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

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---
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.
```sv
<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).
```sv
<script>
export let foo;
// Values that are passed in as props
// are immediately available
console.log({ foo });
</script>
```
---
You can specify a default initial value for a prop. It will be used if the component's consumer doesn't specify the prop on the component (or if its initial value is `undefined`) when instantiating the component. Note that whenever a prop is removed by the consumer, its value is set to `undefined` rather than the initial value.
In development mode (see the [compiler options](docs#svelte_compile)), a warning will be printed if no default initial value is provided and the consumer does not specify a value. To squelch this warning, ensure that a default initial value is specified, even if it is `undefined`.
```sv
<script>
export let bar = 'optional default initial 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.
```sv
<script>
// these are readonly
export const thisIs = 'readonly';
export function greet(name) {
alert(`hello ${name}!`);
}
// this is a prop
export let format = n => n.toFixed(2);
</script>
```
---
You can use reserved words as prop names.
```sv
<script>
let className;
// creates a `class` property, even
// though it is a reserved word
export { className as class };
</script>
```
##### 2. Assignments are 'reactive'
---
To change component state and trigger a re-render, just assign to a locally declared variable.
Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.
Because Svelte's reactivity is based on assignments, using array methods like `.push()` and `.splice()` won't automatically trigger updates. Options for getting around this can be found in the [tutorial](tutorial/updating-arrays-and-objects).
```sv
<script>
let count = 0;
function handleClick () {
// calling this function will trigger an
// update if the markup references `count`
count = count + 1;
}
</script>
```
##### 3. `$:` 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.
```sv
<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 ${title}`);
}
</script>
```
---
Only values which directly appear within the `$:` block will become dependencies of the reactive statement. For example, in the code below `total` will only update when `x` changes, but not `y`.
```sv
<script>
let x = 0;
let y = 0;
function yPlusAValue(value) {
return value + y;
}
$: total = yPlusAValue(x);
</script>
Total: {total}
<button on:click={() => x++}>
Increment X
</button>
<button on:click={() => y++}>
Increment Y
</button>
```
---
If a statement consists entirely of an assignment to an undeclared variable, Svelte will inject a `let` declaration on your behalf.
```sv
<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 an 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.
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 `$` prefix.
```sv
<script>
import { writable } from 'svelte/store';
const count = writable(0);
console.log($count); // logs 0
count.set(1);
console.log($count); // logs 1
$count = 2;
console.log($count); // logs 2
</script>
```
##### Store contract
```js
store = { subscribe: (subscription: (value: any) => void) => (() => void), set?: (value: any) => void }
```
You can create your own stores without relying on [`svelte/store`](docs#svelte_store), by implementing the *store contract*:
1. 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.
2. The `.subscribe` method must return an unsubscribe function. Calling an unsubscribe function must stop its subscription, and its corresponding subscription function must not be called again by the store.
3. 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*.
For interoperability with RxJS Observables, the `.subscribe` method is also allowed to return an object with an `.unsubscribe` method, rather than return the unsubscription function directly. Note however that unless `.subscribe` synchronously calls the subscription (which is not required by the Observable spec), Svelte will see the value of the store as `undefined` until it does.
### &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](docs#svelte_store).
```sv
<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`).
```sv
<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.
```sv
<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>
```
---
If you want to make @keyframes that are accessible globally, you need to prepend your keyframe names with `-global-`.
The `-global-` part will be removed when compiled, and the keyframe then be referenced using just `my-animation-name` elsewhere in your code.
```html
<style>
@keyframes -global-my-animation-name {...}
</style>
```