SvelteRuntime

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(alias) function mount<Props extends Record<string, any>, Exports extends Record<string, any>>(component: ComponentType<SvelteComponent<Props>> | Component<Props, Exports, any>, options: MountOptions<Props>): Exports
import mountMounts 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.
reference } from 'svelte';
import App(alias) type App = SvelteComponent<Record<string, any>, any, any>
(alias) const App: LegacyComponentType
import App from './App.svelte';

const appconst app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any> = mount(alias) mount<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>>(component: ComponentType<SvelteComponent<Record<string, any>, any, any>> | Component<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>, any>, options: MountOptions<...>): {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<...>
import mountMounts 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.
reference(App(alias) const App: LegacyComponentType
import App, {
	target(property) target: Document | Element | ShadowRootTarget element where the component will be mounted.
: documentvar document: Documentwindow.document returns a reference to the document contained in the window.
MDN Reference
.querySelector(method) ParentNode.querySelector<Element>(selectors: string): Element | null (+4 overloads)Returns the first element that is a descendant of node that matches selectors.
MDN Reference
('#app'),
	props(property) props?: Record<string, any> | undefinedComponent properties.
: { some(property) some: string: '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(alias) function mount<Props extends Record<string, any>, Exports extends Record<string, any>>(component: ComponentType<SvelteComponent<Props>> | Component<Props, Exports, any>, options: MountOptions<Props>): Exports
import mountMounts 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.
reference, unmount(alias) function unmount(component: Record<string, any>, options?: {
    outro?: boolean;
} | undefined): Promise<void>
import unmountUnmounts 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 });reference } from 'svelte';
import App(alias) type App = SvelteComponent<Record<string, any>, any, any>
(alias) const App: LegacyComponentType
import App from './App.svelte';

const appconst app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any> = mount(alias) mount<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>>(component: ComponentType<SvelteComponent<Record<string, any>, any, any>> | Component<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>, any>, options: MountOptions<...>): {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<...>
import mountMounts 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.
reference(App(alias) const App: LegacyComponentType
import App, { target(property) target: Document | Element | ShadowRootTarget element where the component will be mounted.
: documentvar document: Documentwindow.document returns a reference to the document contained in the window.
MDN Reference
.body(property) Document.body: HTMLElementThe Document.body property represents the  or  node of the current document, or null if no such element exists.
MDN Reference
 });
// laterunmount(alias) unmount(component: Record<string, any>, options?: {
    outro?: boolean;
} | undefined): Promise<void>
import unmountUnmounts 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 });reference(appconst app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>, { outro(property) outro?: boolean | undefined: 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(alias) 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;
    csp?: Csp;
    transformError?: (error: unknown) => unknown | Promise<unknown>;
}] : [component: Comp extends SvelteComponent<any> ? ComponentType<Comp> : Comp, options: {
    props: Omit<Props, "$$slots" | "$$events">;
    context?: Map<any, any>;
    idPrefix?: string;
    csp?: Csp;
    transformError?: (error: unknown) => unknown | Promise<unknown>;
}]): RenderOutput
import renderOnly 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.
reference } from 'svelte/server';
import App(alias) type App = SvelteComponent<Record<string, any>, any, any>
(alias) const App: LegacyComponentType
import App from './App.svelte';

const resultconst result: RenderOutput = render(alias) render<SvelteComponent<Record<string, any>, any, any>, Record<string, any>>(component: ComponentType<SvelteComponent<Record<string, any>, any, any>>, options?: {
    props?: Omit<Record<string, any>, "$$slots" | "$$events"> | undefined;
    context?: Map<any, any>;
    idPrefix?: string;
    csp?: Csp;
    transformError?: ((error: unknown) => unknown | Promise<unknown>) | undefined;
} | undefined): RenderOutput
import renderOnly 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.
reference(App(alias) const App: LegacyComponentType
import App, {
	props(property) props?: Omit<Record<string, any>, "$$slots" | "$$events"> | undefined: { some(property) some: string: 'property' }
});
resultconst result: RenderOutput.body(property) SyncRenderOutput.body: stringHTML that goes somewhere into the <body>
; // HTML for somewhere in this <body> tag
resultconst result: RenderOutput.head(property) SyncRenderOutput.head: stringHTML that goes into the <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(alias) 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;
    transformError?: (error: unknown) => unknown;
} : {
    target: Document | Element | ShadowRoot;
    props: Props;
    events?: Record<string, (e: any) => any>;
    context?: Map<any, any>;
    intro?: boolean;
    recover?: boolean;
    transformError?: (error: unknown) => unknown;
}): Exports
import hydrateHydrates a component on the given target and returns the exports and potentially the props (if compiled with accessors: true) of the component
reference } from 'svelte';
import App(alias) type App = SvelteComponent<Record<string, any>, any, any>
(alias) const App: LegacyComponentType
import App from './App.svelte';

const appconst app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any> = hydrate(alias) hydrate<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>>(component: ComponentType<SvelteComponent<Record<string, any>, any, any>> | Component<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>, any>, options: {
    ...;
}): {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<...>
import hydrateHydrates a component on the given target and returns the exports and potentially the props (if compiled with accessors: true) of the component
reference(App(alias) const App: LegacyComponentType
import App, {
	target(property) target: Document | Element | ShadowRoot: documentvar document: Documentwindow.document returns a reference to the document contained in the window.
MDN Reference
.querySelector(method) ParentNode.querySelector<Element>(selectors: string): Element | null (+4 overloads)Returns the first element that is a descendant of node that matches selectors.
MDN Reference
('#app'),
	props(property) props?: Record<string, any> | undefined: { some(property) some: string: 'property' }
});

As with mount, effects will not run during hydrate — use flushSync() immediately afterwards if you need them to.

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