xhr sends a request to nodejs for 500 with formData

Here we go again.
I used to submit a request like this:

<form action="http://localhost:3000/api/swiper" method="post" enctype="multipart/form-data">
        <input type="file" name="avatar" id="file" multiple="multiple">
        <button id="btn">上传</button>
        <img src="" alt="" id="img">
</form>

Submitted the request in this way to get the image to the back end was fine, but I didn't get the response data without jquery, so I decided not to do that and put the form in the front end of

<input type="file" name="avatar" id="file" multiple="multiple">
<button id="btn">上传</button>
<img src="" alt="" id="img">
<script>
        const btn = document.querySelector('#btn')
        const inp_file = document.querySelector('#file')
        btn.addEventListener('click',function(){
            var xhr = new XMLHttpRequest();
            xhr.open('POST', 'http://localhost:3000/api/swiper');
            xhr.setRequestHeader('Content-Type', "multipart/form-data; boundary=---------------------------" + Date.now().toString(16));
            let file = inp_file.files[0]
            console.log(file);
            let formdata = new FormData()
            formdata.append('avatar',file)
            xhr.send(formdata);
            xhr.onreadystatechange = function() {
                if(xhr.readyState === 4 && xhr.status === 200) {
                    console.log(xhr.responseText);
                }
            }
        })
    </script>

nodejs:

const express = require('express')
const router = express.Router()
const multer = require('multer')
const mysql = require('../model/orm')
let date = new Date()
const moment = require('moment')
let time = date.getTime()
let originalname;
let random = Math.floor(Math.random() * (1000 - 100 + 1)) + 100
let urlStr; 
let obj;
let arr = [];
let storage = multer.diskStorage({
    destination: function(req, file, cb) {
        cb(null, 'public/upload'); 
    },
    filename: function(req, file, cb) {
        originalname = Buffer.from(file.originalname, "latin1").toString("utf8");
        urlStr = time + '-' + random + '-' + originalname
        time = date.getTime()
        random = Math.floor(Math.random() * (10000 - 100 + 1)) + 100
        obj = {
            swi_url:'http://localhost:3000/upload/' + urlStr,
            swi_qiyong:'false',
            swi_type:"活动",
            swi_time:moment(new Date()).format()
        }
        arr.push(obj)
        cb(null, urlStr)
    }
})
let upload = multer({ storage: storage });
// upload.single('form'),
router.post('/',upload.single('avatar'),(req,res) => {
    try {
        console.log(req.file);
        console.log(req.body);
    } catch (err) {
        console.log(err);
        res.status(500).send(err);
    }
    // console.log(arr);
    // let swiper = mysql.model('swiper')
    // swiper.insert(arr,(err,data) => {
    //     if(err) {
    //         console.log(err);
    //         return 
    //     }
    //     res.send({
    //         code:200,
    //         msg:"post请求处理完毕"
    //     })
    // })
})
module.exports = router

After this, the back end report 500, and then change to change, or req.body no data or is 500, Haizi technology is not good, poor base, head almost think rotten, because of what ah, help Haizi