headings = [];

onload = function(){
  headings = document.querySelectorAll('h2');
};

onscroll = function(e){
  var heading = find(window.scrollY);
  if (!heading) return;
  var links = document.querySelectorAll('#menu a')
    , link;

  for (var i = 0, len = links.length; i < len; ++i) {
    link = links[i];
    link.className = link.getAttribute('href') == '#' + heading.id
      ? 'active'
      : '';
  }
};

function find(y) {
  var i = headings.length
    , heading;

  while (i--) {
    heading = headings[i];
    if (y > heading.offsetTop) {
      return heading;
    }
  }
}