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Rich Harris 2 years ago
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---
title: .svelte files
---
Components are the building blocks of Svelte applications. They are written into `.svelte` files, using a superset of HTML.
All three sections — script, styles and markup — are optional.
<!-- prettier-ignore -->
```svelte
/// file: MyComponent.svelte
<script module>
// module-level logic goes here
// (you will rarely use this)
</script>
<script>
// instance-level logic goes here
</script>
<!-- markup (zero or more items) goes here -->
<style>
/* styles go here */
</style>
```
## `<script>`
A `<script>` block contains JavaScript (or TypeScript, when adding the `lang="ts"` attribute) that runs when a component instance is created. Variables declared (or imported) at the top level can be referenced in the component's markup.
In addition to normal JavaScript, you can use _runes_ to declare [component props]($props) and add reactivity to your component. Runes are covered in the next section.
<!-- TODO describe behaviour of `export` -->
## `<script module>`
A `<script>` tag with a `module` attribute runs once when the module first evaluates, rather than for each component instance. Variables declared in this block can be referenced elsewhere in the component, but not vice versa.
You can `export` bindings from this block, and they will become exports of the compiled module. You cannot `export default`, since the default export is the component itself.
```svelte
<script module>
let total = 0;
</script>
<script>
total += 1;
console.log(`instantiated ${total} times`);
</script>
```
## `<style>`
CSS inside a `<style>` block will be scoped to that component.
```svelte
<style>
p {
/* this will only affect <p> elements in this component */
color: burlywood;
}
</style>
```
For more information regarding styling, read the documentation around [styles and classes](styles-and-classes).

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---
title: .svelte.js and .svelte.ts files
---
Besides `.svelte` files, Svelte also operates on `.svelte.js` and `.svelte.ts` files.
These behave like any other `.js` or `.ts` module, except that you can use runes. This is useful for creating reusable reactive logic, or sharing reactive state across your app.

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---
title: Public API of a component
---
### Public API of a component
Svelte uses the `$props` rune to declare _properties_ or _props_, which means describing the public interface of the component which becomes accessible to consumers of the component.
> [!NOTE] `$props` is one of several runes, which are special hints for Svelte's compiler to make things reactive.
```svelte
<script>
let { foo, bar, baz } = $props();
// Values that are passed in as props
// are immediately available
console.log({ foo, bar, baz });
</script>
```
You can specify a fallback value for a prop. It will be used if the component's consumer doesn't specify the prop on the component when instantiating the component, or if the passed value is `undefined` at some point.
```svelte
<script>
let { foo = 'optional default initial value' } = $props();
</script>
```
To get all properties, use rest syntax:
```svelte
<script>
let { a, b, c, ...everythingElse } = $props();
</script>
```
You can use reserved words as prop names.
```svelte
<script>
// creates a `class` property, even
// though it is a reserved word
let { class: className } = $props();
</script>
```
If you're using TypeScript, you can declare the prop types:
```svelte
<script lang="ts">
interface Props {
required: string;
optional?: number;
[key: string]: unknown;
}
let { required, optional, ...everythingElse }: Props = $props();
</script>
```
If you're using JavaScript, you can declare the prop types using JSDoc:
```svelte
<script>
/** @type {{ x: string }} */
let { x } = $props();
// or use @typedef if you want to document the properties:
/**
* @typedef {Object} MyProps
* @property {string} y Some documentation
*/
/** @type {MyProps} */
let { y } = $props();
</script>
```
If you export a `const`, `class` or `function`, it is readonly from outside the component.
```svelte
<script>
export const thisIs = 'readonly';
export function greet(name) {
alert(`hello ${name}!`);
}
</script>
```
Readonly props can be accessed as properties on the element, tied to the component using [`bind:this` syntax](bindings#bind:this).
### Reactive variables
To change component state and trigger a re-render, just assign to a locally declared variable that was declared using the `$state` rune.
Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.
```svelte
<script>
let count = $state(0);
function handleClick() {
// calling this function will trigger an
// update if the markup references `count`
count = count + 1;
}
</script>
```
Svelte's `<script>` blocks are run only when the component is created, so assignments within a `<script>` block are not automatically run again when a prop updates.
```svelte
<script>
let { person } = $props();
// this will only set `name` on component creation
// it will not update when `person` does
let { name } = person;
</script>
```
If you'd like to react to changes to a prop, use the `$derived` or `$effect` runes instead.
```svelte
<script>
let count = $state(0);
let double = $derived(count * 2);
$effect(() => {
if (count > 10) {
alert('Too high!');
}
});
</script>
```
For more information on reactivity, read the documentation around runes.
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