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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.
```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
---
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.
```html
<script>
import { writable } from 'svelte/store';
const count = writable(0);
console.log($count); // logs 0
count.set(1);
console.log($count); // logs 1
</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>
```