Fixing req.user Undefined Error in Express and Passport

The req.user undefined error typically happens in Express applications using Passport for authentication when Passport is not correctly configured or integrated within the application flow. This property is supposed to hold the current user’s data once they are authenticated, but when not set up correctly, it doesn’t get populated, leading to the error.

Reasons Behind the Error

• Not implementing the serialize and deserialize user functions.
• Improper middleware order in Express setup, leading to sessions not being initialized before Passport.
• Failure to integrate Passport session middleware to handle login sessions.
• Issues in the authentication strategy configuration.

Steps to Fix the Error

1. Ensure serialization and deserialization methods are defined:

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

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

2. Verify the middleware order in your app.js (or server.js) file:

const express = require('express');
const session = require('express-session');
const passport = require('passport');

const app = express();

// Session configuration
app.use(session({
    secret: 'secret',
    resave: false,
    saveUninitialized: false
}));

// Initialize Passport and its session handling
app.use(passport.initialize());
app.use(passport.session());

// ... the rest of your routes and middleware

3. Check the Passport strategy setup and ensure proper verification:

const LocalStrategy = require('passport-local').Strategy;

passport.use(new LocalStrategy(
    (username, password, done) => {
        User.findOne({ username: username }, (err, user) => {
            if (err) { 
                return done(err); 
            }
            if (!user) {
                return done(null, false, { message: 'Incorrect username.' });
            }
            if (!user.validPassword(password)) {
                return done(null, false, { message: 'Incorrect password.' });
            }
            return done(null, user);
        });
    }
));

4. Ensure your login route is calling req.login with a callback that handles errors:

app.post('/login', 
    passport.authenticate('local', { failureRedirect: '/login' }), 
    (req, res, next) => { // Added 'next' in the parameter list for error handling
        req.login(req.user, function(err) {
            if (err) { 
                return next(err); 
            }
            return res.redirect('/user/profile');
        });
    }
);

5. Confirm that you’re using a persistent store for production environments, like connect-redis or connect-mongo, to handle sessions.

// Example using connect-redis with express-session
const RedisStore = require('connect-redis')(session);

app.use(session({
    store: new RedisStore({ client: redisClient }),
    secret: 'secret',
    resave: false,
    saveUninitialized: false
}));

Running Example

Below is a simple Express server setup that incorporates the steps above:

const express = require('express');
const session = require('express-session');
const passport = require('passport');

// Passport strategy setup
require('./passport-setup'); 

const app = express();

// Middlewares
app.use(express.urlencoded({ extended: false }));
app.use(session({
    secret: 'secret',
    resave: false,
    saveUninitialized: false
}));
app.use(passport.initialize());
app.use(passport.session());

// Routes
app.post('/login', 
    passport.authenticate('local', { failureRedirect: '/login' }), 
    (req, res) => {
        res.redirect('/user/profile');
    }
);

app.get('/user/profile', (req, res) => {
    if (req.isAuthenticated()) {
        res.status(200).json({ user: req.user });
    } else {
        res.status(401).send('You are not authenticated');
    }
});

// Start the server
app.listen(3000, () => {
    console.log('Server started on port 3000');
});

Make sure to replace ./passport-setup with the path to your passport configuration file, and fill in the strategies and serialize/deserialize functions as shown in the steps above.