Javascript 广告定位

<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title></title>
<style type="text/css">
  #ad{
   position:absolute;
   top:120px;
   left:50px;
   100px;
   height:200px;
   background-color:#333;
   color:#fff;
  }
</style>

<script type="text/javascript">
  window.onload=init;
  window.onscroll=go;
  var oldTop;

  function init(){

   var ad=document.getElementById("ad");

   //取对象原始的top值
   if(window.getComputedStyle){
    oldTop= parseInt( window.getComputedStyle(ad,null).top );
   }else{
    //IE
    oldTop= parseInt(ad.currentStyle.top);
   }
  }
  function go(){
   //浏览器滚动的距离
   var top=document.documentElement.scrollTop;

   document.getElementById("ad").style.top= parseInt(top)+oldTop +"px";

  }
  //关闭广告
  function closeAd(){
   var ad=document.getElementById("ad");
   ad.parentNode.removeChild(ad);
  }

</script>
</head>
<body>
<div id="ad"><a href="javascript:;" onclick="closeAd()">X</a><a class="close" onclick="this.parentNode.parentNode.removeChild(this.parentNode)">close</a>广告</div>
<div style="height:5000px;">sss</div>

</body>
</html>