ajax 发送的 formdata,后台为什么收不到

ajax 发送 formdata，用 formidable 来处理接收的数据，然后存到本地； 用 axios 发送的请求，可以正常运行，但是用原生 ajax 的就无效，是哪里写错了吗

<div id="app"> <input type="file" name="" id="fileSelect" /> <button type="submit" id="fileUpload">fileUpload</button> </div> <script> const fileSelect = document.getElementById('fileSelect'); const fileUpload = document.getElementById('fileUpload'); fileUpload.addEventListener('click', picUpload); function picUpload() { let formdata = new FormData(); console.log('filelist: ', fileSelect.files); formdata.append('myfile', fileSelect.files[0]); let xhr = new XMLHttpRequest(); xhr.open('POST', 'http://127.0.0.1:3000', true); xhr.setRequestHeader('Content-Type', 'multipart/form-data'); xhr.send(formdata); } </script> 

const http = require('http'), formidable = require('formidable'); path = require('path'); const server = http.createServer(function (req, res) { if (req.method.toLowerCase() === 'post') { const form = formidable({ multiples: true, uploadDir: path.resolve(__dirname, 'upload'), }); form.parse(req, (err, fields, files) => { console.log('fields:', fields); console.log('files:', files); res.writeHead(200, { 'content-type': 'application/json', 'Access-Control-Allow-Origin': '*', }); res.end(JSON.stringify({ fields, files }, null, 2)); }); return; } res.writeHead(200, { 'content-type': 'text/html', 'Access-Control-Allow-Origin': '*', }); res.end('Done!'); }); server.listen(3000); 

打印出来 fields 和 files 都是空的