在div中移动已存在另一个div的div

<div id="container">
  <div id="movingDiv"></div>
</div>

#container {
  position: relative;
}

#movingDiv {
  position: absolute;
  top: 0;
  left: 0;
}

var movingDiv = document.getElementById("movingDiv");

movingDiv.addEventListener("mousedown", function(event) {
  var initialX = event.clientX;
  var initialY = event.clientY;

  document.addEventListener("mousemove", moveDiv);

  document.addEventListener("mouseup", function() {
    document.removeEventListener("mousemove", moveDiv);
  });

  function moveDiv(event) {
    var deltaX = event.clientX - initialX;
    var deltaY = event.clientY - initialY;

    movingDiv.style.top = movingDiv.offsetTop + deltaY + "px";
    movingDiv.style.left = movingDiv.offsetLeft + deltaX + "px";

    initialX = event.clientX;
    initialY = event.clientY;
  }
});