mirror of https://github.com/sveltejs/svelte
feat: allow objects/arrays for class attribute (#14714)
* WIP * missed * fix * fix * rename, smooth over incompatibilities * spread support + test * docs * types * implement CSS pruning for array/object expressions * beefier static analysis * lint * rename doc * move class after all directive docs * tweak docs - clarify top-level falsy values, stagger examples, demonstrate composition, discourage class: more strongly * changeset * fix * Update documentation/docs/03-template-syntax/18-class.md Co-authored-by: Conduitry <git@chor.date> * Apply suggestions from code review --------- Co-authored-by: Rich Harris <rich.harris@vercel.com> Co-authored-by: Conduitry <git@chor.date>pull/14825/head
Simon H
9 months ago
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GitHub
parent
38a3ae321f
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015210a1a8
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---
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'svelte': minor
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---
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feat: allow `class` attribute to be an object or array, using `clsx`
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@ -1,23 +0,0 @@
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---
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title: class:
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---
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The `class:` directive is a convenient way to conditionally set classes on elements, as an alternative to using conditional expressions inside `class` attributes:
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```svelte
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<!-- These are equivalent -->
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<div class={isCool ? 'cool' : ''}>...</div>
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<div class:cool={isCool}>...</div>
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```
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As with other directives, we can use a shorthand when the name of the class coincides with the value:
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```svelte
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<div class:cool>...</div>
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```
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Multiple `class:` directives can be added to a single element:
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```svelte
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<div class:cool class:lame={!cool} class:potato>...</div>
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```
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@ -0,0 +1,90 @@
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---
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title: class
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---
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There are two ways to set classes on elements: the `class` attribute, and the `class:` directive.
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## Attributes
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Primitive values are treated like any other attribute:
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```svelte
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<div class={large ? 'large' : 'small'}>...</div>
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```
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> [!NOTE]
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> For historical reasons, falsy values (like `false` and `NaN`) are stringified (`class="false"`), though `class={undefined}` (or `null`) cause the attribute to be omitted altogether. In a future version of Svelte, all falsy values will cause `class` to be omitted.
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### Objects and arrays
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Since Svelte 5.16, `class` can be an object or array, and is converted to a string using [clsx](https://github.com/lukeed/clsx).
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If the value is an object, the truthy keys are added:
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```svelte
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<script>
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let { cool } = $props();
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</script>
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<!-- results in `class="cool"` if `cool` is truthy,
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`class="lame"` otherwise -->
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<div class={{ cool, lame: !cool }}>...</div>
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```
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If the value is an array, the truthy values are combined:
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```svelte
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<!-- if `faded` and `large` are both truthy, results in
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`class="saturate-0 opacity-50 scale-200"` -->
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<div class={[faded && 'saturate-0 opacity-50', large && 'scale-200']}>...</div>
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```
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Note that whether we're using the array or object form, we can set multiple classes simultaneously with a single condition, which is particularly useful if you're using things like Tailwind.
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Arrays can contain arrays and objects, and clsx will flatten them. This is useful for combining local classes with props, for example:
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```svelte
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<!--- file: Button.svelte --->
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<script>
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let props = $props();
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</script>
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<button {...props} class={['cool-button', props.class]}>
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{@render props.children?.()}
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</button>
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```
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The user of this component has the same flexibility to use a mixture of objects, arrays and strings:
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```svelte
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<!--- file: App.svelte --->
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<script>
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import Button from './Button.svelte';
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let useTailwind = $state(false);
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</script>
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<Button
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onclick={() => useTailwind = true}
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class={{ 'bg-blue-700 sm:w-1/2': useTailwind }}
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>
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Accept the inevitability of Tailwind
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</Button>
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```
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## The `class:` directive
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Prior to Svelte 5.16, the `class:` directive was the most convenient way to set classes on elements conditionally.
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```svelte
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<!-- These are equivalent -->
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<div class={{ cool, lame: !cool }}>...</div>
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<div class:cool={cool} class:lame={!cool}>...</div>
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```
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As with other directives, we can use a shorthand when the name of the class coincides with the value:
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```svelte
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<div class:cool class:lame={!cool}>...</div>
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```
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> [!NOTE] Unless you're using an older version of Svelte, consider avoiding `class:`, since the attribute is more powerful and composable.
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import { test } from '../../test';
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export default test({
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warnings: [
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{
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code: 'css_unused_selector',
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message: 'Unused CSS selector ".unused"\nhttps://svelte.dev/e/css_unused_selector',
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start: {
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line: 24,
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column: 1,
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character: 548
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},
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end: {
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line: 24,
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column: 8,
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character: 555
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}
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}
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]
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});
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.used1.svelte-xyz { color: green; }
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.used2.svelte-xyz { color: green; }
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.used3.svelte-xyz { color: green; }
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.used4.svelte-xyz { color: green; }
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.used5.svelte-xyz { color: green; }
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.used6.svelte-xyz { color: green; }
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.used7.svelte-xyz { color: green; }
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.used8.svelte-xyz { color: green; }
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.used9.svelte-xyz { color: green; }
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/* (unused) .unused { color: red; }*/
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<script>
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let condition = Math.random() < 0.5;
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</script>
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<p class={['used1']}></p>
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<p class={[{ used2: true }]}></p>
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<p class={{ used3: true }}></p>
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<p class={{ 'used4 used5': true }}></p>
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<p class={{ used6 }}></p>
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<p class={[condition ? 'used7' : 'used8']}></p>
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<p class={[condition && 'used9']}></p>
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<style>
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.used1 { color: green; }
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.used2 { color: green; }
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.used3 { color: green; }
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.used4 { color: green; }
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.used5 { color: green; }
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.used6 { color: green; }
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.used7 { color: green; }
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.used8 { color: green; }
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.used9 { color: green; }
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.unused { color: red; }
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</style>
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.x.svelte-xyz { color: green; }
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<h1 class={[foo]}>hello world</h1>
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<style>
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.x { color: green; }
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</style>
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.x.svelte-xyz { color: green; }
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<h1 class={{ foo: true, ...rest }}>hello world</h1>
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<style>
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.x { color: green; }
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</style>
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.x.svelte-xyz { color: green; }
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<h1 class={{ [foo]: true }}>hello world</h1>
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<style>
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.x { color: green; }
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</style>
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import { test } from '../../test';
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export default test({
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html: `
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<button>update</button>
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`,
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test({ assert, target }) {
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const button = target.querySelector('button');
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button?.click();
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assert.htmlEqual(
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target.innerHTML,
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`
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo svelte-owbekl"></div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<div class="foo">child</div>
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<button>update</button>
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`
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);
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}
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});
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<script>
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let { class: cls } = $props();
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</script>
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<div class={cls}>child</div>
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<script>
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import Child from "./child.svelte";
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let foo = $state('foo');
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let bar = $state(null);
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let spread = { class: { foo: true, bar: false } };
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</script>
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<div class={{ foo: true, bar: false }}></div>
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<div class={['foo', false && 'bar']}></div>
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<div class={{ foo, bar }}></div>
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<div class={[ foo, bar ]}></div>
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<div {...spread}></div>
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<Child class={{ foo: true, bar: false }} />
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<Child class={['foo', false && 'bar']} />
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<Child class={{ foo, bar }} />
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<Child class={[ foo, bar ]} />
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<Child {...spread} />
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<button onclick={() => {
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foo = null;
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bar = 'bar';
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}}>update</button>
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<style>
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.foo {
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color: red;
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}
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.bar {
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color: blue;
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}
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</style>
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