写一个可拖曳的div

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用mouse事件写一个可拖曳的div
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<div id='app'></div>
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#app{
  border: 1px solid red;
  height: 100px;
  width: 100px;
  position: absolute;
  top: 0;
  left: 0;
}
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let dragging = false
let position = null

let app = document.getElementById('app')
app.addEventListener('mousedown', e => {
  dragging = true
  position = [e.clientX, e.clientY]
})

document.addEventListener('mousemove', e => {
  if(dragging){
    const x = e.clientX
    const y = e.clientY
    const deltaX = x - position[0]
    const deltaY = y - position[1]
    const left = parseInt(app.style.left || 0)
    const top = parseInt(app.style.top || 0)
    app.style.left = left + deltaX + 'px'
    app.style.top = top + deltaY + 'px'
    position = [x, y]
  }
})

document.addEventListener('mouseup', e => {
  dragging = false
})