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

<div transition:foo></div>

<script>
	export default {
		data() {
			return { x: 42 };
		},

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