可拖曳 div

<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
<div id="xxx"></div>

var dragging = false
var position = null

xxx.addEventListener('mousedown',function(e){
  dragging = true
  position = [e.clientX, e.clientY]
})
<script>
var dragging = false
var position = null

xxx.addEventListener('mousemove', function(e){
  dragging = true // 正在移动
  position = [e.clinetX, e.clientY]
})

document.addEventListener('mousemove', function(e){
  if(dragging === false){return} // 没有移动就别拖曳
  console.log('hi')
  const x = e.clientX
  const y = e.clientY
  const deltaX = x - position[0]
  const deltaY = y - position[1]
  const left = parseInt(xxx.style.left || 0)
  const top = parseInt(xxx.style.top || 0)
  xxx.style.left = left + deltaX + 'px'
  xxx.style.top = top + deltaY + 'px'
  position = [x, y]
})
document.addEventListener('mouseup', function(e){
  dragging = false
})
</script>
</body>
</html>

使用 transform 会比 top/left 性能更好，因为可以避免 reflow 和 repaint