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---
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title: Imperative component API
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---
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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.
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## Creating a component
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```ts
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// @noErrors
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const component = new Component(options);
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```
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A client-side component — that is, a component compiled with `generate: 'dom'` (or the `generate` option left unspecified) is a JavaScript class.
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```ts
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// @noErrors
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import App from './App.svelte';
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const app = new App({
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target: document.body,
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props: {
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// assuming App.svelte contains something like
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// `export let answer`:
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answer: 42
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}
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});
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```
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The following initialisation options can be provided:
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| option | default | description |
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| --------- | ----------- | ---------------------------------------------------------------------------------------------------- |
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| `target` | **none** | An `HTMLElement` or `ShadowRoot` to render to. This option is required |
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| `anchor` | `null` | A child of `target` to render the component immediately before |
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| `props` | `{}` | An object of properties to supply to the component |
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| `context` | `new Map()` | A `Map` of root-level context key-value pairs to supply to the component |
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| `hydrate` | `false` | See below |
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| `intro` | `false` | If `true`, will play transitions on initial render, rather than waiting for subsequent state changes |
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Existing children of `target` are left where they are.
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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](/docs/svelte-compiler#compile). 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.
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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`.
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The existing DOM doesn't need to match the component — Svelte will 'repair' the DOM as it goes.
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```ts
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/// file: index.js
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// @noErrors
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import App from './App.svelte';
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const app = new App({
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target: document.querySelector('#server-rendered-html'),
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hydrate: true
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});
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```
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> [!NOTE]
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> In Svelte 5+, use [`mount`](svelte#mount) instead
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## `$set`
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```ts
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// @noErrors
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component.$set(props);
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```
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Programmatically sets props on an instance. `component.$set({ x: 1 })` is equivalent to `x = 1` inside the component's `<script>` block.
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Calling this method schedules an update for the next microtask — the DOM is _not_ updated synchronously.
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```ts
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// @noErrors
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component.$set({ answer: 42 });
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```
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> [!NOTE]
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> In Svelte 5+, use `$state` instead to create a component props and update that
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>
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> ```js
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> // @noErrors
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> let props = $state({ answer: 42 });
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> const component = mount(Component, { props });
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> // ...
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> props.answer = 24;
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> ```
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## `$on`
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```ts
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// @noErrors
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component.$on(ev, callback);
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```
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Causes the `callback` function to be called whenever the component dispatches an `event`.
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A function is returned that will remove the event listener when called.
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```ts
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// @noErrors
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const off = component.$on('selected', (event) => {
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console.log(event.detail.selection);
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});
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off();
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```
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> [!NOTE]
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> In Svelte 5+, pass callback props instead
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## `$destroy`
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```js
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// @noErrors
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component.$destroy();
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```
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Removes a component from the DOM and triggers any `onDestroy` handlers.
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> [!NOTE]
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> In Svelte 5+, use [`unmount`](svelte#unmount) instead
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## Component props
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```js
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// @noErrors
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component.prop;
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```
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```js
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// @noErrors
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component.prop = value;
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```
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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(...)`.
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By default, `accessors` is `false`, unless you're compiling as a custom element.
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```js
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// @noErrors
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console.log(component.count);
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component.count += 1;
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```
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> [!NOTE]
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> In Svelte 5+, this concept is obsolete. If you want to make properties accessible from the outside, `export` them
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## Server-side component API
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```js
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// @noErrors
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const result = Component.render(...)
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```
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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.
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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.
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You can import a Svelte component directly into Node using `svelte/register`.
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```js
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// @noErrors
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require('svelte/register');
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const App = require('./App.svelte').default;
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const { head, html, css } = App.render({
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answer: 42
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});
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```
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The `.render()` method accepts the following parameters:
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| parameter | default | description |
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| --------- | ------- | -------------------------------------------------- |
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| `props` | `{}` | An object of properties to supply to the component |
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| `options` | `{}` | An object of options |
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The `options` object takes in the following options:
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| option | default | description |
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| --------- | ----------- | ------------------------------------------------------------------------ |
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| `context` | `new Map()` | A `Map` of root-level context key-value pairs to supply to the component |
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```js
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// @noErrors
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const { head, html, css } = App.render(
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// props
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{ answer: 42 },
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// options
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{
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context: new Map([['context-key', 'context-value']])
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}
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);
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```
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> [!NOTE]
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> In Svelte 5+, use [`render`](svelte-server#render) instead
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