js实现鼠标拖动图片并兼容IE/FF火狐/谷歌等主流浏览器

<script language="javascript" type="text/javascript"> 
var isDrag = false; 
function isIE(){ 
if(navigator.userAgent.indexOf("MSIE")>0){return true;} 
else{return false;} 
} 
function addListener(element,e,fn){ 
if(isIE()){element.attachEvent("on" + e,fn);} 
else{element.addEventListener(e,fn,false);} 
} 
function drag(e){ 
var e = e || window.event; 
var element = e.srcElement || e.target; 
if(e.preventDefault)e.preventDefault(); 
else e.returnvalue=false; 
isDrag = true; 
var relLeft = e.clientX - parseInt(element.style.left); 
var relTop = e.clientY - parseInt(element.style.top); 
element.onmouseup = function(){ isDrag = false; } 
document.onmousemove = function(e_move){ 
var e_move = e_move || window.event; 
if(isDrag){ 
element.style.left=e_move.clientX - relLeft + "px"; 
element.style.top=e_move.clientY - relTop + "px"; 
return false; 
} 
} 
} 
window.onload = function() 
{ 
var element = document.getElementById("elimg"); 
var element2 = document.getElementById("eldiv"); 
addListener(element,"mousedown",drag); 
addListener(element2,"mousedown",drag); 
} 
</script> 
<div id="eldiv" style="width:140px;height:100px;background:#EEE;position:absolute;left:117px;top:124px"></div> 
<img id="elimg" style="left:609px;top:113px;position:absolute" src="http://www.cnit618.com/template/images/logo.gif" />