scrollspy.js

function getPosY(e) {
	var top = 0;
	while (e) {
		top += parseInt(e.offsetTop) || 0;
		e = e.offsetParent;
	}
	return top;
}
var lastTop = null;
var iAmHere;

setInterval(function () { 
	var c = 0,
		as = tocDiv.getElementsByTagName('a'),
		top = parseInt(window.pageYOffset || window.scrollY || document.documentElement.scrollTop) || 0;
	if (top != lastTop) {
		for (var a, i = 0; a = as[i]; i++) {
			var e, hash = a.href.substr(a.href.indexOf('#') +1);
			if (e = document.getElementById(hash)) {
				if ((getPosY(e) <= top+100)) {
					c = i;
				}
			}
		}
		if (iAmHere) iAmHere.id = ''; (iAmHere = as[c]).id = 'here';
		strzalka.style.top = 0 + tocHeight / as.length * c + 'px';
	}
	lastTop = top;
}, 400);