Active menu item on scroll.js

// Active menu on scroll
var lastId,
  topMenu = $('.main-menu'),
  topMenuHeight = topMenu.outerHeight(),
  // All list items
  menuItems = topMenu.find('a'),
  // Anchors corresponding to menu items
  scrollItems = menuItems.map(function(){
    var item = $($(this).attr('href'));
    if (item.length) { return item; }
  });

//Bind click handler to menu items
//so we can get a fancy scroll animation
menuItems.click(function(e){
  var href = $(this).attr('href'),
    offsetTop = href === '#' ? 0 : $(href).offset().top - topMenuHeight;
  $('html, body').stop().animate({ 
    scrollTop: offsetTop
  }, 300);
  e.preventDefault();
});

// Bind to scroll
$(window).scroll(function(){
  // Get container scroll position
  var fromTop = $(this).scrollTop() + topMenuHeight;
  // Get id of current scroll item
  var cur = scrollItems.map(function(){
    if ($(this).offset().top < fromTop)
      return this;
  });
  // Get the id of the current element
  cur = cur[cur.length - 1];
  var id = cur && cur.length ? cur[0].id : "";
  if (lastId !== id) {
    lastId = id;
    // Set/remove active class
    menuItems
    .parent().removeClass('active')
    .end().filter("[href='#" + id + "']").parent().addClass('active');
  }
});