要点：
- 注意监听范围，不能只监听div
- 不要使用drag事件，很难用
- 使用transform会比top/left性能更好，因为可以避免reflow和repaint
<!DOCTYPE html>
<html lang="en">
<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>Document</title>
<style>
#xxx {
width: 50px;
height: 50px;
border: 1px solid red;
position: absolute;
}
</style>
</head>
<body>
<div id="xxx"></div>
<script>
const xxx = document.querySelector('#xxx')
let dragging = false // 拖曳状态确认
let position = null // 鼠标当前位置记录
xxx.addEventListener('mousedown', (e) => {
dragging = true //记录当前状态为拖曳
position = [e.clientX, e.clientY] // 记录当前鼠标位置
})
document.addEventListener('mousemove', function (e) {
if (dragging === false) {
return
} // 如果不处于拖曳状态则退出
const x = e.clientX
const y = e.clientY
const deltaX = x - position[0] // 计算移动中的鼠标和原始坐标的差值
const deltaY = y - position[1]
const left = parseInt(xxx.style.left || 0) //获得div本身的left和top
const top = parseInt(xxx.style.top || 0)
xxx.style.left = left + deltaX + 'px' //赋值给新的left和top
xxx.style.top = top + deltaY + 'px'
position = [x, y] //记录新的鼠标位置
})
document.addEventListener('mouseup', function (e) {
dragging = false // 鼠标放开时候，取消拖曳状态
})
</script>
</body>
</html>
