手写可拖拽的div

<style>
    * {
      margin: 0;
      padding: 0;
    }
    div{
      border: 1px solid red;
      position: absolute;
      top: 0;
      left: 0;
      width: 100px;
      height: 100px;
    }
</style>

<div id="xxx"></div>

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

    document.addEventListener('mousemove', function(e){
      if(dragging === false) return
      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)
      // 使用 transform 会比 top / left 性能更好，因为可以避免 reflow 和 repaint
      xxx.style.left = left + deltaX + 'px'
      xxx.style.top = top + deltaY + 'px'
      position = [x, y]
    })

    document.addEventListener('mouseup', function(e){
      dragging = false
    })
</script>