<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title></title>
<style type="text/css">
#box {
height: 300px;
width: 300px;
background-color: green;
position: absolute;
left: 0;
top: 0;
}
.border {
border: 2px dashed red;
position: absolute;
}
</style>
</head>
<body>
<div id="box">
</div>
</body>
<script type="text/javascript">
var box = document.getElementById("box");
var x = 0;
var y = 0;
box.onmousedown = function(ev) {
var oEvent = ev || event;
x = oEvent.clientX - box.offsetLeft;
y = oEvent.clientY - box.offsetTop;
//产生一个新的div，即为拖拽的框
var div = document.createElement("div");
div.className = "border";
div.style.width = box.offsetWidth + "px";
div.style.height = box.offsetHeight + "px";
div.style.left = box.offsetLeft + "px";
div.style.top = box.offsetTop + "px";
document.body.appendChild(div)
//鼠标移动的函数
document.onmousemove = function(ev) {
var oEvent = ev || event;
var L = oEvent.clientX - x;
var T = oEvent.clientY - y;
div.style.left = L + "px";
div.style.top = T + "px";
}
//鼠标抬起的函数
document.onmouseup = function() {
this.onmousemove = null;
this.onmouseup = null;
box.style.left = div.offsetLeft + "px";
box.style.top = div.offsetTop + "px";
document.body.removeChild(div);
}
return false;
}
</script>
</html>
来源：https://www.cnblogs.com/niuniudashijie/p/6129787.html