加载页面后设置下拉值

<select id="mySelect">
  <option value="option1">Option 1</option>
  <option value="option2">Option 2</option>
  <option value="option3">Option 3</option>
</select>

window.onload = function() {
  var selectElement = document.getElementById("mySelect");
  selectElement.innerHTML = ""; // 清空原有选项

  // 动态添加新的选项
  var option1 = document.createElement("option");
  option1.value = "newOption1";
  option1.text = "New Option 1";
  selectElement.add(option1);

  var option2 = document.createElement("option");
  option2.value = "newOption2";
  option2.text = "New Option 2";
  selectElement.add(option2);

  // 设置默认选中项
  selectElement.value = "newOption1";
};