本例可返回下拉列表中所选的选项的所属表单id - 在线编辑器

          <!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>蜜蜂教程(mifengjc.com)</title>
  <script>
    function displayResult() {
      var x = document.getElementById("mySelect").form.id;
      alert(x);
    }
  </script>
</head>
<body>

  <form id="form1">
    选择你最喜欢的水果:
    <select id="mySelect">
  <option>Apple</option>
  <option>Orange</option>
  <option>Pineapple</option>
  <option>Banana</option>
</select>
  </form>
  <button type="button" onclick="displayResult()">显示所属的表单ID</button>

</body>
</html>
        