var menu = document.getElementById('menu');
var link_list = menu.getElementsByTagName('a');

for (var i = 0; i < link_list.length; i++) {
  var this_link = link_list[i].id;
  var copy = this_link.substring(this_link.indexOf("_") + 1);

  if (link_list[i].className == 'active' ||
      location == link_list[i].href) {
    link_list[i].className = 'active'
    break;
  }
}