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.

<script>
	// logic goes here
</script>

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

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

<script>

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:

<script>
	// these properties can be set externally
	export let foo;
	export let bar = 'optional default value';

	// Values that are passed in as props
	// are immediately available
	console.log(foo, bar);

	// function declarations cannot be set externally,
	// but can be accessed from outside
	export function instanceMethod() {
		alert(foo);
	}
</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.

<script>
	let count = 0;

	function handleClick () {
		// calling this function will trigger a re-render
		// 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 $: label. Reactive statements run immediately before the component updates, whenever the values that they depend on have changed.

<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.

<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

---

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.

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.

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

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

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

<script context="module">

---

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.

<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>

<style>

---

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).

<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.

<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>