代码示例:(标识:eg_js_events_onchange_dropdown)
<!DOCTYPE html>
<html>
<head>
<script>
function preferedBrowser() {
  prefer = document.forms[0].browsers.value;
  alert("你喜欢使用 " + prefer + " 在网上冲浪!");
}
</script>
</head>
<body>

<form>
请选择您喜欢的浏览器:
  <select id="browsers" onchange="preferedBrowser()">
    <option value="Chrome">Chrome</option>
    <option value="Internet Explorer">Internet Explorer</option>
    <option value="Firefox">Firefox</option>
  </select>
</form>

</body>
</html>
运行结果: