smoothAsButter.js

function getYfromElem(query) {
	return window.pageYOffset + document.querySelector(query).getBoundingClientRect().top;
}

function smoothAsButter(elem, dur) {
	var startY = window.pageYOffset;
	var elemY = getYfromElem(elem);
	var resultY = document.body.scrollHeight - elemY < window.innerHeight ? document.body.scrollHeight - window.innerHeight : elemY;
	var diff = resultY - startY;

	var easing = function(t) {
		return t < .5 ? 4 * t * t * t : (t-1) * (2 * t - 2) * (2 * t - 2) + 1;
	}

	var start;

	if (!diff) {
		return;
	}

	window.requestAnimationFrame(function step(ts) {
		if (!start) {
			start = ts;
		}

		var time = ts - start;
		var percentage = Math.min(time / dur, 1);

		percentage = easing(percentage);

		window.scrollTo(0, startY + diff * percentage);

		if (time < dur) {
			window.requestAnimationFrame(step);
		}
	});
}

// Usage
// smoothAsButter('#targetidhere', duration in ms here)