var dragFlag = false;

var position = null;

drag.addEventListener('mousedown', function(e) {

  dragFlag = true;

  position = [e.clientX, e.clientY];

})

​

// mousemove 与 mouseup 不能监听在元素上 快速拖到会有问题

document.addEventListener('mousemove', function(e) {

 if (dragFlag) {

    return;

  }

  const x = e.clientX;

  const y = e.clientY;

  const deltaX = x - position[0];

  const deltaY = y - position[1];

  // parseInt('')空转为 NaN

  // drag.style.left初始值为'0px'

  const left = parseInt(drag.style.left || 0, 10);

  const top = parseInt(drag.style.top || 0, 10);

  drag.style.left = left + deltaX + 'px';

  drag.style.top = top + deltaY + 'px';

  position= [x, y];

})

​

document.addEventListener('mouseup', function() {

    dragFlag = false;

})