js修改select的默认值

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

document.getElementById('mySelect').selectedIndex = 1; // 设置第二个选项为默认选中

document.getElementById('mySelect').value = 'option3'; // 设置值为'option3'的选项为默认选中

// 假设需要根据某些条件动态添加选项并设置默认值
var selectElement = document.getElementById('mySelect');
selectElement.innerHTML = ''; // 清空原有选项

// 添加新选项
var newOption1 = document.createElement('option');
newOption1.value = 'newOption1';
newOption1.text = 'New Option 1';
selectElement.appendChild(newOption1);

var newOption2 = document.createElement('option');
newOption2.value = 'newOption2';
newOption2.text = 'New Option 2';
selectElement.appendChild(newOption2);

// 设置默认值
selectElement.value = 'newOption2'; // 设置新添加的第二个选项为默认选中

window.onload = function() {
  document.getElementById('mySelect').value = 'option2';
};