var prevScrollpos = window.pageYOffset;
      window.onscroll=function(){scrollevent()};
      function scrollevent()
      {
           var currentScrollPos = window.pageYOffset;
           if (prevScrollpos < currentScrollPos)
           {
               document.getElementById("navbar").className = "navup";
           }
           else
           {
               document.getElementById("navbar").className = "navdown";
           }
           prevScrollpos = currentScrollPos;
      }