$$props and $$restProps

In runes mode, getting an object containing all the props that were passed in is easy, using the $props rune.

In legacy mode, we use $$props and $$restProps:

• $$props contains all the props that were passed in, including ones that are not individually declared with the export keyword
• $$restProps contains all the props that were passed in except the ones that were individually declared

For example, a <Button> component might need to pass along all its props to its own <button> element, except the variant prop:

<script> export let variant; </script>  <button {...$$restProps} class="variant-{variant} {$$props.class ?? ''}"> click me </button>  <style> .variant-danger { background: red; } </style>

In Svelte 3/4 using $$props and $$restProps creates a modest performance penalty, so they should only be used when needed.