效果如下:

 

 


<!DOCTYPE html>
<html>
<head>
	<title></title>
	<style type="text/css">
		body{position:relative;}
		*{padding:0;margin:0;}
		.dialog{width:400px;height:400px;border:1px solid #ddd;position:fixed;top:100px;left:200px;}
		.dialog h2{height:40px;background:purple;cursor:move;}
		.mask{position:fixed;width:0;height:0;left:0;top:0;display:none;border: 3px solid #eee;box-shadow: 0px 0px 10px 5px rgba(0, 0, 0, 0.07);cursor:move;}
	</style>
</head>
<body>
	<div class="dialog" id="box">
		<h2 id='digTit'></h2>
	</div>
	<div class="mask"></div>
	<script type="text/javascript">
		var dragEle = document.getElementById('digTit')
		var box = document.getElementById('box')
		var x,y // 物体离上边，下边的距离
		var moveBox = document.querySelector('.mask') // 移动的是这层
		// 设置居中
		var oldX = (document.documentElement.clientWidth-box.offsetWidth)/2
		var oldY = (document.documentElement.clientHeight-box.offsetHeight)/2
		box.style.left = oldX + 'px'
		box.style.top = oldY + 'px'
		moveBox.style.left = oldX + 'px'
		moveBox.style.top = oldY + 'px'
		moveBox.style.width = box.offsetWidth + 'px'
		moveBox.style.height = box.offsetHeight + 'px'
				

		digTit.addEventListener('mousedown', function (e) {
			moveBox.style.display = 'block'
			var beginX = e.clientX
			var beginY = e.clientY

			document.addEventListener('mousemove', move, false)
			document.addEventListener('mouseup', up, false)

			function move(e) {
				x = oldX + e.clientX - beginX
				y = oldY + e.clientY - beginY
				moveBox.style.left = x + 'px'
				moveBox.style.top = y + 'px'
			}

			function up(e) {
				box.style.left = x + 'px'
				box.style.top = y + 'px'
				oldX = x
				oldY = y
				moveBox.style.display = 'none'
				document.removeEventListener('mousemove', move, false)
				document.removeEventListener('mouseup', up, false)
			}
		}, false)
	</script>
</body>
</html>