如何在下载时向用户提供Ajax POST响应？

<form id="download-form">
 <input type="text" name="filename" placeholder="请输入文件名">
 <button type="submit">下载文件</button>
</form>

document.getElementById('download-form').addEventListener('submit', function(event) {
  event.preventDefault();

  var formData = new FormData(event.target);

  var xhr = new XMLHttpRequest();
  xhr.open('POST', '/download');
  xhr.responseType = 'blob';

  xhr.onload = function() {
    if (xhr.status === 200) {
      var blob = xhr.response;
      var url = URL.createObjectURL(blob);
      var a = document.createElement('a');
      a.href = url;
      a.download = 'download.txt';
      document.body.appendChild(a);
      a.click();
      setTimeout(function() {
        document.body.removeChild(a);
        URL.revokeObjectURL(url);
      }, 100);
    }
  };

  xhr.send(formData);
});

const express = require('express');
const fs = require('fs');
const app = express();

app.use(express.urlencoded({ extended: true }));

app.post('/download', function(req, res) {
  var filename = req.body.filename || 'download.txt';
  var filePath = './files/' + filename;

  fs.readFile(filePath, function(err, data) {
    if (err) {
      res.status(404).send('文件不存在');
    } else {
      res.setHeader('Content-disposition', 'attachment; filename=' + filename);
      res.setHeader('Content-type', 'text/plain');
      res.send(data);
    }
  });
});

app.listen(3000, function() {
  console.log('服务器已启动');
});