Team:CIEI-BJ/js/nav/slow fix

       //缓动隐藏功能实现
       var new_scroll_position = 0;
       var last_scroll_position;
       var header = document.getElementById("header");

       window.addEventListener('scroll', function(e) {
           last_scroll_position = window.scrollY;

           // Scrolling down
           if (new_scroll_position < last_scroll_position && last_scroll_position > 80) {
               // header.removeClass('slideDown').addClass('slideUp');
               header.classList.remove("slideDown");
               header.classList.add("slideUp");

               // Scrolling up
           } else if (new_scroll_position > last_scroll_position) {
               // header.removeClass('slideUp').addClass('slideDown');
               header.classList.remove("slideUp");
               header.classList.add("slideDown");
           }

           new_scroll_position = last_scroll_position;
       });