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196 lines
4.9 KiB
196 lines
4.9 KiB
---
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title: Component format
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---
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---
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Components are the building blocks of Svelte applications. They are written into `.svelte` files, using a superset of HTML.
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All three sections — script, styles and markup — are optional.
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```html
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<script>
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// logic goes here
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</script>
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<style>
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/* styles go here */
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</style>
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<!-- markup (zero or more items) goes here -->
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```
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### <script>
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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:
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##### 1. `export` creates a component prop
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---
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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:
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```html
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<script>
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// these properties can be set externally
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export let foo;
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export let bar = 'optional default value';
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// Values that are passed in as props
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// are immediately available
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console.log(foo, bar);
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// function declarations cannot be set externally,
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// but can be accessed from outside
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export function instanceMethod() {
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alert(foo);
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}
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</script>
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```
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##### 2. Assignments are 'reactive'
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---
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To change component state and trigger a re-render, just assign to a locally declared variable.
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Update expressions (`count += 1`) and property assignments (`obj.x = y`) have the same effect.
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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).
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```html
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<script>
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let count = 0;
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function handleClick () {
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// calling this function will trigger a re-render
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// if the markup references `count`
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count = count + 1;
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}
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</script>
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```
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##### 3. `$:` marks a statement as reactive
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---
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Any top-level statement (i.e. not inside a block or a function) can be made reactive by prefixing it with the `$:` label. Reactive statements run immediately before the component updates, whenever the values that they depend on have changed.
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```html
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<script>
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export let title;
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// this will update `document.title` whenever
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// the `title` prop changes
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$: document.title = title;
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$: {
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console.log(`multiple statements can be combined`);
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console.log(`the current title is ${title}`);
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}
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</script>
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```
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---
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If a statement consists entirely of an assignment to an undeclared variable, Svelte will inject a `let` declaration on your behalf.
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```html
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<script>
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export let num;
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// we don't need to declare `squared` and `cubed`
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// — Svelte does it for us
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$: squared = num * num;
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$: cubed = squared * num;
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</script>
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```
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##### 4. Prefix stores with `$` to access their values
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---
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Any time you have a reference to a store, you can access its value inside a component by prefixing it with the `$` character. This causes Svelte to declare the prefixed variable, and set up a store subscription that will be unsubscribed when appropriate.
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Note that the store must be declared at the top level of the component — not inside an `if` block or a function, for example.
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Local variables (that do not represent store values) must *not* have a `$` prefix.
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```html
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<script>
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import { writable } from 'svelte/store';
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const count = writable(0);
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console.log($count); // logs 0
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count.set(1);
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console.log($count); // logs 1
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</script>
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```
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### <script context="module">
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---
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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.
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You can `export` bindings from this block, and they will become exports of the compiled module.
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You cannot `export default`, since the default export is the component itself.
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```html
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<script context="module">
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let totalComponents = 0;
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// this allows an importer to do e.g.
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// `import Example, { alertTotal } from './Example.svelte'`
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export function alertTotal() {
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alert(totalComponents);
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}
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</script>
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<script>
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totalComponents += 1;
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console.log(`total number of times this component has been created: ${totalComponents}`);
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</script>
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```
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### <style>
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---
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CSS inside a `<style>` block will be scoped to that component.
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This works by adding a class to affected elements, which is based on a hash of the component styles (e.g. `svelte-123xyz`).
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```html
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<style>
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p {
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/* this will only affect <p> elements in this component */
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color: burlywood;
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}
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</style>
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```
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---
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To apply styles to a selector globally, use the `:global(...)` modifier.
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```html
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<style>
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:global(body) {
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/* this will apply to <body> */
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margin: 0;
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}
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div :global(strong) {
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/* this will apply to all <strong> elements, in any
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component, that are inside <div> elements belonging
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to this component */
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color: goldenrod;
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}
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</style>
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```
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