https://www.martenjulian.com/wp-content/themes/website-theme/js/overlay.js

1(function() {
2	var triggerBttn = document.getElementById( 'nav-open' ),
3		overlay = document.querySelector( 'div.overlay' ),
4		closeBttn = overlay.querySelector( 'button.overlay-close' );
5		transEndEventNames = {
6			'WebkitTransition': 'webkitTransitionEnd',
7			'MozTransition': 'transitionend',
8			'OTransition': 'oTransitionEnd',
9			'msTransition': 'MSTransitionEnd',
10			'transition': 'transitionend'
11		},
12		transEndEventName = transEndEventNames[ Modernizr.prefixed( 'transition' ) ],
13		support = { transitions : Modernizr.csstransitions };
14
15	function toggleOverlay() {
16		if( classie.has( overlay, 'open' ) ) {
17			classie.remove( overlay, 'open' );
18			classie.add( overlay, 'close' );
19			var onEndTransitionFn = function( ev ) {
20				if( support.transitions ) {
21					if( ev.propertyName !== 'visibility' ) return;
22					this.removeEventListener( transEndEventName, onEndTransitionFn );
23				}
24				classie.remove( overlay, 'close' );
25			};
26			if( support.transitions ) {
27				overlay.addEventListener( transEndEventName, onEndTransitionFn );
28			}
29			else {
30				onEndTransitionFn();
31			}
32		}
33		else if( !classie.has( overlay, 'close' ) ) {
34			classie.add( overlay, 'open' );
35		}
36	}
37
38	triggerBttn.addEventListener( 'click', toggleOverlay );
39	closeBttn.addEventListener( 'click', toggleOverlay );
40})();