.

### state\_proxy\_equality\_mismatch

    Reactive `$state(...)` proxies and the values they proxy have different identities. Because of this, comparisons with `%operator%` will produce unexpected results

`$state(...)` creates a proxy of the value it is passed. The proxy and the value have different identities, meaning equality checks will always return `false`:

    <script>
    	let value = { foo: 'bar' };
    	let proxy = $state(value);
    
    	value === proxy; // always false
    </script>

To resolve this, ensure you’re comparing values where both values were created with `$state(...)`, or neither were. Note that `$state.raw(...)` will _not_ create a state proxy.

### transition\_slide\_display

    The `slide` transition does not work correctly for elements with `display: %value%`

The slide transition works by animating the `height` of the element, which requires a `display` style like `block`, `flex` or `grid`. It does not work for:

*   `display: inline` (which is the default for elements like `<span>`), and its variants like `inline-block`, `inline-flex` and `inline-grid`
*   `display: table` and `table-[name]`, which are the defaults for elements like `<table>` and `<tr>`
*   `display: contents`

### dynamic\_void\_element\_content

    `<svelte:element this="%tag%">` is a void element — it cannot have content

Elements such as `<input>` cannot have content, any children passed to these elements will be ignored.

### state\_snapshot\_uncloneable

    Value cannot be cloned with `$state.snapshot` — the original value was returned

    The following properties cannot be cloned with `$state.snapshot` — the return value contains the originals:
    
    %properties%

`$state.snapshot` tries to clone the given value in order to return a reference that no longer changes. Certain objects may not be cloneable, in which case the original value is returned. In the following example, `property` is cloned, but `window` is not, because DOM elements are uncloneable:

    const object = $state({ property: 'this is cloneable', window })
    const snapshot = $state.snapshot(object);

---

## Page: https://svelte.dev/docs/svelte/legacy-overview

Svelte 5 introduced some significant changes to Svelte’s API, including runes, snippets 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.

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.

---

## Page: https://svelte.dev/docs/svelte/legacy-let

In runes mode, reactive state is explicitly declared with the `$state` rune.

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>

---

## Page: https://svelte.dev/docs/svelte/legacy-reactive-assignments

In runes mode, reactions to state updates are handled with the `$derived` and `$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. 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:

    $: {
    	// 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:

    $: ({ 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:

    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:

    $: if (browser) {
    	document.title = title;
    }

---

## Page: https://svelte.dev/docs/svelte/legacy-export-let

In runes mode, component props are declared with the `$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.

> 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:

Greeter

    <script>
    	export function greet(name) {
    		alert(`hello ${name}!`);
    	}
    </script>

    <script lang="ts">
    	export function greet(name) {
    		alert(`hello ${name}!`);
    	}
    </script>

App

    <script>
    	import Greeter from './Greeter.svelte';
    
    	let greeter;
    </script>
    
    <Greeter bind:this={greeter} />
    
    <button on:click={() => greeter.greet('world')}>
    	greet
    </button>

    <script lang="ts">
    	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:

App

    <script>
    	/** @type {string} */
    	let className;
    
    	// creates a `class` property, even
    	// though it is a reserved word
    	export { className as class };
    </script>

    <script lang="ts">
    	let className: string;
    
    	// creates a `class` property, even
    	// though it is a reserved word
    	export { className as class };
    </script>

---

## Page: https://svelte.dev/docs/svelte/legacy-$$props-and-$$restProps

In runes mode, getting an object containing all the props that were passed in is easy, using the `$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.

---

## Page: https://svelte.dev/docs/svelte/legacy-on

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

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

App

    <script>
    	let count = 0;
    
    	/** @param {MouseEvent} event */
    	function handleClick(event) {
    		count += 1;
    	}
    </script>
    
    <button on:click={handleClick}>
    	count: {count}
    </button>

    <script lang="ts">
    	let count = 0;
    	function handleClick(event: MouseEvent) {
    		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:

App

    <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>

    <script lang="ts">
    	let count = 0;
    
    	function increment() {
    		count += 1;
    	}
    	function log(event: MouseEvent) {
    		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:

Stepper

    <script>
    	import { createEventDispatcher } from 'svelte';
    	const dispatch = createEventDispatcher();
    </script>
    
    <button on:click={() => dispatch('decrement')}>decrement</button>
    <button on:click={() => dispatch('increment')}>increment</button>

    <script lang="ts">
    	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`. 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.

> If you’re planning an eventual migration to Svelte 5, use callback props instead. This will make upgrading easier as `createEventDispatcher` is deprecated:
> 
> Stepper
> 
>     <script>
>     	export let decrement;
>     	export let increment;
>     </script>
>     
>     <button on:click={decrement}>decrement</button>
>     <button on:click={increment}>increment</button>
> 
>     <script lang="ts">
>     	export let decrement;
>     	export let increment;
>     </script>
>     
>     <button on:click={decrement}>decrement</button>
>     <button on:click={increment}>increment</button>

---

## Page: https://svelte.dev/docs/svelte/legacy-slots

In Svelte 5, content can be passed to components in the form of snippets and rendered using render tags.

In legacy mode, content inside component tags is considered _slotted content_, which can be rendered by the component using a `<slot>` element:

App

    <script>
    	import Modal from './Modal.svelte';
    </script>
    
    <Modal>This is some slotted content</Modal>

    <script lang="ts">
    	import Modal from './Modal.svelte';
    </script>
    
    <Modal>This is some slotted content</Modal>

Modal

    <div class="modal">
    	<slot></slot>
    </div>

> If you want to render a regular `<slot>` element, you can use `<svelte:element this={'slot'} />`.

## Named slots

A component can have _named_ slots in addition to the default slot. On the parent side, add a `slot="..."` attribute to an element, component or `<svelte:fragment>` directly inside the component tags.

App

    <script>
    	import Modal from './Modal.svelte';
    
    	let open = true;
    </script>
    
    {#if open}
    	<Modal>
    		This is some slotted content
    
    		<div slot="buttons">
    			<button on:click={() => open = false}>
    				close
    			</button>
    		</div>
    	</Modal>
    {/if}

    <script lang="ts">
    	import Modal from './Modal.svelte';
    
    	let open = true;
    </script>
    
    {#if open}
    	<Modal>
    		This is some slotted content
    
    		<div slot="buttons">
    			<button on:click={() => open = false}>
    				close
    			</button>
    		</div>
    	</Modal>
    {/if}

On the child side, add a corresponding `<slot name="...">` element:

Modal

    <div class="modal">
    	<slot></slot>
    	<hr>
    	<slot name="buttons"></slot>
    </div>

## Fallback content

If no slotted content is provided, a component can define fallback content by putting it inside the `<slot>` element:

    <slot>
    	This will be rendered if no slotted content is provided
    </slot>

## Passing data to slotted content

Slots can be rendered zero or more times and can pass values _back_ to the parent using props. The parent exposes the values to the slot template using the `let:` directive.

FancyList

    <ul>
    	{#each items as data}
    		<li class="fancy">
    			<!-- 'item' here... -->
    			<slot item={process(data)} />
    		</li>
    	{/each}
    </ul>

App

    <!-- ...corresponds to 'item' here: -->
    <FancyList {items} let:item={processed}>
    	<div>{processed.text}</div>
    </FancyList>

The usual shorthand rules apply — `let:item` is equivalent to `let:item={item}`, and `<slot {item}>` is equivalent to `<slot item={item}>`.

Named slots can also expose values. The `let:` directive goes on the element with the `slot` attribute.

FancyList

    <ul>
    	{#each items as item}
    		<li class="fancy">
    			<slot name="item" item={process(data)} />
    		</li>
    	{/each}
    </ul>
    
    <slot name="footer" />

App

    <FancyList {items}>
    	<div slot="item" let:item>{item.text}</div>
    	<p slot="footer">Copyright (c) 2019 Svelte Industries</p>
    </FancyList>

---

## Page: https://svelte.dev/docs/svelte/legacy-$$slots

In runes mode, we know which snippets 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.

Card

    <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>

App

    <Card>
    	<h1 slot="title">Blog Post Title</h1>
    	<!-- No slot named "description" was provided so the optional slot will not be rendered. -->
    </Card>

---

## Page: https://svelte.dev/docs/svelte/legacy-svelte-fragment

The `<svelte:fragment>` element allows you to place content in a named slot without wrapping it in a container DOM element. This keeps the flow layout of your document intact.

Widget

    <div>
    	<slot name="header">No header was provided</slot>
    	<p>Some content between header and footer</p>
    	<slot name="footer" />
    </div>

App

    <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>

    <script lang="ts">
    	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>

> In Svelte 5+, this concept is obsolete, as snippets don’t create a wrapping element

---

## Page: https://svelte.dev/docs/svelte/legacy-svelte-component

In runes mode, `<MyComponent>` will re-render if the value of `MyComponent` changes. See the Svelte 5 migration guide 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.

---

## Page: https://svelte.dev/docs/svelte/legacy-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}

> This concept is obsolete, as components can import themselves:
> 
> App
> 
>     <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}
> 
>     <script lang="ts">
>     	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}

---

## Page: https://svelte.dev/docs/svelte/legacy-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

    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.

    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. 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.

index

    import App from './App.svelte';
    
    const app = new App({
    	target: document.querySelector('#server-rendered-html'),
    	hydrate: true
    });

> In Svelte 5+, use `mount` instead

## $set

    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.

    component.$set({ answer: 42 });

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

## $on

    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.

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

> In Svelte 5+, pass callback props instead

## $destroy

    component.$destroy();

Removes a component from the DOM and triggers any `onDestroy` handlers.

> In Svelte 5+, use `unmount` instead

## Component props

    component.prop;

    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.

    console.log(component.count);
    component.count += 1;

> In Svelte 5+, this concept is obsolete. If you want to make properties accessible from the outside, `export` them

## Server-side component API

    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`.

    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 |

    const { head, html, css } = App.render(
    	// props
    	{ answer: 42 },
    	// options
    	{
    		context: new Map([['context-key', 'context-value']])
    	}
    );

> In Svelte 5+, use `render` instead

---

## Page: https://svelte.dev/docs/svelte-store

    import {
    	derived,
    	fromStore,
    	get,
    	readable,
    	readonly,
    	toStore,
    	writable
    } from 'svelte/store';

## derived

Derived value store by synchronizing one or more readable stores and applying an aggregation function over its input values.

    function derived<S extends Stores, T>(
    	stores: S,
    	fn: (
    		values: StoresValues<S>,
    		set: (value: T) => void,
    		update: (fn: Updater<T>) => void
    	) => Unsubscriber | void,
    	initial_value?: T | undefined
    ): Readable<T>;

    function derived<S extends Stores, T>(
    	stores: S,
    	fn: (values: StoresValues<S>) => T,
    	initial_value?: T | undefined
    ): Readable<T>;

## fromStore

    function fromStore<V>(store: Writable<V>): {
    	current: V;
    };

    function fromStore<V>(store: Readable<V>): {
    	readonly current: V;
    };

## get

Get the current value from a store by subscribing and immediately unsubscribing.

    function get<T>(store: Readable<T>): T;

## readable

Creates a `Readable` store that allows reading by subscription.

    function readable<T>(
    	value?: T | undefined,
    	start?: StartStopNotifier<T> | undefined
    ): Readable<T>;

## readonly

Takes a store and returns a new one derived from the old one that is readable.

    function readonly<T>(store: Readable<T>): Readable<T>;

## toStore

    function toStore<V>(
    	get: () => V,
    	set: (v: V) => void
    ): Writable<V>;

    function toStore<V>(get: () => V): Readable<V>;

## writable

Create a `Writable` store that allows both updating and reading by subscription.

    function writable<T>(
    	value?: T | undefined,
    	start?: StartStopNotifier<T> | undefined
    ): Writable<T>;

## Readable

Readable interface for subscribing.

    interface Readable<T> {…}

    subscribe(this: void, run: Subscriber<T>, invalidate?: () => void): Unsubscriber;

*   `run` subscription callback
*   `invalidate` cleanup callback

Subscribe on value changes.

## StartStopNotifier

Start and stop notification callbacks. This function is called when the first subscriber subscribes.

    type StartStopNotifier<T> = (
    	set: (value: T) => void,
    	update: (fn: Updater<T>) => void
    ) => void | (() => void);

## Subscriber

Callback to inform of a value updates.

    type Subscriber<T> = (value: T) => void;

## Unsubscriber

Unsubscribes from value updates.

    type Unsubscriber = () => void;

## Updater

Callback to update a value.

    type Updater<T> = (value: T) => T;

## Writable

Writable interface for both updating and subscribing.

    interface Writable<T> extends Readable<T> {…}

    set(this: void, value: T): void;

*   `value` to set

Set value and inform subscribers.

    update(this: void, updater: Updater<T>): void;

*   `updater` callback

Update value using callback and inform subscribers.

---

## Page: https://svelte.dev/docs/svelte-compiler#compile

    import {
    	VERSION,
    	compile,
    	compileModule,
    	migrate,
    	parse,
    	preprocess,
    	walk
    } from 'svelte/compiler';

## VERSION

The current version, as set in package.json.

    const VERSION: string;

## compile

`compile` converts your `.svelte` source code into a JavaScript module that exports a component

    function compile(
    	source: string,
    	options: CompileOptions
    ): CompileResult;

## compileModule

`compileModule` takes your JavaScript source code containing runes, and turns it into a JavaScript module.

    function compileModule(
    	source: string,
    	options: ModuleCompileOptions
    ): CompileResult;

## migrate

Does a best-effort migration of Svelte code towards using runes, event attributes and render tags. May throw an error if the code is too complex to migrate automatically.

    function migrate(
    	source: string,
    	{
    		filename,
    		use_ts
    	}?:
    		| {
    				filename?: string;
    				use_ts?: boolean;
    		  }
    		| undefined
    ): {
    	code: string;
    };

## parse

The parse function parses a component, returning only its abstract syntax tree.

The `modern` option (`false` by default in Svelte 5) makes the parser return a modern AST instead of the legacy AST. `modern` will become `true` by default in Svelte 6, and the option will be removed in Svelte 7.

    function parse(
    	source: string,
    	options: {
    		filename?: string;
    		modern: true;
    		loose?: boolean;
    	}
    ): AST.Root;

    function parse(
    	source: string,
    	options?:
    		| {
    				filename?: string;
    				modern?: false;
    				loose?: boolean;
    		  }
    		| undefined
    ): Record<string, any>;

## preprocess

The preprocess function provides convenient hooks for arbitrarily transforming component source code. For example, it can be used to convert a `<style lang="sass">` block into vanilla CSS.

    function preprocess(
    	source: string,
    	preprocessor: PreprocessorGroup | PreprocessorGroup[],
    	options?:
    		| {
    				filename?: string;
    		  }
    		| undefined
    ): Promise<Processed>;

## walk

> Replace this with `import { walk } from 'estree-walker'`

    function walk(): never;

## AST

    namespace AST {
    	export interface BaseNode {
    		type: string;
    		start: number;
    		end: number;
    	}
    
    	export interface Fragment {
    		type: 'Fragment';
    		nodes: Array<
    			Text | Tag | ElementLike | Block | Comment
    		>;
    	}
    
    	export interface Root extends BaseNode {
    		type: 'Root';
    		/**
    		 * Inline options provided by `<svelte:options>` — these override options passed to `compile(...)`
    		 */
    		options: SvelteOptions | null;
    		fragment: Fragment;
    		/** The parsed `<style>` element, if exists */
    		css: AST.CSS.StyleSheet | null;
    		/** The parsed `<script>` element, if exists */
    		instance: Script | null;
    		/** The parsed `<script module>` element, if exists */
    		module: Script | null;
    	}
    
    	export interface SvelteOptions {
    		// start/end info (needed for warnings and for our Prettier plugin)
    		start: number;
    		end: number;
    		// options
    		runes?: boolean;
    		immutable?: boolean;
    		accessors?: boolean;
    		preserveWhitespace?: boolean;
    		namespace?: Namespace;
    		css?: 'injected';
    		customElement?: {
    			tag?: string;
    			shadow?: 'open' | 'none';
    			props?: Record<
    				string,
    				{
    					attribute?: string;
    					reflect?: boolean;
    					type?:
    						| 'Array'
    						| 'Boolean'
    						| 'Number'
    						| 'Object'
    						| 'String';
    				}
    			>;
    			/**
    			 * Is of type
    			 * ```ts
    			 * (ceClass: new () => HTMLElement) => new () => HTMLElement
    			 * ```
    			 */
    			extend?: ArrowFunctionExpression | Identifier;
    		};
    		attributes: Attribute[];
    	}
    
    	/** Static text */
    	export interface Text extends BaseNode {
    		type: 'Text';
    		/** Text with decoded HTML entities */
    		data: string;
    		/** The original text, with undecoded HTML entities */
    		raw: string;
    	}
    
    	/** A (possibly reactive) template expression — `{...}` */
    	export interface ExpressionTag extends BaseNode {
    		type: 'ExpressionTag';
    		expression: Expression;
    	}
    
    	/** A (possibly reactive) HTML template expression — `{@html ...}` */
    	export interface HtmlTag extends BaseNode {
    		type: 'HtmlTag';
    		expression: Expression;
    	}
    
    	/** An HTML comment */
    	// TODO rename to disambiguate
    	export interface Comment extends BaseNode {
    		type: 'Comment';
    		/** the contents of the comment */
    		data: string;
    	}
    
    	/** A `{@const ...}` tag */
    	export interface ConstTag extends BaseNode {
    		type: 'ConstTag';
    		declaration: VariableDeclaration & {
    			declarations: [
    				VariableDeclarator & {
    					id: Pattern;
    					init: Expression;
    				}
    			];
    		};
    	}
    
    	/** A `{@debug ...}` tag */
    	export interface DebugTag extends BaseNode {
    		type: 'DebugTag';
    		identifiers: Identifier[];
    	}
    
    	/** A `{@render foo(...)} tag */
    	export interface RenderTag extends BaseNode {
    		type: 'RenderTag';
    		expression:
    			| SimpleCallExpression
    			| (ChainExpression & {
    					expression: SimpleCallExpression;
    			  });
    	}
    
    	/** An `animate:` directive */
    	export interface AnimateDirective extends BaseNode {
    		type: 'AnimateDirective';
    		/** The 'x' in `animate:x` */
    		name: string;
    		/** The y in `animate:x={y}` */
    		expression: null | Expression;
    	}
    
    	/** A `bind:` directive */
    	export interface BindDirective extends BaseNode {
    		type: 'BindDirective';
    		/** The 'x' in `bind:x` */
    		name: string;
    		/** The y in `bind:x={y}` */
    		expression:
    			| Identifier
    			| MemberExpression
    			| SequenceExpression;
    	}
    
    	/** A `class:` directive */
    	export interface ClassDirective extends BaseNode {
    		type: 'ClassDirective';
    		/** The 'x' in `class:x` */
    		name: 'class';
    		/** The 'y' in `class:x={y}`, or the `x` in `class:x` */
    		expression: Expression;
    	}
    
    	/** A `let:` directive */
    	export interface LetDirective extends BaseNode {
    		type: 'LetDirective';
    		/** The 'x' in `let:x` */
    		name: string;
    		/** The 'y' in `let:x={y}` */
    		expression:
    			| null
    			| Identifier
    			| ArrayExpression
    			| ObjectExpression;
    	}
    
    	/** An `on:` directive */
    	export interface OnDirective extends BaseNode {
    		type: 'OnDirective';
    		/** The 'x' in `on:x` */
    		name: string;
    		/** The 'y' in `on:x={y}` */
    		expression: null | Expression;
    		modifiers: string[];
    	}
    
    	/** A `style:` directive */
    	export interface StyleDirective extends BaseNode {
    		type: 'StyleDirective';
    		/** The 'x' in `style:x` */
    		name: string;
    		/** The 'y' in `style:x={y}` */
    		value:
    			| true
    			| ExpressionTag
    			| Array<ExpressionTag | Text>;
    		modifiers: Array<'important'>;
    	}
    
    	// TODO have separate in/out/transition directives
    	/** A `transition:`, `in:` or `out:` directive */
    	export interface TransitionDirective extends BaseNode {
    		type: 'TransitionDirective';
    		/** The 'x' in `transition:x` */
    		name: string;
    		/** The 'y' in `transition:x={y}` */
    		expression: null | Expression;
    		modifiers: Array<'local' | 'global'>;
    		/** True if this is a `transition:` or `in:` directive */
    		intro: boolean;
    		/** True if this is a `transition:` or `out:` directive */
    		outro: boolean;
    	}
    
    	/** A `use:` directive */
    	export interface UseDirective extends BaseNode {
    		type: 'UseDirective';
    		/** The 'x' in `use:x` */
    		name: string;
    		/** The 'y' in `use:x={y}` */
    		expression: null | Expression;
    	}
    
    	interface BaseElement extends BaseNode {
    		name: string;
    		attributes: Array<
    			Attribute | SpreadAttribute | Directive
    		>;
    		fragment: Fragment;
    	}
    
    	export interface Component extends BaseElement {
    		type: 'Component';
    	}
    
    	export interface TitleElement extends BaseElement {
    		type: 'TitleElement';
    		name: 'title';
    	}
    
    	export interface SlotElement extends BaseElement {
    		type: 'SlotElement';
    		name: 'slot';
    	}
    
    	export interface RegularElement extends BaseElement {
    		type: 'RegularElement';
    	}
    
    	export interface SvelteBody extends BaseElement {
    		type: 'SvelteBody';
    		name: 'svelte:body';
    	}
    
    	export interface SvelteComponent extends BaseElement {
    		type: 'SvelteComponent';
    		name: 'svelte:component';
    		expression: Expression;
    	}
    
    	export interface SvelteDocument extends BaseElement {
    		type: 'SvelteDocument';
    		name: 'svelte:document';
    	}
    
    	export interface SvelteElement extends BaseElement {
    		type: 'SvelteElement';
    		name: 'svelte:element';
    		tag: Expression;
    	}
    
    	export interface SvelteFragment extends BaseElement {
    		type: 'SvelteFragment';
    		name: 'svelte:fragment';
    	}
    
    	export interface SvelteBoundary extends BaseElement {
    		type: 'SvelteBoundary';
    		name: 'svelte:boundary';
    	}
    
    	export interface SvelteHead extends BaseElement {
    		type: 'SvelteHead';
    		name: 'svelte:head';
    	}
    
    	/** This is only an intermediate representation while parsing, it doesn't exist in the final AST */
    	export interface SvelteOptionsRaw extends BaseElement {
    		type: 'SvelteOptions';
    		name: 'svelte:options';
    	}
    
    	export interface SvelteSelf extends BaseElement {
    		type: 'SvelteSelf';
    		name: 'svelte:self';
    	}
    
    	export interface SvelteWindow extends BaseElement {
    		type: 'SvelteWindow';
    		name: 'svelte:window';
    	}
    
    	/** An `{#each ...}` block */
    	export interface EachBlock extends BaseNode {
    		type: 'EachBlock';
    		expression: Expression;
    		/** The `entry` in `{#each item as entry}`. `null` if `as` part is omitted */
    		context: Pattern | null;
    		body: Fragment;
    		fallback?: Fragment;
    		index?: string;
    		key?: Expression;
    	}
    
    	/** An `{#if ...}` block */
    	export interface IfBlock extends BaseNode {
    		type: 'IfBlock';
    		elseif: boolean;
    		test: Expression;
    		consequent: Fragment;
    		alternate: Fragment | null;
    	}
    
    	/** An `{#await ...}` block */
    	export interface AwaitBlock extends BaseNode {
    		type: 'AwaitBlock';
    		expression: Expression;
    		// TODO can/should we move these inside the ThenBlock and CatchBlock?
    		/** The resolved value inside the `then` block */
    		value: Pattern | null;
    		/** The rejection reason inside the `catch` block */
    		error: Pattern | null;
    		pending: Fragment | null;
    		then: Fragment | null;
    		catch: Fragment | null;
    	}
    
    	export interface KeyBlock extends BaseNode {
    		type: 'KeyBlock';
    		expression: Expression;
    		fragment: Fragment;
    	}
    
    	export interface SnippetBlock extends BaseNode {
    		type: 'SnippetBlock';
    		expression: Identifier;
    		parameters: Pattern[];
    		body: Fragment;
    	}
    
    	export interface Attribute extends BaseNode {
    		type: 'Attribute';
    		name: string;
    		/**
    		 * Quoted/string values are represented by an array, even if they contain a single expression like `"{x}"`
    		 */
    		value:
    			| true
    			| ExpressionTag
    			| Array<Text | ExpressionTag>;
    	}
    
    	export interface SpreadAttribute extends BaseNode {
    		type: 'SpreadAttribute';
    		expression: Expression;
    	}
    
    	export interface Script extends BaseNode {
    		type: 'Script';
    		context: 'default' | 'module';
    		content: Program;
    		attributes: Attribute[];
    	}
    
    	export type AttributeLike =
    		| Attribute
    		| SpreadAttribute
    		| Directive;
    
    	export type Directive =
    		| AST.AnimateDirective
    		| AST.BindDirective
    		| AST.ClassDirective
    		| AST.LetDirective
    		| AST.OnDirective
    		| AST.StyleDirective
    		| AST.TransitionDirective
    		| AST.UseDirective;
    
    	export type Block =
    		| AST.EachBlock
    		| AST.IfBlock
    		| AST.AwaitBlock
    		| AST.KeyBlock
    		| AST.SnippetBlock;
    
    	export type ElementLike =
    		| AST.Component
    		| AST.TitleElement
    		| AST.SlotElement
    		| AST.RegularElement
    		| AST.SvelteBody
    		| AST.SvelteBoundary
    		| AST.SvelteComponent
    		| AST.SvelteDocument
    		| AST.SvelteElement
    		| AST.SvelteFragment
    		| AST.SvelteHead
    		| AST.SvelteOptionsRaw
    		| AST.SvelteSelf
    		| AST.SvelteWindow
    		| AST.SvelteBoundary;
    
    	export type Tag =
    		| AST.ExpressionTag
    		| AST.HtmlTag
    		| AST.ConstTag
    		| AST.DebugTag
    		| AST.RenderTag;
    
    	export type TemplateNode =
    		| AST.Root
    		| AST.Text
    		| Tag
    		| ElementLike
    		| AST.Attribute
    		| AST.SpreadAttribute
    		| Directive
    		| AST.Comment
    		| Block;
    
    	export type SvelteNode =
    		| Node
    		| TemplateNode
    		| AST.Fragment
    		| _CSS.Node;
    
    	export type { _CSS as CSS };
    }

## CompileError

    interface CompileError extends ICompileDiagnostic {}

## CompileOptions

    interface CompileOptions extends ModuleCompileOptions {…}

    name?: string;

Sets the name of the resulting JavaScript class (though the compiler will rename it if it would otherwise conflict with other variables in scope). If unspecified, will be inferred from `filename`

    customElement?: boolean;

*   default `false`

If `true`, tells the compiler to generate a custom element constructor instead of a regular Svelte component.

    accessors?: boolean;

*   default `false`
*   deprecated This will have no effect in runes mode

If `true`, getters and setters will be created for the component’s props. If `false`, they will only be created for readonly exported values (i.e. those declared with `const`, `class` and `function`). If compiling with `customElement: true` this option defaults to `true`.

    namespace?: Namespace;

*   default `'html'`

The namespace of the element; e.g., `"html"`, `"svg"`, `"mathml"`.

    immutable?: boolean;

*   default `false`
*   deprecated This will have no effect in runes mode

If `true`, tells the compiler that you promise not to mutate any objects. This allows it to be less conservative about checking whether values have changed.

    css?: 'injected' | 'external';

*   `'injected'`: styles will be included in the `head` when using `render(...)`, and injected into the document (if not already present) when the component mounts. For components compiled as custom elements, styles are injected to the shadow root.
*   `'external'`: the CSS will only be returned in the `css` field of the compilation result. Most Svelte bundler plugins will set this to `'external'` and use the CSS that is statically generated for better performance, as it will result in smaller JavaScript bundles and the output can be served as cacheable `.css` files. This is always `'injected'` when compiling with `customElement` mode.

    cssHash?: CssHashGetter;

*   default `undefined`

A function that takes a `{ hash, css, name, filename }` argument and returns the string that is used as a classname for scoped CSS. It defaults to returning `svelte-${hash(css)}`.

    preserveComments?: boolean;

*   default `false`

If `true`, your HTML comments will be preserved in the output. By default, they are stripped out.

    preserveWhitespace?: boolean;

*   default `false`

If `true`, whitespace inside and between elements is kept as you typed it, rather than removed or collapsed to a single space where possible.

    runes?: boolean | undefined;

*   default `undefined`

Set to `true` to force the compiler into runes mode, even if there are no indications of runes usage. Set to `false` to force the compiler into ignoring runes, even if there are indications of runes usage. Set to `undefined` (the default) to infer runes mode from the component code. Is always `true` for JS/TS modules compiled with Svelte. Will be `true` by default in Svelte 6. Note that setting this to `true` in your `svelte.config.js` will force runes mode for your entire project, including components in `node_modules`, which is likely not what you want. If you’re using Vite, consider using dynamicCompileOptions instead.

    discloseVersion?: boolean;

*   default `true`

If `true`, exposes the Svelte major version in the browser by adding it to a `Set` stored in the global `window.__svelte.v`.

    compatibility?: {…}

*   deprecated Use these only as a temporary solution before migrating your code

    componentApi?: 4 | 5;

*   default `5`

Applies a transformation so that the default export of Svelte files can still be instantiated the same way as in Svelte 4 — as a class when compiling for the browser (as though using `createClassComponent(MyComponent, {...})` from `svelte/legacy`) or as an object with a `.render(...)` method when compiling for the server

    sourcemap?: object | string;

*   default `null`

An initial sourcemap that will be merged into the final output sourcemap. This is usually the preprocessor sourcemap.

    outputFilename?: string;

*   default `null`

Used for your JavaScript sourcemap.

    cssOutputFilename?: string;

*   default `null`

Used for your CSS sourcemap.

    hmr?: boolean;

*   default `false`

If `true`, compiles components with hot reloading support.

    modernAst?: boolean;

*   default `false`

If `true`, returns the modern version of the AST. Will become `true` by default in Svelte 6, and the option will be removed in Svelte 7.

## CompileResult

The return value of `compile` from `svelte/compiler`

    interface CompileResult {…}

    js: {…}

The compiled JavaScript

    code: string;

The generated code

    map: SourceMap;

A source map

    css: null | {
    	/** The generated code */
    	code: string;
    	/** A source map */
    	map: SourceMap;
    };

The compiled CSS

    warnings: Warning[];

An array of warning objects that were generated during compilation. Each warning has several properties:

*   `code` is a string identifying the category of warning
*   `message` describes the issue in human-readable terms
*   `start` and `end`, if the warning relates to a specific location, are objects with `line`, `column` and `character` properties

    metadata: {…}

Metadata about the compiled component

    runes: boolean;

Whether the file was compiled in runes mode, either because of an explicit option or inferred from usage. For `compileModule`, this is always `true`

    ast: any;

The AST

## MarkupPreprocessor

A markup preprocessor that takes a string of code and returns a processed version.

    type MarkupPreprocessor = (options: {
    	/**
    	 * The whole Svelte file content
    	 */
    	content: string;
    	/**
    	 * The filename of the Svelte file
    	 */
    	filename?: string;
    }) => Processed | void | Promise<Processed | void>;

## ModuleCompileOptions

    interface ModuleCompileOptions {…}

    dev?: boolean;

*   default `false`

If `true`, causes extra code to be added that will perform runtime checks and provide debugging information during development.

    generate?: 'client' | 'server' | false;

*   default `'client'`

If `"client"`, Svelte emits code designed to run in the browser. If `"server"`, Svelte emits code suitable for server-side rendering. If `false`, nothing is generated. Useful for tooling that is only interested in warnings.

    filename?: string;

Used for debugging hints and sourcemaps. Your bundler plugin will set it automatically.

    rootDir?: string;

*   default `process.cwd() on node-like environments, undefined elsewhere`

Used for ensuring filenames don’t leak filesystem information. Your bundler plugin will set it automatically.

    warningFilter?: (warning: Warning) => boolean;

A function that gets a `Warning` as an argument and returns a boolean. Use this to filter out warnings. Return `true` to keep the warning, `false` to discard it.

## Preprocessor

A script/style preprocessor that takes a string of code and returns a processed version.

    type Preprocessor = (options: {
    	/**
    	 * The script/style tag content
    	 */
    	content: string;
    	/**
    	 * The attributes on the script/style tag
    	 */
    	attributes: Record<string, string | boolean>;
    	/**
    	 * The whole Svelte file content
    	 */
    	markup: string;
    	/**
    	 * The filename of the Svelte file
    	 */
    	filename?: string;
    }) => Processed | void | Promise<Processed | void>;

## PreprocessorGroup

A preprocessor group is a set of preprocessors that are applied to a Svelte file.

    interface PreprocessorGroup {…}

    name?: string;

Name of the preprocessor. Will be a required option in the next major version

    markup?: MarkupPreprocessor;

    style?: Preprocessor;

    script?: Preprocessor;

## Processed

The result of a preprocessor run. If the preprocessor does not return a result, it is assumed that the code is unchanged.

    interface Processed {…}

    code: string;

The new code

    map?: string | object;

A source map mapping back to the original code

    dependencies?: string[];

A list of additional files to watch for changes

    attributes?: Record<string, string | boolean>;

Only for script/style preprocessors: The updated attributes to set on the tag. If undefined, attributes stay unchanged.

    toString?: () => string;

## Warning

    interface Warning extends ICompileDiagnostic {}