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svelte/documentation/docs/06-runtime/04-imperative-component-api.md

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title
Imperative component API

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

mount

Instantiates a component and mounts it to the given target:

// @errors: 2322
import { mount } from 'svelte';
import App from './App.svelte';

const app = mount(App, {
	target: document.querySelector('#app'),
	props: { some: 'property' }
});

You can mount multiple components per page, and you can also mount from within your application, for example when creating a tooltip component and attaching it to the hovered element.

Note that unlike calling new App(...) in Svelte 4, things like effects (including onMount callbacks, and action functions) will not run during mount. If you need to force pending effects to run (in the context of a test, for example) you can do so with flushSync().

unmount

Unmounts a component that was previously created with mount 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:

// @errors: 2724 2305 2307
import { render } from 'svelte/server';
import App from './App.svelte';

const result = render(App, {
	props: { some: 'property' }
});
result.body; // HTML for somewhere in this <body> tag
result.head; // HTML for somewhere in this <head> tag

hydrate

Like mount, but will reuse up any HTML rendered by Svelte's SSR output (from the render function) inside the target and make it interactive:

// @errors: 2322
import { hydrate } from 'svelte';
import App from './App.svelte';

const app = hydrate(App, {
	target: document.querySelector('#app'),
	props: { some: 'property' }
});

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