{#if visible}
	<div transition:foo='{k: duration}'>fades in</div>
{/if}

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