svelte/test/runtime/samples/transition-js-if-else-block-intro/main.html

{#if x}
	<div ref:yes in:foo>yes</div>
{:else}
	<div ref:no in:foo>no</div>
{/if}

<script>
	export default {
		transitions: {
			foo: function ( node, params ) {
				return {
					duration: 400,
					tick: t => {
						node.foo = t;
					}
				};
			}
		}
	};
</script>