class

There are two ways to set classes on elements: the class attribute, and the class: directive.

Attributes

Primitive values are treated like any other attribute:

<div class={large ? 'large' : 'small'}>...</div>

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.

Objects and arrays

Since Svelte 5.16, class can be an object or array, and is converted to a string using clsx.

If the value is an object, the truthy keys are added:

<script>
	let { cool } = $props();
</script>

<!-- results in `class="cool"` if `cool` is truthy,
	 `class="lame"` otherwise -->
<div class={{ cool, lame: !cool }}>...</div>

If the value is an array, the truthy values are combined:

<!-- if `faded` and `large` are both truthy, results in
	 `class="saturate-0 opacity-50 scale-200"` -->
<div class={[faded && 'saturate-0 opacity-50', large && 'scale-200']}>...</div>

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.

Arrays can contain arrays and objects, and clsx will flatten them. This is useful for combining local classes with props, for example:

<script>
	let props = $props();
</script>

<button {...props} class={['cool-button', props.class]}>
	{@render props.children?.()}
</button>

<script lang="ts">
	let props = $props();
</script>

<button {...props} class={['cool-button', props.class]}>
	{@render props.children?.()}
</button>

The user of this component has the same flexibility to use a mixture of objects, arrays and strings:

<script>
	import Button from './Button.svelte';
	let useTailwind = $state(false);
</script>

<Button
	onclick={() => useTailwind = true}
	class={{ 'bg-blue-700 sm:w-1/2': useTailwind }}
>
	Accept the inevitability of Tailwind
</Button>

<script lang="ts">
	import Button from './Button.svelte';
	let useTailwind = $state(false);
</script>

<Button
	onclick={() => useTailwind = true}
	class={{ 'bg-blue-700 sm:w-1/2': useTailwind }}
>
	Accept the inevitability of Tailwind
</Button>

The class: directive

Prior to Svelte 5.16, the class: directive was the most convenient way to set classes on elements conditionally.

<!-- These are equivalent -->
<div class={{ cool, lame: !cool }}>...</div>
<div class:cool={cool} class:lame={!cool}>...</div>

As with other directives, we can use a shorthand when the name of the class coincides with the value:

<div class:cool class:lame={!cool}>...</div>

Unless you’re using an older version of Svelte, consider avoiding class:, since the attribute is more powerful and composable.