Refresh token concept in Express - Mini Project: Build & Apply

Implementing Refresh Token Concept in Express

📖 Scenario: You are building a simple authentication system for a web app using Express. You want to keep users logged in securely by using access tokens and refresh tokens.Access tokens expire quickly, so you need a way to get a new access token without asking the user to log in again. This is done using refresh tokens.

🎯 Goal: Build a basic Express server that issues access tokens and refresh tokens, stores refresh tokens, and allows clients to get new access tokens by sending a valid refresh token.

📋 What You'll Learn

Create an Express app with a route to login and issue tokens

Store refresh tokens in memory

Create a route to accept a refresh token and issue a new access token

Create a route to logout and remove the refresh token

💡 Why This Matters

🌍 Real World

Refresh tokens are used in real web apps to keep users logged in without asking for credentials repeatedly. This improves user experience and security.

💼 Career

Understanding refresh tokens is important for backend developers working on authentication and security in web applications.

1

Setup Express app and login route

Create an Express app by requiring express and calling express(). Then create a POST route /login that sends a JSON response with accessToken set to 'access123' and refreshToken set to 'refresh123'.

Express

const express = require('express');
const app = express();

// Create POST /login route
// Your code here

Hint

Use express() to create the app. Use app.post('/login', (req, res) => { ... }) to create the login route.

2

Add in-memory storage for refresh tokens

Create a variable called refreshTokens and set it to an empty array []. Then, inside the /login route, add the string 'refresh123' to the refreshTokens array.

Express

const express = require('express');
const app = express();

// Create refreshTokens array
// Your code here

app.use(express.json());

app.post('/login', (req, res) => {
  // Add 'refresh123' to refreshTokens
  // Your code here
  res.json({ accessToken: 'access123', refreshToken: 'refresh123' });
});

app.listen(3000);

Hint

Use const refreshTokens = [] to create the array. Use refreshTokens.push('refresh123') inside the login route.

3

Create /token route to refresh access token

Add a POST route /token that reads refreshToken from req.body. Check if refreshTokens includes this refreshToken. If yes, respond with JSON containing accessToken set to 'newAccess123'. If not, respond with status 403 and JSON message { error: 'Forbidden' }.

Express

const express = require('express');
const app = express();

const refreshTokens = [];

app.use(express.json());

app.post('/login', (req, res) => {
  refreshTokens.push('refresh123');
  res.json({ accessToken: 'access123', refreshToken: 'refresh123' });
});

// Create POST /token route
// Your code here

app.listen(3000);

Hint

Use app.post('/token', (req, res) => { ... }). Read refreshToken from req.body. Use refreshTokens.includes(refreshToken) to check validity.

4

Add /logout route to delete refresh token

Create a POST route /logout that reads refreshToken from req.body. Remove this refreshToken from the refreshTokens array using filter. Respond with status 204 and no content.

Express

const express = require('express');
const app = express();

const refreshTokens = [];

app.use(express.json());

app.post('/login', (req, res) => {
  refreshTokens.push('refresh123');
  res.json({ accessToken: 'access123', refreshToken: 'refresh123' });
});

app.post('/token', (req, res) => {
  const refreshToken = req.body.refreshToken;
  if (refreshTokens.includes(refreshToken)) {
    res.json({ accessToken: 'newAccess123' });
  } else {
    res.status(403).json({ error: 'Forbidden' });
  }
});

// Create POST /logout route
// Your code here

app.listen(3000);

Hint

Use app.post('/logout', (req, res) => { ... }). Remove the token from refreshTokens using splice. Send status 204 with res.sendStatus(204).

Practice

1. What is the main purpose of a refresh token in an Express app using authentication?

easy

A. To encrypt the access token

B. To store user passwords securely

C. To log out the user automatically after a timeout

D. To get a new access token without asking the user to log in again

Solution

1. Step 1: Understand the role of refresh tokens

   Refresh tokens allow the app to request new access tokens without user interaction.

2. Step 2: Compare options with refresh token purpose

   Only To get a new access token without asking the user to log in again correctly describes this purpose; others describe unrelated functions.

3. Final Answer:

   To get a new access token without asking the user to log in again -> Option D

4. Quick Check:

   Refresh token purpose = get new access token without login [OK]

Hint: Refresh tokens renew access tokens silently [OK]

Common Mistakes:

• Confusing refresh token with access token
• Thinking refresh token stores passwords
• Assuming refresh token logs out users

2. Which of the following is the correct way to send a refresh token in an Express response header?

easy

A. res.setHeader('refresh-token', token);

B. res.sendRefreshToken(token);

C. res.refreshToken = token;

D. res.header('Authorization', token);

Solution

1. Step 1: Recall Express method to set headers

   Express uses res.setHeader(name, value) to set response headers.

2. Step 2: Match correct syntax for refresh token header

   res.setHeader('refresh-token', token); uses correct method and header name; others are invalid or use wrong header.

3. Final Answer:

   res.setHeader('refresh-token', token); -> Option A

4. Quick Check:

   Set header with res.setHeader(name, value) [OK]

Hint: Use res.setHeader to send custom headers [OK]

Common Mistakes:

• Using non-existent res.sendRefreshToken method
• Assigning token directly to res property
• Using wrong header like 'Authorization' for refresh token

3. Given this Express route snippet, what will be the output if the refresh token is valid?

app.post('/token', (req, res) => {
  const refreshToken = req.body.token;
  if (!refreshToken) return res.status(401).send('No token');
  if (refreshToken !== 'validtoken') return res.status(403).send('Invalid token');
  res.json({ accessToken: 'newAccessToken123' });
});

medium

A. Status 401 with message 'No token'

B. Status 403 with message 'Invalid token'

C. JSON response with new access token

D. Empty response with status 200

Solution

1. Step 1: Check token presence and validity

   If refreshToken is missing, returns 401; if invalid, returns 403.

2. Step 2: For valid token, send new access token JSON

   When token equals 'validtoken', response sends JSON with new access token.

3. Final Answer:

   JSON response with new access token -> Option C

4. Quick Check:

   Valid token returns new access token JSON [OK]

Hint: Valid token returns JSON with new access token [OK]

Common Mistakes:

• Confusing status codes for missing vs invalid token
• Expecting empty response instead of JSON
• Ignoring token validation logic

4. Identify the bug in this Express refresh token handler:

app.post('/refresh', (req, res) => {
  const token = req.body.refreshToken;
  if (!token) res.status(401).send('Missing token');
  if (token !== 'secret') res.status(403).send('Forbidden');
  res.json({ accessToken: 'newToken' });
});

medium

A. Missing return statements after res.status calls causing multiple responses

B. Incorrect property name for token in request body

C. Using res.json instead of res.send

D. No bug, code works fine

Solution

1. Step 1: Check response flow after status calls

   Without return, code continues after sending response, causing errors.

2. Step 2: Confirm need for return to stop execution

   Adding return after res.status(...).send(...) prevents multiple responses.

3. Final Answer:

   Missing return statements after res.status calls causing multiple responses -> Option A

4. Quick Check:

   Return after res.status to stop code [OK]

Hint: Always return after sending response to avoid errors [OK]

Common Mistakes:

• Not returning after res.status sends response
• Assuming res.json is wrong here
• Thinking property name is incorrect

5. You want to implement refresh token rotation in Express to improve security. Which approach correctly applies this concept?

hard

A. Keep the same refresh token forever to avoid user re-login

B. Issue a new refresh token on each use and invalidate the old one

C. Send refresh token only once during login and never again

D. Store refresh tokens in localStorage on the client side

Solution

1. Step 1: Understand refresh token rotation

   Rotation means issuing a new refresh token each time the old one is used and invalidating the old token.

2. Step 2: Evaluate options for security best practice

   Issue a new refresh token on each use and invalidate the old one matches rotation concept; others either reduce security or misuse storage.

3. Final Answer:

   Issue a new refresh token on each use and invalidate the old one -> Option B

4. Quick Check:

   Refresh token rotation = new token each use [OK]

Hint: Rotate refresh tokens by replacing old with new each use [OK]

Common Mistakes:

• Reusing same refresh token indefinitely
• Not invalidating old refresh tokens
• Storing tokens insecurely on client side