选择框显示ajax调用后不变的值

<select id="mySelect">
  <option value="default">请选择</option>
</select>

var selectElement = document.getElementById("mySelect");

// 使用ajax发送请求
var xhr = new XMLHttpRequest();
xhr.open("GET", "url/to/backend", true);
xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    var response = JSON.parse(xhr.responseText);
    
    // 填充选择框
    for (var i = 0; i < response.length; i++) {
      var option = document.createElement("option");
      option.value = response[i].value;
      option.text = response[i].text;
      selectElement.appendChild(option);
    }
  }
};
xhr.send();

var selectedValue = "";

selectElement.addEventListener("change", function() {
  selectedValue = selectElement.value;
});

xhr.onreadystatechange = function() {
  if (xhr.readyState === 4 && xhr.status === 200) {
    var response = JSON.parse(xhr.responseText);
    
    // 填充选择框
    for (var i = 0; i < response.length; i++) {
      var option = document.createElement("option");
      option.value = response[i].value;
      option.text = response[i].text;
      selectElement.appendChild(option);
    }
    
    // 重新选中之前选择的值
    selectElement.value = selectedValue;
  }
};