svelte/site/content/docs/01-component-format.md

---
title: Component format
---

---

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.

```html
<script>
	// logic goes here
</script>

<style>
	/* styles go here */
</style>

<!-- markup (zero or more items) goes here -->
```

### &lt;script&gt;

A `<script>` block contains JavaScript that runs when a component instance is created. Variables declared (or imported) at the top level are 'visible' from the component's markup. There are four additional rules:

##### 1. `export` creates a component prop

---

Svelte uses the `export` keyword to mark a variable declaration as a *property* or *prop*, which means it becomes accessible to consumers of the component (see the section on [attributes and props](docs#Attributes_and_props) for more information).

```html
<script>
	export let foo;

	// Values that are passed in as props
	// are immediately available
	console.log({ foo });
</script>
```

---

You can specify a default value, which will be used if the component's consumer doesn't specify a prop.

In development mode (see the [compiler options](docs#svelte_compile)), a warning will be printed if no default is provided and the consumer does not specify a value. To squelch this warning, ensure that a default is specified, even if it is `undefined`.

```html
<script>
	export let bar = 'optional default value';
	export let baz = undefined;
</script>
```

---

If you export a `const`, `class` or `function`, it is readonly from outside the component. Function *expressions* are valid props, however.

```html
<script>
	// these are readonly
	export const thisIs = 'readonly';

	export function greet(name) {
		alert(`hello ${name}!`);
	}

	// this is a prop
	export let format = n => n.toFixed(2);
</script>
```

---

You can use reserved words as prop names.

```html
<script>
	let className;

	// creates a `class` property, even
	// though it is a reserved word
	export { className as class };
</script>
```

##### 2. Assignments are 'reactive'

---

To change component state and trigger a re-render, just assign to a locally declared variable.

Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.

Because Svelte's reactivity is based on assignments, using array methods like `.push()` and `.splice()` won't automatically trigger updates. Options for getting around this can be found in the [tutorial](tutorial/updating-arrays-and-objects).

```html
<script>
	let count = 0;

	function handleClick () {
		// calling this function will trigger an
		// update if the markup references `count`
		count = count + 1;
	}
</script>
```

##### 3. `$:` marks a statement as reactive

---

Any top-level statement (i.e. not inside a block or a function) can be made reactive by prefixing it with the `$:` [JS label syntax](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label). Reactive statements run immediately before the component updates, whenever the values that they depend on have changed.

```html
<script>
	export let title;

	// this will update `document.title` whenever
	// the `title` prop changes
	$: document.title = title;

	$: {
		console.log(`multiple statements can be combined`);
		console.log(`the current title is ${title}`);
	}
</script>
```

---

If a statement consists entirely of an assignment to an undeclared variable, Svelte will inject a `let` declaration on your behalf.

```html
<script>
	export let num;

	// we don't need to declare `squared` and `cubed`
	// — Svelte does it for us
	$: squared = num * num;
	$: cubed = squared * num;
</script>
```

##### 4. Prefix stores with `$` to access their values

---

A *store* is any object that allows reactive access to a value via a simple *store contract*.

The [`svelte/store` module](docs#svelte_store) contains minimal store implementations which fulfil this contract. You can use these as the basis for your own stores, or you can implement your stores from scratch.

A store must contain a `.subscribe` method, which must accept as its argument a subscription function. This subscription function must be immediately and synchronously called with the store's current value upon calling `.subscribe`. All of a store's active subscription functions must later be synchronously called whenever the store's value changes. The `.subscribe` method must also return an unsubscription function. Calling an unsubscription function must stop its subscription, and its corresponding subscription function must not be called again by the store.

A store may optionally contain a `.set` method, which must accept as its argument a new value for the store, and which synchronously calls all of the store's active subscription functions. Such a store is called a *writable store*.

```js
const unsubscribe = store.subscribe(value => {
	console.log(value);
}); // logs `value`

// later...
unsubscribe();
```

---

Any time you have a reference to a store, you can access its value inside a component by prefixing it with the `$` character. This causes Svelte to declare the prefixed variable, and set up a store subscription that will be unsubscribed when appropriate.

Assignments to `$`-prefixed variables require that the variable be a writable store, and will result in a call to the store's `.set` method.

Note that the store must be declared at the top level of the component — not inside an `if` block or a function, for example.

Local variables (that do not represent store values) must *not* have a `$` prefix.

```html
<script>
	import { writable } from 'svelte/store';

	const count = writable(0);
	console.log($count); // logs 0

	count.set(1);
	console.log($count); // logs 1

	$count = 2;
	console.log($count); // logs 2
</script>
```


### &lt;script context="module"&gt;

---

A `<script>` tag with a `context="module"` attribute runs once when the module first evaluates, rather than for each component instance. Values declared in this block are accessible from a regular `<script>` (and the component markup) 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.

> Variables defined in `module` scripts are not reactive — reassigning them will not trigger a rerender even though the variable itself will update. For values shared between multiple components, consider using a [store](https://svelte.dev/docs#svelte_store).

```html
<script context="module">
	let totalComponents = 0;

	// this allows an importer to do e.g.
	// `import Example, { alertTotal } from './Example.svelte'`
	export function alertTotal() {
		alert(totalComponents);
	}
</script>

<script>
	totalComponents += 1;
	console.log(`total number of times this component has been created: ${totalComponents}`);
</script>
```


### &lt;style&gt;

---

CSS inside a `<style>` block will be scoped to that component.

This works by adding a class to affected elements, which is based on a hash of the component styles (e.g. `svelte-123xyz`).

```html
<style>
	p {
		/* this will only affect <p> elements in this component */
		color: burlywood;
	}
</style>
```

---

To apply styles to a selector globally, use the `:global(...)` modifier.

```html
<style>
	:global(body) {
		/* this will apply to <body> */
		margin: 0;
	}

	div :global(strong) {
		/* this will apply to all <strong> elements, in any
			 component, that are inside <div> elements belonging
			 to this component */
		color: goldenrod;
	}
</style>
```