本例可更改下拉列表中的被选选项 - 在线编辑器

          <!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>蜜蜂教程(mifengjc.com)</title>
  <script>
    function displayResult() {
      document.getElementById("orange").selected = true;
    }
  </script>
</head>
<body>

  <form>
    选择你最喜欢的水果:
    <select>
  <option id="apple">Apple</option>
  <option id="orange">Orange</option>
  <option id="pineapple" selected="selected">Pineapple</option>
  <option id="banana">Banana</option>
</select>
  </form>

  <button type="button" onclick="displayResult()">选择 Orange</button>

</body>
</html>
        