smooth_scroll.js

function smoothScroll(scrollTargetY, speed) {
	var nextTick,
		currentTime	= 0,
		scrollY		= window.scrollY || document.documentElement.scrollTop,
		time		= Math.max(0.1, Math.min(Math.abs(scrollY - scrollTargetY) / speed, 0.8));

	nextTick = window.requestAnimationFrame ||
		window.webkitRequestAnimationFrame ||
		window.mozRequestAnimationFrame ||
		function (callback) {
			window.setTimeout(callback, 1000 / 60);
		};

	function easeInOutQuint(pos) {
		if ((pos /= 0.5) < 1) {
			return 0.5 * Math.pow(pos, 5);
		}
		return 0.5 * (Math.pow((pos - 2), 5) + 2);
	}

	(function tick() {
		var p, t;

		currentTime += 1 / 60;

		p = currentTime / time;
		t = easeInOutQuint(p);

		if (p < 1) {
			nextTick(tick);
			window.scrollTo(0, scrollY + ((scrollTargetY - scrollY) * t));
		} else {
			window.scrollTo(0, scrollTargetY);
		}
	})();
}

smoothScroll(0, 1500);