ajax无法捕获multer中Node Express的响应

const express = require('express');
const multer = require('multer');

const app = express();

// 配置multer中间件
const storage = multer.diskStorage({
  destination: function (req, file, cb) {
    cb(null, 'uploads/'); // 设置文件保存的目录
  },
  filename: function (req, file, cb) {
    cb(null, file.originalname);
  }
});

const upload = multer({ storage: storage });

// 处理文件上传的路由
app.post('/upload', upload.single('file'), function (req, res) {
  // 在multer的回调函数中返回自定义的响应
  if (req.file) {
    res.json({ success: true, message: '文件上传成功' });
  } else {
    res.json({ success: false, message: '文件上传失败' });
  }
});

app.listen(3000, function () {
  console.log('Server is running on port 3000');
});

// 使用ajax发送文件上传请求
var fileInput = document.getElementById('fileInput');
var file = fileInput.files[0];

var formData = new FormData();
formData.append('file', file);

var xhr = new XMLHttpRequest();
xhr.open('POST', '/upload', true);
xhr.onload = function () {
  if (xhr.status === 200) {
    var response = JSON.parse(xhr.responseText);
    if (response.success) {
      console.log(response.message);
    } else {
      console.log(response.message);
    }
  }
};
xhr.send(formData);