svelte/documentation/docs/99-legacy/21-legacy-$$slots.md

title
$$slots

$$slots is an object whose keys are the names of the slots passed into the component by the parent. If the parent does not pass in a slot with a particular name, that name will not be present in $$slots. This allows components to render a slot (and other elements, like wrappers for styling) only if the parent provides it.

Note that explicitly passing in an empty named slot will add that slot's name to $$slots. For example, if a parent passes <div slot="title" /> to a child component, $$slots.title will be truthy within the child.

<!-- Card.svelte -->
<div>
	<slot name="title" />
	{#if $$slots.description}
		<!-- This <hr> and slot will render only if a slot named "description" is provided. -->
		<hr />
		<slot name="description" />
	{/if}
</div>

<!-- App.svelte -->
<Card>
	<h1 slot="title">Blog Post Title</h1>
	<!-- No slot named "description" was provided so the optional slot will not be rendered. -->
</Card>

[!NOTE] In Svelte 5+, this concept is obsolete, as you pass snippets as component props and can check whether or not that prop is set