User login flow in Express - Mini Project: Build & Apply

User login flow

📖 Scenario: You are building a simple user login flow for a website using Express.js. Users will send their username and password to the server, and the server will check if the credentials are correct.

🎯 Goal: Create an Express.js server that accepts login requests, checks user credentials from a predefined list, and responds with success or failure messages.

📋 What You'll Learn

Create a users object with exact username-password pairs

Add a variable to hold the login success message

Write a POST route handler for '/login' that checks credentials

Send the correct response message based on login success or failure

💡 Why This Matters

🌍 Real World

User login flows are essential for websites and apps to authenticate users securely.

💼 Career

Understanding how to build login routes with Express is a key skill for backend web developers.

1

DATA SETUP: Create users data

Create a constant object called users with these exact entries: 'alice': 'wonderland', 'bob': 'builder', 'charlie': 'chocolate'.

Express

// Your code here

Hint

Use a JavaScript object with usernames as keys and passwords as values.

2

CONFIGURATION: Add success message variable

Add a constant string variable called successMessage and set it to 'Login successful!'.

Express

const users = {
  'alice': 'wonderland',
  'bob': 'builder',
  'charlie': 'chocolate'
};

// Your code here

Hint

Use const to create a string variable for the success message.

3

CORE LOGIC: Create login POST route

Write an Express POST route handler for '/login' that extracts username and password from req.body, checks if users[username] equals password, and stores the result in a boolean variable called isValidUser.

Express

const users = {
  'alice': 'wonderland',
  'bob': 'builder',
  'charlie': 'chocolate'
};

const successMessage = 'Login successful!';

const express = require('express');
const app = express();
app.use(express.json());

// Your code here

Hint

Use app.post with a callback that destructures username and password from req.body.

4

COMPLETION: Send response based on login result

Inside the /login route handler, send a JSON response with { message: successMessage } if isValidUser is true, otherwise send { message: 'Invalid username or password' }. Then, start the Express server listening on port 3000.

Express

const users = {
  'alice': 'wonderland',
  'bob': 'builder',
  'charlie': 'chocolate'
};

const successMessage = 'Login successful!';

const express = require('express');
const app = express();
app.use(express.json());

app.post('/login', (req, res) => {
  const { username, password } = req.body;
  const isValidUser = users[username] === password;
  // Your code here
});

// Your code here

Hint

Use res.json() to send the response and app.listen(3000) to start the server.

Practice

1. What is the main purpose of a user login flow in an Express app?

easy

A. To verify the user's identity before granting access

B. To display the homepage content

C. To log server errors

D. To serve static files like images

Solution

1. Step 1: Understand the login flow goal

   The login flow is designed to check who the user is by verifying credentials.

2. Step 2: Identify the correct purpose

   Granting access only after verification matches the login flow's main purpose.

3. Final Answer:

   To verify the user's identity before granting access -> Option A

4. Quick Check:

   Login flow = Verify user identity [OK]

Hint: Login flow means checking user identity first [OK]

Common Mistakes:

• Confusing login flow with serving static files
• Thinking login flow logs errors
• Assuming login flow shows homepage content

2. Which Express route method is best suited to securely receive login form data?

easy

A. app.put('/login', ...)

B. app.get('/login', ...)

C. app.post('/login', ...)

D. app.delete('/login', ...)

Solution

1. Step 1: Recall HTTP methods for form data

   POST is used to send data securely from forms, unlike GET which appends data in URL.

2. Step 2: Match method to login data handling

   Login forms should use POST to keep credentials hidden and secure.

3. Final Answer:

   app.post('/login', ...) -> Option C

4. Quick Check:

   Use POST for login data [OK]

Hint: Use POST to send login data securely [OK]

Common Mistakes:

• Using GET exposes credentials in URL
• PUT and DELETE are not for login forms
• Confusing route methods for form submission

3. What will be the output if the following Express code is used for login and the user provides correct credentials?

app.post('/login', (req, res) => {
  const { username, password } = req.body;
  if(username === 'user' && password === 'pass') {
    req.session.user = username;
    res.send('Login successful');
  } else {
    res.status(401).send('Invalid credentials');
  }
});

medium

A. "Login successful" message sent and session user set

B. Server crashes due to missing session setup

C. "Invalid credentials" message sent always

D. Redirects to homepage without message

Solution

1. Step 1: Analyze the login condition

   The code checks if username is 'user' and password is 'pass'. If true, it sets session user and sends success message.

2. Step 2: Understand the output for correct credentials

   When correct, it sends 'Login successful' and stores username in session.

3. Final Answer:

   "Login successful" message sent and session user set -> Option A

4. Quick Check:

   Correct credentials = success message + session set [OK]

Hint: Correct login sends success and sets session [OK]

Common Mistakes:

• Assuming server crashes without session middleware
• Thinking invalid message shows on correct login
• Confusing redirect with send response

4. Identify the error in this Express login route code:

app.post('/login', (req, res) => {
  const { username, password } = req.body;
  if(username == 'admin' && password == '1234') {
    res.session.user = username;
    res.send('Welcome admin');
  } else {
    res.send('Access denied');
  }
});

medium

A. Using '==' instead of '===' for comparison

B. No error, code works fine

C. Missing res.status(401) for failed login

D. Assigning session to 'res.session' instead of 'req.session'

Solution

1. Step 1: Check session assignment

   Session data should be stored on req.session, not res.session.

2. Step 2: Confirm correct session usage

   Using res.session will cause undefined error; req.session is correct.

3. Final Answer:

   Assigning session to 'res.session' instead of 'req.session' -> Option D

4. Quick Check:

   Session stored on req, not res [OK]

Hint: Session is on req, not res object [OK]

Common Mistakes:

• Confusing req and res objects
• Ignoring missing status code on failure
• Thinking '==' causes error here

5. You want to keep users logged in across pages after login in Express. Which approach correctly implements this using sessions?

1. Use express-session middleware
2. On successful login, save username in req.session
3. On other routes, check if req.session.user exists
4. If exists, allow access; else redirect to login

hard

A. Sessions should not be used; use cookies only

B. This approach is correct and follows best practices

C. Store user info in res.locals instead of session

D. Use GET method to store session data

Solution

1. Step 1: Understand session usage in Express

   express-session middleware manages sessions; storing user info in req.session keeps login state.

2. Step 2: Verify access control logic

   Checking req.session.user on other routes to allow or redirect is standard practice.

3. Final Answer:

   This approach is correct and follows best practices -> Option B

4. Quick Check:

   Sessions + req.session.user check = persistent login [OK]

Hint: Use express-session and req.session.user for login persistence [OK]

Common Mistakes:

• Thinking cookies alone handle login state securely
• Using res.locals which resets each request
• Trying to store session data via GET method