Why is id not defined (Express + Passport - Express-session)?

Question

Node.js

I'm trying to make an authorization based on this code
. But the console gives an error
. I do almost everything, as in the code from the link. Tell me what could be the problem?
server.js

const express = require('express');
const path = require('path');
const flash = require('connect-flash');
const session = require('express-session');
const bodyParser = require('body-parser');
const passport = require('passport');
const mongoose = require('mongoose');
const ejs = require('ejs');

//initialize app
const app = express();

//Set Moment.js
app.locals.moment = require('moment');

//Load Routes
const mainRoutes = require('./routes/main');

//passport config
require('./config/passport')(passport);

// Connect to MongoDB
mongoose.connect('mongodb://name:[email protected]:27017/articles')
    .then(() =>  console.log('Successfully connected'))
    .catch(err => console.log(err));

//ejs middleware
app.engine('ejs', require('ejs').renderFile);
app.set('view engine', 'ejs');
app.set('views', path.join(__dirname, 'views'));

// body parser middleware
app.use(bodyParser.urlencoded({ extended: false }));
app.use(bodyParser.json());

//static folder middleware
app.use(express.static(path.join(__dirname, 'public')));

// session middleware
app.use(session({
    secret: 'secret',
    resave: true,
    saveUninitialized: true
}));

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

//flash middleware
app.use(flash());

//global variables
app.use((req, res, next)=> {
    res.locals.success_msg = req.flash('success_msg');
    res.locals.err_msg = req.flash('err_msg');
    res.locals.error = req.flash('error');
    res.locals.user = req.user || null;
    next();
});

//use Routes
app.use('/',  mainRoutes);

const port = 8080;
app.listen(port, ()=>{
    console.log(`App started at port ${port}`);
});

passport.js

const LocalStrategy = require('passport-local').Strategy;
const mongoose = require('mongoose');
const User = mongoose.model('user');
const bcrypt = require('bcryptjs');


module.exports = passport => {
    passport.use(new LocalStrategy({
        usernameField : 'login'
        },
        (login, password, done) => {
            //match user
            User.findOne({
                login : login
            })
                .then(user => {
                    if(!user){
                        return done(null, false, {message : 'no user found'});
                    }

                    //match passwords
                    bcrypt.compare(password, user.password, (err, isMatch)=>{
                        if (err) throw err;
                        if (isMatch) {
                            return done(null, user);
                        } else {
                            return done(null, false, {message : 'password incorrect'});
                        }
                    })
                })
                .catch(err => console.log(err));
        }
    ));

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

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

Routes

//login
router.get('/login', (req, res) => {
    res.render('main/login.ejs');
});

//  POST api/login
router.post('/login', (req, res, next) => {
    passport.authenticate('local', {
        successRedirect : '/admin',
        failureRedirect : '/login',
        failureFlash : true
    })(req, res, next);
});

router.get('/admin', (req, res) => {
    res.render('main/add-article.ejs')
    });

Answer 1

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

whence the variableid