在拖动的元素进入到放置目标时执行 JavaScript - 在线编辑器

          <!DOCTYPE HTML>
<html>
<head>
  <meta charset="utf-8">
  <title>蜜蜂教程(mifengjc.com)</title>
  <style>
    .droptarget {
      float: left;
      width: 100px;
      height: 35px;
      margin: 15px;
      margin-right: 100px;
      padding: 10px;
      border: 1px solid #aaaaaa;
    }
  </style>
</head>
<body>

  <p>在两个矩形框中来回拖动 p 元素:</p>
  <div class="droptarget" ondrop="drop(event)" ondragenter="dragEnter(event)" ondragleave="dragLeave(event)" ondragover="allowDrop(event)">
    <p ondragstart="dragStart(event)" draggable="true" id="dragtarget">拖动我!</p>
  </div>
  <div class="droptarget" ondragenter="dragEnter(event)" ondragleave="dragLeave(event)" ondrop="drop(event)" ondragover="allowDrop(event)"></div>
  <p style="clear:both;"><strong>注意：</strong>Internet Explorer 8 及更早 IE 版本或 Safari 5.1 及更早版本的浏览器不支持 drag 事件。</p>
  <p id="demo"></p>
  <script>
    function dragStart(event) {
      event.dataTransfer.setData("Text", event.target.id);
    }

    function dragEnter(event) {
      document.getElementById("demo").innerHTML = "进入放置区域";
      if (event.target.className == "droptarget") {
        event.target.style.border = "3px dotted red";
      }
    }

    function dragLeave(event) {
      document.getElementById("demo").innerHTML = "离开放置区域";
      if (event.target.className == "droptarget") {
        event.target.style.border = "";
      }
    }

    function allowDrop(event) {
      event.preventDefault();
    }

    function drop(event) {
      event.preventDefault();
      var data = event.dataTransfer.getData("Text");
      event.target.appendChild(document.getElementById(data));
    }
  </script>

</body>
</html>
        