Basic offset function, replacing jQuery's offset.

offset.js

function offset(elt) {
	var rect = elt.getBoundingClientRect(), bodyElt = document.body;

	return {
 		top: rect.top + bodyElt .scrollTop,
		left: rect.left + bodyElt .scrollLeft
	}
}

//use:
var element = getElementById('element');
var offsetElement = offset(element);

Sometimes document.body will not give scrollTop properly. If you replace document.body with document.documentElement it works more reliably. jQuery does the same.