鼠标滚轮事件

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
"http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
    <meta http-equiv="content-type" content="text/html; charset=UTF-8"/>
    <title>无标题文档</title>
    <style type="text/css">
        div{
            height: 800px;
        }
    </style>
    <script type="text/javascript" src="http://apps.bdimg.com/libs/jquery/2.1.4/jquery.min.js"></script>
    <script type="text/javascript" src="jquery-3.1.1.js"></script>
    <script type="text/javascript">
        $('document').ready(function(){
            var lastScrollTop = 0;
            $(window).scroll(function(event){
               var st = $(this).scrollTop();
               if (st > lastScrollTop){
                   // downscroll code
                   alert('向下滚动');
               } else {
                  // upscroll code
                   alert('向上滚动');
               }
               lastScrollTop = st;
            });
        });
    </script>
</head>
<body>
    <div></div>
</body>
<html>