sending formdata through POST throws an error at second time - node.js

Question

I am making item list that I can add items into a data.json file from input:text.
nodejs will receive POST request and store formdata into data.json.
input:text and submit button are inside form.
I put some text in input:text.
click submit button and addEventListener has e.preventDefault(). so page won't refresh.
nodejs receives POST request then function itemManager will deal with formdata and run writeFile and readFile.
That's my logic but this won't work at second button click.
At second button click, page will show me this picture.
router.js
const express = require('express');
const router = express.Router();
const itemController = require('../controllers/itemController');
router.get('/', itemController);
router.post('/', itemController);
module.exports = router;
itemController.js
const fs = require('fs');
function itemManager(req, res) {
console.log(Object.keys(req.body).length);
if (Object.keys(req.body).length) {
console.log(req.body);
let json = JSON.stringify(req.body);
json = json + "\n";
fs.writeFile('./data/data.json', json, {
encoding: "utf-8",
flag: "a+"
}, (err) => {
if (err) console.log(err);
else {
console.log(`success!`);
}
});
}
fs.readFile('./data/data.json', 'utf8', (err, data) => {
if (err) res.render('index');
else {
const dataArray = data.split('\n');
dataArray.pop();
for (let i in dataArray) {
dataArray.push(JSON.parse(dataArray.shift()));
}
console.log(`data Array!!!!! : `, dataArray);
res.render('index', {dataArray});
}
})
}
module.exports = itemManager;