<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style>
#aa{
width:500px;
height:500px;
border:red 1px solid;
margin:100px auto;
position:relative;
/*padding:2px;*/
}
#dd{
width:100px;
height:100px;
background:green;
/*margin:2px;*/
position:absolute;
}
</style>
</head>
<body>
<div id="aa">
<div id="dd"></div>
</div>
</body>
</html>
<script>
var $=function(id){
return document.getElementById(id)
}
var aa=$("aa");
var dd=$("dd");
dd.onmousedown=function(ev){
var ev=ev||window.event;
var left=aa.offsetLeft
var top=aa.offsetTop
var x=ev.clientX-left-dd.offsetLeft;
var y=ev.clientY-top-dd.offsetTop;
document.onmousemove=function(ev){
var ev=ev||window.event;
dd.style.left=ev.clientX-left-x+"px";
dd.style.top=ev.clientY-top-y+"px"
if(ev.clientX-left-x<=0){
dd.style.left=0+"px";
}
if(ev.clientY-top-y<=0){
dd.style.top=0+"px";
}
if(ev.clientX-left-x>=aa.offsetWidth-dd.offsetWidth){
dd.style.left=aa.offsetWidth-dd.offsetWidth+"px";
}
if(ev.clientY-top-y>=aa.offsetHeight-dd.offsetHeight){
dd.style.top=aa.offsetHeight-dd.offsetHeight+"px";
}
}
return false;
}
document.onmouseup=function(){
document.onmousemove=null;
}
</script>
原文地址：https://www.cnblogs.com/shangjun6/p/10375234.html