scrollToPos ( yPos, opts, after );

scroll.js

/* 
Props to emilejs for nice handling of easing and callbacks.

TODO:	
opts.el - calculate positions
(in our case, we do that outside of this function already) 
IE8+ ... el = (typeof el === 'string') ? document.querySelector(el) : el

use window.requestAnimationFrame instead of setInterval
- with polyfill of course

*/
function scrollToPos(yPos, opts, after){
	
	var opts = opts || {},
		duration = opts.duration || 600, 
		threshold = opts.threshold || 100,
		easing = opts.easing || function(pos){return(-Math.cos(pos*Math.PI)/2)+0.5;},
		startTime = +new Date, 
		finish = startTime + duration,
		startY = isNaN(window.pageYOffset) ? document.documentElement.scrollTop : window.pageYOffset,
		distance = yPos - startY;

	if (Math.abs(distance) < threshold) {
		window.scrollTo(0, yPos);
		return;
	}

	var interval = setInterval(function(){
		var time = +new Date, 
			pos = time > finish ? 1 : (time-startTime) / duration;
		
		window.scrollTo(0, startY + distance * easing(pos));

		if(time > finish) { 
			clearInterval(interval);
			opts.after && opts.after();
			after && setTimeout(after,1);
		}
	}, 15);
};