JS: Highlight current menu item on fixed nav as user scrolls

JS: Highlight current menu item on fixed nav as user scrolls

// Highlight current page menu item as user scrolls
var lastId,
    pageMenu = $("nav"),
    pageMenuHeight = pageMenu.outerHeight(),
    // All list items
    menuItems = pageMenu.find("a"),
    // Anchors corresponding to menu items
    scrollItems = menuItems.map(function(){
      var item = $($(this).attr("href"));
      if (item.length) { return item; }
    });

// Bind to scroll
$(window).scroll(function(){
   // Get container scroll position
   var fromTop = $(this).scrollTop()+pageMenuHeight;
   
   // 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");
   }                   
});