jQuery判断鼠标滚动方向

 var scrolltop = new Array();
 var index = 0;
 scrolltop[0] = 0;
 $(document).scroll(function(){
      index ++;
      scrolltop[index] = $(document).scrollTop();
      if (scrolltop[index] > scrolltop[index-1]) {
             console.log("scroll down");
      }else{
             console.log("scroll up");
      };
 })