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svelte • Docs • Svelte

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

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

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.

• 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.

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> {}

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;

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;

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: null; // 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>;

Create a snippet programmatically

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

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;

Returns an AbortSignal that aborts when the current derived or effect re-runs or is destroyed.

Must be called while a derived or effect is running.

<script>
	import { getAbortSignal } from 'svelte';

	let { id } = $props();

	async function getData(id) {
		const response = await fetch(`/items/${id}`, {
			signal: getAbortSignal()
		});

		return await response.json();
	}

	const data = $derived(await getData(id));
</script>

function getAbortSignal(): AbortSignal;

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;

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;

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;

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;

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;

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, 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;

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;

Available since 5.36

Returns a promise that resolves once any state changes, and asynchronous work resulting from them, have resolved and the DOM has been updated

function settled(): Promise<void>;

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

function tick(): Promise<void>;

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>;

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;

Can be used to create strongly typed Svelte components.

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

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;

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;

Internal implementation details that vary between environments

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

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;

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;
};

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;

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;
		});

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;

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