svelte/test/runtime/samples/transition-js-deferred/main.html

<script>
	export let visible;

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

{#if visible}
	<div transition:foo></div>
{/if}