closest is a polyfill for #Element.closest.

The #Element.closest method returns the closest ancestor of the current element (or the current element itself) which matches the selectors given in parameter. If there isn't such an ancestor, it returns null.

element.closest(selectorString) //=> element

This is especially useful for delegating events.

document.addEventListener('click', function (event) {
	// find nearest element up the tree that is an <a>
	var link = event.target.closest('a');

	if (link) {
		// do something with the <a>
		event.preventDefault();
	}
});

The also polyfills #Element.matches, which is widely supported but often vendor-prefixed.

element.matches(selectorString) //=> boolean

matches is especially useful for short-handing hasAttribute or classList.contains with selectors.

var widget = document.querySelector('.custom-widget');

if (widget.matches('[data-active]') || widget.matches('.widget--active')) {
	// do something with the active widget
}

closest is especially useful for delegating events, but remember that Internet Explorer 8 does not support #Element.addEventListener.