拖拽的原理：三个事件 onmousedown、onmousemove、onmousemove
1、鼠标按下，触发onmousedown，获取鼠标座标x,y，获取元素座标x,y
通过event.clientX、event.clientY获取鼠标位置的座标
let x = e.clientX - box.offsetLeft; //鼠标点击座标距离盒子左边缘的距离
let y = e.clientY - box.offsetTop; //鼠标点击座标距离盒子上边缘的距离
2、设置元素left、top值，（元素要设置position:absolute）
box.style.left = ev.clientX - x + 'px';
box.style.top = ev.clientY - y + 'px';
3、放开鼠标取消dom事件
下面是详细代码：我只开了横向移动
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<meta name="viewport" content="width=device-width, initial-scale=1.0">
<meta http-equiv="X-UA-Compatible" content="ie=edge">
<title>Document</title>
<style>
#box {
width: 100px;
height: 100px;
background: red;
position: absolute
}
</style>
</head>
<body style="position: relative;overflow: hidden;">
<div id="box">
</div>
<script>
window.onload = function () {
let box = document.getElementById('box')
box.onmousedown = function (ev) {
let e = ev || event;
let x = e.clientX - box.offsetLeft; //鼠标点击座标距离盒子左边缘的距离
let y = e.clientY - box.offsetTop; //鼠标点击座标距离盒子上边缘的距离
document.onmousemove = function (ev) {
let e = ev || event;
box.style.left = ev.clientX - x + 'px';
box.style.top = ev.clientY - y + 'px';
let bodyScreenX = ev.screenX
let bodyClientWidth = document.body.clientWidth
document.onmouseup = function (ev) {
if (ev.clientX - x < 0) {
box.style.left = 0
} else if (bodyScreenX > bodyClientWidth) {
box.style.right = 0
box.style.left = bodyClientWidth - 100 + 'px'
}
document.onmousemove = null;
document.onmouseup = null;
}
}
}
}
</script>
</body>
</html>