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SvelteRuntime

Imperative component API

better title needed?

  • mount
  • unmount
  • render
  • hydrate
  • how they interact with each other

Every Svelte application starts by imperatively creating a root component. On the client this component is mounted to a specific element. On the server, you want to get back a string of HTML instead which you can render. The following functions help you achieve those tasks.

mount

Instantiates a component and mounts it to the given target:

import { 
function mount<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;
    anchor?: Node;
    props?: Props;
    events?: Record<string, (e: any) => any>;
    context?: Map<any, any>;
    intro?: boolean;
} : {
    target: Document | Element | ShadowRoot;
    props: Props;
    anchor?: Node;
    events?: Record<string, (e: any) => any>;
    context?: Map<any, any>;
    intro?: 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.


mount } from 'svelte';
import 
type App = SvelteComponent<Record<string, any>, any, any>
const App: ComponentType
App from './App.svelte';

const 
const app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>
app = 
mount<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<...>>(component: ComponentType<...> | Component<...>, options: {
    ...;
}): {
    ...;
} & Record<...>
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.


mount(const App: ComponentTypeApp, {
	target: Document | Element | ShadowRoottarget: var document: Document
MDN Reference


document.ParentNode.querySelector<Element>(selectors: string): Element | null (+4 overloads)
Returns the first element that is a descendant of node that matches selectors.


MDN Reference


querySelector('#app'),
	props?: Record<string, any> | undefinedprops: { some: stringsome: '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 created with mount or hydrate:

import { 
function mount<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;
    anchor?: Node;
    props?: Props;
    events?: Record<string, (e: any) => any>;
    context?: Map<any, any>;
    intro?: boolean;
} : {
    target: Document | Element | ShadowRoot;
    props: Props;
    anchor?: Node;
    events?: Record<string, (e: any) => any>;
    context?: Map<any, any>;
    intro?: 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.


mount, function unmount(component: Record<string, any>): void
Unmounts a component that was previously mounted using mount or hydrate.


unmount } from 'svelte';
import 
type App = SvelteComponent<Record<string, any>, any, any>
const App: ComponentType
App from './App.svelte';

const 
const app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>
app = 
mount<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<...>>(component: ComponentType<...> | Component<...>, options: {
    ...;
}): {
    ...;
} & Record<...>
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.


mount(const App: ComponentTypeApp, {...});

// later
function unmount(component: Record<string, any>): void
Unmounts a component that was previously mounted using mount or hydrate.


unmount(
const app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>
app);

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


render } from 'svelte/server';
import 
type App = SvelteComponent<Record<string, any>, any, any>
const App: ComponentType
App from './App.svelte';

const const result: RenderOutputresult = 
render<SvelteComponent<Record<string, any>, any, any>, Record<string, any>>(component: ComponentType<SvelteComponent<Record<string, any>, any, any>>, options?: {
    ...;
} | undefined): RenderOutput
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.


render(const App: ComponentTypeApp, {
	props?: Omit<Record<string, any>, "$$slots" | "$$events"> | undefinedprops: { some: stringsome: 'property' }
});
const result: RenderOutputresult.RenderOutput.body: string
HTML that goes somewhere into the &#x3C;body>


body; // HTML for somewhere in this <body> tag
const result: RenderOutputresult.RenderOutput.head: string
HTML that goes into the &#x3C;head>


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


hydrate } from 'svelte';
import 
type App = SvelteComponent<Record<string, any>, any, any>
const App: ComponentType
App from './App.svelte';

const 
const app: {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<string, any>
app = 
hydrate<Record<string, any>, {
    $on?(type: string, callback: (e: any) => void): () => void;
    $set?(props: Partial<Record<string, any>>): void;
} & Record<...>>(component: ComponentType<...> | Component<...>, options: {
    ...;
}): {
    ...;
} & Record<...>
Hydrates a component on the given target and returns the exports and potentially the props (if compiled with accessors: true) of the component


hydrate(const App: ComponentTypeApp, {
	target: Document | Element | ShadowRoottarget: var document: Document
MDN Reference


document.ParentNode.querySelector<Element>(selectors: string): Element | null (+4 overloads)
Returns the first element that is a descendant of node that matches selectors.


MDN Reference


querySelector('#app'),
	props?: Record<string, any> | undefinedprops: { some: stringsome: 'property' }
});

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

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