一、非面向对象的写法

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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<script type="text/javascript">
window.onload=function()
{
var oDiv=document.getElementById('div1');
oDiv.onmousedown=function(ev)
{
var oEvent=ev||event;
var x=0;
var y=0;
x=oEvent.clientX-oDiv.offsetLeft;
y=oEvent.clientY-oDiv.offsetTop;
document.onmousemove=function(ev)
{
var oEvent=ev||event;
var out1=oEvent.clientX-x;
var out2=oEvent.clientY-y;

var oWidth=document.documentElement.clientWidth-oDiv.offsetWidth;
var oHeight=document.documentElement.clientHeight-oDiv.offsetHeight;

if(out1<0)
{out1=0;}
else if (out1>oWidth)
{
out1=oWidth;
}

if(out2<0)
{out2=0;}
else if (out2>oHeight)
{
out2=oHeight;
}

oDiv.style.left=out1+'px';
oDiv.style.top=out2+'px';
}
document.onmouseup=function()
{
document.onmousemove=null;
document.onmouseup=null;
}
return false;//解决firefox低版本的bug问题
}
}
</script>
</head>

<body>
<div id="div1" style="width:100px; height:100px; background:#060; border:1px solid #039; position:absolute;">
</div>
</body>
</html>

&nbsp;

二、面向对象的写法

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<!DOCTYPE HTML>
<html>
<head>
<meta charset='utf-8'>
<title></title>
<style type="text/css">
*{ margin:0; padding:0;}
.drag{ width:100px; height: 100px; background:red; position: absolute;}
</style>
<script type="text/javascript" src='jquery-1.11.1.min.js'></script>
<script type="text/javascript">
window.onload=function(){
var dg=new Drag('drag');
};
function Drag(obj){
this.oDiv=document.getElementById(obj);
this.x=0;
this.y=0;
var _this=this;
this.oDiv.onmousedown=function(ev){
_this.cc(ev);
return false;
};
};
Drag.prototype.cc=function(ev){
var oEvent=ev||event;
var _this=this;
this.x=oEvent.clientX-this.oDiv.offsetLeft;
this.y=oEvent.clientY-this.oDiv.offsetTop;
document.onmousemove=function(ev){
_this.aa(ev);
};
document.onmouseup=function(){
_this.bb();
};
};
Drag.prototype.aa=function(ev){
var oEvent=ev||event;
var outX=oEvent.clientX-this.x;
var outY=oEvent.clientY-this.y;
if (outX<=0) {
outX=0;
}else if(outX>document.documentElement.clientWidth-this.oDiv.offsetWidth){
outX=document.documentElement.clientWidth-this.oDiv.offsetWidth;
};
if (outY<=0) {
outY=0;
}else if(outY>document.documentElement.clientHeight-this.oDiv.offsetHeight){
outY=document.documentElement.clientHeight-this.oDiv.offsetHeight;
};

this.oDiv.style.left=outX+'px';
this.oDiv.style.top=outY+'px';
};
Drag.prototype.bb=function(ev){
document.onmousemove=null;
document.onmouseout=null;
};
</script>
</head>
<body>
<div class="drag" id='drag'></div>
</body>
</html>