svelte/test/runtime/samples/transition-js-each-block-keyed-intro-outro/main.html

{#each things as thing (thing.name)}
	<div transition:foo>{thing.name}</div>
{/each}

<script>
	export default {
		transitions: {
			foo: function ( node, params ) {
				return {
					duration: 100,
					tick: t => {
						node.foo = t;
					}
				};
			}
		}
	};
</script>