app.js

const express = require('express');
const app = express();
const cookieParser = require('cookie-parser');
const mongoose = require('mongoose');
app.use(cookieParser());
app.use(express.json());

app.use(function (req, res, next) {

    // Website you wish to allow to connect
    res.setHeader('Access-Control-Allow-Origin', 'http://localhost:3000');

    // Request methods you wish to allow
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');

    // Request headers you wish to allow
    res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With, content-type');

    // Set to true if you need the website to include cookies in the requests sent
    // to the API (e.g. in case you use sessions)
    res.setHeader('Access-Control-Allow-Credentials', true);

    // Pass to next layer of middleware
    next();
});

mongoose.connect('mongodb://localhost:27017/mernauth', { useNewUrlParser: true, useUnifiedTopology: true }, () => {
    console.log('successfully connected to database');
});

const userRouter = require('./routes/User');
app.use('/user', userRouter);

app.listen(5000, () => {
    console.log('express server started');
});