js拖动原理

<!DOCTYPE html>
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>js拖动div修正版</title>
<script type="text/javascript">
var _move = false;
var _x,_y;
window.onload=function(event){
   document.getElementById('layer').onmousedown=function(event){
   event = event ? event : window.event;
//判断浏览器类型（针对ie）
   var which = navigator.userAgent.indexOf('MSIE') > 1 ? (event.button == 1 ? 1 : 0) : (event.which == 1 ? 1 : 0) ;
   if(which) {
     _move = true;
     _x = event.clientX - parseInt(document.getElementById('layer').style.left);
    _y = event.clientY - parseInt(document.getElementById('layer').style.top);
    }
}
}

document.onmousemove=function(event){
event = event ? event : window.event;
     if(_move) {
         var x = event.clientX - _x;
         var y = event.clientY - _y;
         document.getElementById('layer').style.left = x+'px';
         document.getElementById('layer').style.top = y+'px';
     }
}

document.onmouseup=function(){ _move = false; }
</script>
</head>
<body>
<div id="layer" style="position:absolute; 200px; height:200px; background-color:#3333FF; left:425px; top:134px"></div>
</body>
</html>