Google Sign up/in in NodeJS and MongoDB

authRoutes.js

// /routes/passport.js

const passport = require('passport');

module.exports = (app) => {

    /**
     * SignIn.
     */
    app.get('/auth/google', passport.authenticate('google', {
        scope: ['profile', 'email']
    }));

    /**
     * Callback from Google.
     */
    app.get('/auth/google/callback', passport.authenticate('google'));

    /**
     * Logout.
     */
    app.get('/api/logout', (req, res) => {
        req.logout();
        res.send(req.user);
    });

    /**
     * Get current user.
     */
    app.get('/api/current_user', (req, res) => {
        res.send(req.user);
    });
};

GoogleSignIn.js

const express = require('express');
const keys = require('./configs/keys');
const mongoose = require('mongoose');
const cookieSession = require('cookie-session');
const passport = require('passport');
require('./models/user');
require('./service/passport');
mongoose.connect(keys.mongoURI);
const app = express();

app.use(cookieSession({
        maxAge: 30 * 24 * 60 * 60 * 1000,
        keys: [keys.cookieKey],
    }));

app.use(passport.initialize());
app.use(passport.session());

require('./routes/authRoutes')(app);

app.listen(process.env.PORT || 3000);

packages.json

{
  "name": "express",
  "version": "0.0.0",
  "private": true,
  "main": "app.js",
  "scripts": {
    "start": "node app.js"
  },
  "engines": {
    "node": "10.15.3",
    "npm": "6.4.1"
  },
  "dependencies": {
    "cookie-parser": "~1.4.3",
    "cookie-session": "^1.3.3",
    "debug": "~2.6.9",
    "express": "~4.16.0",
    "http-errors": "~1.6.2",
    "mongoose": "^5.5.3",
    "morgan": "~1.9.0",
    "passport": "^0.4.0",
    "passport-google-oauth20": "^2.0.0",
    "pug": "^2.0.3"
  }
}

passport.js

// /services/passport.js

const passport = require('passport');
const mongoose = require('mongoose');
const GoogleStrategy = require('passport-google-oauth20').Strategy;
const keys = require('../configs/keys');
const User = mongoose.model('user');

passport.serializeUser((user, done) => {
    done(null, user.id);
});

passport.deserializeUser((id, done) => {
    User.findById(id).then(user => {
        done(null, user);
    })
});

passport.use(new GoogleStrategy({
    clientID: keys.googleClientID,
    clientSecret: keys.googleClientSecret,
    callbackURL: '/auth/google/callback'
}, (accessToken, refreshToken, profile, done) => {
    User.findOne({googleId: profile.id}).then(existingUser => {
        if (existingUser) {
            done(null, existingUser);
        } else {
            new User({googleId: profile.id}).save().then(user => done(null, user));
        }
    });
}));

user.js

// /models/user.js

const mongoose = require('mongoose');
const {Schema} = mongoose;

const userSchema = new Schema({
    googleId: String,
});

mongoose.model('user', userSchema);