StiflerProger

const express = require('express');
const jsonParser = express.json();

module.exports.register = [jsonParser, function (req, res) {
    if (!req.body) return res.sendStatus(400);
    console.log(req.body);
    res.json(
        {
            serverErr: '',
            fieldsErr: {
                username: 'Тест',
            }
        }
    );
})]

const express = require('express');
const router = express.Router();

const controller = require('../controllers/regController');

router.post('/',  controller.register)

module.exports = router;

app.use(function (req, res) {
    var send = res.send;
    res.send = function (body) {
        // тут что надо делай
        send.call(this, body);
    };
});