aync.example.js

// Let's get hold of those elements
var outer = document.querySelector('#outer');
var inner = document.querySelector('#inner');

// Let's listen for attribute changes on the
// outer element
new MutationObserver(() => {
	console.log('mutate');
}).observe(outer, {
	attributes: true
});

// Here's a click listener…
function onClick() {
	console.log('click');

	setTimeout(() => {
		console.log('timeout');
	}, 0);

	Promise.resolve().then(() => {
		console.log('promise');
	});
	outer.setAttribute('data-random', Math.random());
}

// attach handler to both elements
inner.addEventListener('click', onClick);
outer.addEventListener('click', onClick);