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

{{#if visible}}
	<div in:fade>fades in</div>
{{/if}}

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