var _els = document.getElementsByClassName('parallax'),
				_layers = [],
				_scrollY = window.scrollY;

			for (var i = 0; i < _els.length; i++) {
				var el = _els[i];
				var offset = el.dataset.offset;
				_layers.push({el: el, y: 0, offset: offset});
			};

			window.addEventListener('scroll', handle_scroll);


			function handle_scroll(e) {
				_scrollY = window.scrollY;
				animate();
			}

			function animate() {
				for (var i = 0; i < _layers.length; i++) {
					var oldY = _layers[i].y;
					_layers[i].y = _scrollY * _layers[i].offset;

					if (oldY !== _layers[i].y) {
						TweenMax.to(_layers[i].el, 0.5, {
							y: _layers[i].y,
							overwrite: 'all'
						});
					}
				}
			}