拖拽的实现是通过给元素设置 position 为相对定位，而后在鼠标按下的时候记录下地位，而后监测文档内鼠标的挪动情况，来更改元素款式的 left 和 top 值来实现了拖动的成果。大佬博客指路。代码如下：
<!DOCTYPE html>
<html lang="cn">
<head>
<meta charset="UTF-8">
<meta http-equiv="X-UA-Compatible" content="IE=edge">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<title> 拖拽 </title>
</head>
<style>
.container{
width: 100%;
position: relative;
height: 100%;
}
#drag{
width: 50px;
height: 50px;
background-color: red;
position: absolute;
cursor: move;
}
</style>
<body>
<div class="container">
<div id="drag"></div>
</div>
<script>
let drag = document.querySelector('#drag')
drag.onmousedown = function(event){
// 获取到事件对象
// 从中能够获取到鼠标所在的地位，计算出 dom 元素的地位
console.log(event);
var diffX = event.clientX - drag.offsetLeft;
var diffY = event.clientY - drag.offsetTop;
if(typeof drag.setCapture !== 'undefined'){drag.setCapture();
}
document.onmousemove = function(event){
var event = event || window.event;
var moveX = event.clientX - diffX;
var moveY = event.clientY - diffY;
if(moveX < 0){moveX = 0}else if(moveX > window.innerWidth - drag.offsetWidth){moveX = window.innerWidth - drag.offsetWidth}
if(moveY < 0){moveY = 0}else if(moveY > window.innerHeight - drag.offsetHeight){moveY = window.innerHeight - drag.offsetHeight}
drag.style.left = moveX + 'px';
drag.style.top = moveY + 'px'
}
document.onmouseup = function(event){
this.onmousemove = null;
this.onmouseup = null;
// 修复低版本 ie bug
if(typeof drag.releaseCapture!='undefined'){drag.releaseCapture();
}
}
}
</script>
</body>
</html>另外博客还介绍了一种 H5 的拖拽，通过给元素设置属性 draggable=”true” 来容许元素拖拽，而后监听元素的拖拽事件，来实现。