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");
     };
})