svelte/documentation/examples/14-classes/00-classes/App.svelte

<script>
	let current = 'foo';
</script>

<button class:active={current === 'foo'} on:click={() => (current = 'foo')}>foo</button>

<button class:active={current === 'bar'} on:click={() => (current = 'bar')}>bar</button>

<button class:active={current === 'baz'} on:click={() => (current = 'baz')}>baz</button>

<style>
	button {
		display: block;
	}

	.active {
		background-color: #ff3e00;
		color: white;
	}
</style>