下面的例子可检测所选项是否是默认选项 - 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").selectedIndex;
      var y = document.getElementsByTagName("option");
      alert("" + y[x].text + " 是默认选项吗? " + y[x].defaultSelected);
    }
  </script>
</head>
<body>

  <form>
    选择你最喜欢的水果:
    <select id="mySelect">
  <option>Apple</option>
  <option>Orange</option>
  <option selected="selected">Pineapple</option>
  <option>Banana</option>
</select>
  </form>
  <button type="button" onclick="displayResult()">选中的是默认的水果吗?</button>

</body>
</html>
        