Js，PHP WITH AJAX:如何使用javascript with ajax确定学生的字母等级？

<form id="gradeForm">
  <label for="name">学生姓名:</label>
  <input type="text" id="name" name="name">
  <br>
  <label for="score">学生成绩:</label>
  <input type="number" id="score" name="score">
  <br>
  <button type="submit">确定</button>
</form>
<p id="result"></p>

document.getElementById('gradeForm').addEventListener('submit', function(e) {
  e.preventDefault(); // 阻止表单的默认提交行为

  var name = document.getElementById('name').value;
  var score = document.getElementById('score').value;

  // 创建XMLHttpRequest对象
  var xhr = new XMLHttpRequest();
  xhr.open('POST', 'grade.php', true);
  xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
  xhr.onreadystatechange = function() {
    if (xhr.readyState === XMLHttpRequest.DONE) {
      if (xhr.status === 200) {
        document.getElementById('result').textContent = xhr.responseText;
      } else {
        console.error('请求失败');
      }
    }
  };
  xhr.send('name=' + encodeURIComponent(name) + '&score=' + score);
});

<?php
$name = $_POST['name'];
$score = $_POST['score'];
$grade = '';

if ($score >= 90) {
  $grade = 'A';
} elseif ($score >= 80) {
  $grade = 'B';
} elseif ($score >= 70) {
  $grade = 'C';
} elseif ($score >= 60) {
  $grade = 'D';
} else {
  $grade = 'E';
}

echo $name . '的字母等级是：' . $grade;
?>