两行JavaScript代码的QQ窗口抖动效果

<img id="win" style='position:relative' src="/article/UploadPic/20089/6/200896124734762.jpg"> <br /><br /> <button onclick="zd()">振动</button> <script > function zd(u){ var a=['top','left'],b=0; u=setInterval(function(){ document.getElementById('win').style[a[b%2]]=(b++)%4<2?0:4; if(b>15){clearInterval(u);b=0} },32) } </script> [提示：你可先修改部分代码，再按运行]