In this app, we have a <Hoverable> component that tracks whether the mouse is currently over it. It needs to pass that data back to the parent component, so that we can update the slotted contents.

For this, we use slot props. In Hoverable.svelte, pass the hovering value into the slot:

<div on:mouseenter={enter} on:mouseleave={leave}>
  <slot hovering={hovering}></slot>
</div>

Remember you can also use the {hovering} shorthand, if you prefer.

Then, to expose hovering to the contents of the <Hoverable> component, we use the let directive:

<Hoverable let:hovering={hovering}>
  <div class:active={hovering}>
    {#if hovering}
      <p>I am being hovered upon.</p>
    {:else}
      <p>Hover over me!</p>
    {/if}
  </div>
</Hoverable>

You can rename the variable, if you want — let's call it active in the parent component:

<Hoverable let:hovering={active}>
  <div class:active>
    {#if active}
      <p>I am being hovered upon.</p>
    {:else}
      <p>Hover over me!</p>
    {/if}
  </div>
</Hoverable>

You can have as many of these components as you like, and the slotted props will remain local to the component where they're declared.

Named slots can also have props; use the let directive on an element with a slot="..." attribute, instead of on the component itself.