Nav menu active class change.js

//Nav menu active class change
var lastId
    , topMenu = $(".menu")
    , topMenuHeight = topMenu.outerHeight() + 15
    ,menuItems = topMenu.find("a")
    ,scrollItems = menuItems.map(function () {
        var item = $($(this).attr("href"));
        if (item.length) {
            return item;
        }
    });

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