js 对select框赋值

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

<script>
  document.getElementById('mySelect').value = '2'; // 选中 "Option 2"
</script>

document.getElementById('mySelect').selectedIndex = 1; // 选中第二个选项，即 "Option 2"

var selectElement = document.getElementById('mySelect');
for (var i = 0; i < selectElement.options.length; i++) {
  if (selectElement.options[i].text === 'Option 2') {
    selectElement.options[i].selected = true;
    break;
  }
}

window.onload = function() {
  document.getElementById('mySelect').value = '2';
};

// 假设你添加了一个新的选项
var newOption = document.createElement('option');
newOption.value = '4';
newOption.text = 'Option 4';
document.getElementById('mySelect').appendChild(newOption);

// 然后重新设置值
document.getElementById('mySelect').value = '4';