svelte/site/content/tutorial/13-advanced-styling/01-classes/text.md

title
The class directive

Like any other attribute, you can specify classes with a JavaScript attribute, seen here:

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

This is such a common pattern in UI development that Svelte includes a special directive to simplify it:

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

The selected class is added to the element whenever the value of the expression is truthy, and removed when it's falsy.