javascript 的 clientX用法

<!DOCTYPE html>
<html lang="en">
<head>
   <meta charset="UTF-8">
   <title>测试</title>
</head>
<body >

<div id="div1" style=" 100px; height: 100px; position: absolute;"></div>

<script>
   window.onload=function(){
       var oDiv=document.getElementById('div1');
       document.onmousemove=function(ev){
           var oEvent=ev||event;
           var scrollTop=document.documentElement.scrollTop||document.body.scrollTop; //解决chrome下scrollTop兼容性问题
           var scrollLeft=document.documentElement.scrollLeft||document.body.scrollLeft;
            oDiv.style.top=oEvent.clientY+scrollTop+'px'; //clientX与scrollTop一般要同时使用
            oDiv.style.left=oEvent.clientX+scrollLeft+'px';
       }
   }
</script>
</body>
</html>