获取div的页面位置

<style type="text/css">
 body { margin:0px; }
 .map { width:100px; height:100px; position:absolute; background-color:#476D78; position:absolute; top:200px; left:500px; }
</style> 
<script type="text/javascript">
 document.onmousedown=function(){
  // 获取当前div的坐标
  var div=document.getElementById("mydiv");
  var top,left;
  if(div.currentStyle){left=div.currentStyle.left;top=div.currentStyle.top;}
  else if(window.getComputedStyle){left=window.getComputedStyle(div,null).left;top=window.getComputedStyle(div,null).top;}
  alert(left+","+top);
 }
</script>
<div class="map" id="mydiv">
 asdfasdfasdfasdf
</div>