JS根据在表单上选择的年龄显示消息

<form>
  <label for="age">选择年龄：</label>
  <select id="age">
    <option value="0-18">0-18岁</option>
    <option value="19-30">19-30岁</option>
    <option value="31-50">31-50岁</option>
    <option value="51+">51岁以上</option>
  </select>
  <button onclick="showMessage()">显示消息</button>
</form>

function showMessage() {
  var age = document.getElementById("age").value;
  var message = "";

  switch (age) {
    case "0-18":
      message = "你还年轻，好好学习吧！";
      break;
    case "19-30":
      message = "你正值壮年，要努力工作！";
      break;
    case "31-50":
      message = "你已经成熟稳重，继续保持！";
      break;
    case "51+":
      message = "你经验丰富，享受生活吧！";
      break;
    default:
      message = "请选择年龄！";
  }

  alert(message);
}

window.onload = function() {
  var button = document.querySelector("button");
  button.addEventListener("click", showMessage);
}