拖拽的原理：三個事件 onmousedown、onmousemove、onmousemove
1、鼠標按下，觸發onmousedown，獲取鼠標座標x,y，獲取元素座標x,y
通過event.clientX、event.clientY獲取鼠標位置的座標
let x = e.clientX - box.offsetLeft; //鼠標點擊座標距離盒子左邊緣的距離
let y = e.clientY - box.offsetTop; //鼠標點擊座標距離盒子上邊緣的距離
2、設置元素left、top值，（元素要設置position:absolute）
box.style.left = ev.clientX - x + 'px';
box.style.top = ev.clientY - y + 'px';
3、放開鼠標取消dom事件
下面是詳細代碼：我只開了橫向移動
<!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; //鼠標點擊座標距離盒子左邊緣的距離
let y = e.clientY - box.offsetTop; //鼠標點擊座標距離盒子上邊緣的距離
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>