svelte/test/runtime/samples/event-handler-custom-context/main.html

<script>
	export let z = '???';
	export let answer = '42';

	function tap(node, callback) {
		const clickHandler = event => {
			callback({ answer });
		};

		node.addEventListener( 'click', clickHandler, false );

		return {
			destroy () {
				node.addEventListener( 'click', clickHandler, false );
			}
		};
	}
</script>

<button on:tap='{event => z = event.answer}'>{z}</button>