creating accounts with passportJs

I’m currently building an application using Passport and bcrypt for user authentication. I’ve created the registration part. Is there anything I could improve on? I’m still getting the hang of writing in JavaScript and I’m not sure if I’m using arrow functions correctly.

require('dotenv').config();
const express = require('express');
const bodyParser = require('body-parser');
const ejs = require("ejs");
const mongoose = require('mongoose'),
  Schema = mongoose.Schema,
  bcrypt = require('bcrypt'),
  saltRounds = 10;
const session = require('express-session');
const passport = require('passport');
const passportLocalMongoose = require("passport-local-mongoose");
const GoogleStrategy = require('passport-google-oauth20').Strategy;
const findOrCreate = require('mongoose-findorcreate')

const port = process.env.PORT;

const app = express();

app.use(express.static(__dirname + '/public'));
app.set('view engine', 'ejs');
app.use(bodyParser.urlencoded({
  extended: true
}));

app.use(session({
  secret: process.env.SECRET,
  resave: false,
  saveUninitialized: false
}));

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

mongoose.connect('mongodb://localhost:27017/userDB', {
  useNewUrlParser: true,
  useUnifiedTopology: true
});
mongoose.set('useCreateIndex', true);

const userSchema = new Schema({
  username: {
    type: String,
    unique: true,
    lowercase: true,
    required: true
  },
  password: String,
  googleId: String,
});

userSchema.pre('save', function(next) {
  const user = this;

  // only hash the password if it has been modified (or is new)
  if (!user.isModified('password')) return next();

  // generate a salt
  bcrypt.genSalt(saltRounds, (err, salt) => {
    if (err) return next(err);

    // hash the password using our new salt
    bcrypt.hash(req.body.password, saltRounds, (err, hash) => {
      if (err) return next(err);

      // override the cleartext password with the hashed one
      req.body.password = hash;
      next();
    });
  });
});

userSchema.methods.comparePassword = function(candidatePassword, cb) {
  bcrypt.compare(candidatePassword, this.password, function(err, isMatch) {
    if (err) return cb(err);
    cb(null, isMatch);
  });
};

userSchema.plugin(passportLocalMongoose);
userSchema.plugin(findOrCreate);

const User = new mongoose.model('User', userSchema);

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

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

passport.use(new GoogleStrategy({
    clientID: process.env.CLIENT_ID,
    clientSecret: process.env.CLIENT_SECRET,
    callbackURL: 'http://localhost:3000/auth/google/welcome',
    userProfileURL: 'https://www.googleapis.com/oauth2/v3/userinfo'
  },
  function(accessToken, refreshToken, profile, cb) {
    User.findOrCreate({
      googleId: profile.id
    }, (err, user) => {
      return cb(err, user);
    });
  }
));

app.get('/', (req, res) => {
  res.render('pages/home');
});

app.get('/auth/google', (req, res) => {
  passport.authenticate('google', {
    scope: ['profile']
  })
});

app.get('/auth/google/welcome',
  passport.authenticate('google', {
    failureRedirect: '/login'
  }), (req, res) => {
    // Successful authentication, redirect welcome page.
    res.redirect('/welcome');
  });

app.get('/login', (req, res) => {
  res.render('pages/login');
});

app.get('/register', (req, res) => {
  res.render('pages/register');
});

app.get('/welcome', (req, res) => {
  res.render('pages/welcome');
});

app.get('/welcome', (req, res) => {
  if (req.isAuthenticated()) {
    res.render('/register');
  } else {
    res.redirect('/welcome')
  }
});

app.post('/register', (req, res) => {

  User.register({
    username: req.body.username
  }, req.body.password, (err, user) => {
    if (err) {
      console.log(err);
      res.redirect('/register');
    } else {
      passport.authenticate('local')(req, res, () => {
        res.redirect('/welcome');
      });
    }
  });
});

app.post('/login', (req, res) => {

  const user = new User({
    username: req.body.username,
    password: req.body.password
  });

  req.login(user, (err) => {
    if (err) {
      console.log(err);
    } else {
      passport.authenticate('local')(req, res, () => {
        res.redirect('pages/welcome');
      });
    }
  });
});

app.listen(port, () => {
  console.log("Server has started.");
});