req.user is undefined in PassportJs

Question

I'm am trying to establish an authentication using passportjs, I don't have a big knowledge with this middleware, but the course that I'm following uses it and when I wanted to use it it didn't work for me, I've tried almost every possible solution, but none of them helped , and when I tried to log the value of req.user, it returned undefined.

so we have 3 files to perform this operation:

1 . the user.js MongoDb schema :

const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const passportLocalMongoose = require('passport-local-mongoose');

const User = new Schema({
  admin: {
    type: Boolean,
    default: false,
  },
});

User.plugin(passportLocalMongoose);

module.exports = mongoose.model('User', User);

2. the authenticate.js file

var passport = require('passport');
var LocalStrategy = require('passport-local').Strategy;
var User = require('./models/users');

exports.local = passport.use(new LocalStrategy(User.authenticate()));
passport.serializeUser(User.serializeUser());
passport.deserializeUser(User.deserializeUser());

3.And the app.js server file

var express = require('express');
var cookieParser = require('cookie-parser');
var session = require('express-session');
var FileStore = require('session-file-store')(session);
const passport = require('passport');
const authenticate = require('./authenticate');
.... other code .....

//Session
app.use(
  session({
    name: 'session-id',
    secret: '12345-67890-09876-54321',
    saveUninitialized: false,
    resave: false,
    store: new FileStore(),
  })
);

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

app.use('/', indexRouter);
app.use('/users', usersRouter);

function auth (req, res, next) {
  console.log(req.user);
  if (!req.user) {
    var err = new Error('You are not authenticated!');
    err.status = 403;
    next(err);
  }
  else {
        next();
  }
}

That's it, I'm using :

"passport": "^0.4.0",
"passport-local": "^1.0.0",
"passport-local-mongoose": "^5.0.1",
"express": "~4.16.0",
"express-session": "^1.15.6",