Не работает jquery.Не пойму в чем проблема

 <script src="https://cdnjs.cloudflare.com/ajax/libs/material-design-lite/1.3.0/material.min.js"></script> 
 <script>// Hide Header on on scroll down 
    var didScroll; 
    var lastScrollTop = 0; 
    var delta = 5; 
    var navbarHeight = $('footer').outerHeight(); 
     
    $(window).scroll(function(event){ 
        didScroll = true; 
    }); 
     
    setInterval(function() { 
        if (didScroll) { 
            hasScrolled(); 
            didScroll = false; 
        } 
    }, 250); 
     
    function hasScrolled() { 
        var st = $(this).scrollTop(); 
         
        // Make sure they scroll more than delta 
        if(Math.abs(lastScrollTop - st) <= delta) 
            return; 
         
        // If they scrolled down and are past the navbar, add class .nav-up. 
        // This is necessary so you never see what is "behind" the navbar. 
        if (st > lastScrollTop && st > navbarHeight){ 
            // Scroll Down 
            $('footer').removeClass('nav-down').addClass('nav-up'); 
        } else { 
            // Scroll Up 
            if(st + $(window).height() < $(document).height()) { 
                $('footer').removeClass('nav-up').addClass('nav-down'); 
            } 
        } 
         
        lastScrollTop = st; 
    } 
 </script>