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88 lines
2.4 KiB
88 lines
2.4 KiB
2 months ago
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---
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title: Reactive $: statements
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---
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In runes mode, reactions to state updates are handled with the [`$derived`]($derived) and [`$effect`]($effect) runes.
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In legacy mode, any top-level statement (i.e. not inside a block or a function) can be made reactive by prefixing it with a `$:` [label](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/label). These statements run after other code in the `<script>` and before the component markup is rendered, then whenever the values that they depend on change.
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```svelte
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<script>
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let a = 1;
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let b = 2;
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// this is a 'reactive statement', and it will re-run
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// when `a`, `b` or `sum` change
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$: console.log(`${a} + ${b} = ${sum}`);
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// this is a 'reactive assignment' — `sum` will be
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// recalculated when `a` or `b` change. It is
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// not necessary to declare `sum` separately
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$: sum = a + b;
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</script>
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```
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Statements are ordered _topologically_ by their dependencies and their assignments: since the `console.log` statement depends on `sum`, `sum` is calculated first even though it appears later in the source.
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Multiple statements can be combined by putting them in a block:
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```js
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// @noErrors
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$: {
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// recalculate `total` when `items` changes
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total = 0;
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for (const item of items) {
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total += item.value;
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}
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}
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```
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The left-hand side of a reactive assignments can be an identifier, or it can be a destructuring assignment:
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```js
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// @noErrors
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$: ({ larry, moe, curly } = stooges);
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```
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## Understanding dependencies
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The dependencies of a `$:` statement are determined at compile time — they are whichever variables are referenced (but not assigned to) inside the statement.
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In other words, a statement like this will _not_ re-run when `count` changes, because the compiler cannot 'see' the dependency:
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```js
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// @noErrors
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let count = 0;
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let double = () => count * 2;
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$: doubled = double();
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```
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Similarly, topological ordering will fail if dependencies are referenced indirectly: `z` will never update, because `y` is not considered 'dirty' when the update occurs. Moving `$: z = y` below `$: setY(x)` will fix it:
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```svelte
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<script>
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let x = 0;
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let y = 0;
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$: z = y;
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$: setY(x);
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function setY(value) {
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y = value;
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}
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</script>
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```
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## Browser-only code
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Reactive statements run during server-side rendering as well as in the browser. This means that any code that should only run in the browser must be wrapped in an `if` block:
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```js
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// @noErrors
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$: if (browser) {
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document.title = title;
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}
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```
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