Refresh token pattern in NestJS - Mini Project: Build & Apply

Implementing Refresh Token Pattern in NestJS

📖 Scenario: You are building a secure authentication system for a web app using NestJS. To keep users logged in without asking for their password repeatedly, you will implement a refresh token pattern. This means the app issues a short-lived access token and a longer-lived refresh token. When the access token expires, the refresh token can get a new access token without logging in again.

🎯 Goal: Build a simple NestJS service that stores a refresh token for a user, validates it, and issues a new access token when requested.

📋 What You'll Learn

Create a service with a dictionary to store refresh tokens by user ID

Add a configuration variable for refresh token expiration time

Implement a method to validate a refresh token and generate a new access token

Complete the service by adding a method to save a refresh token for a user

💡 Why This Matters

🌍 Real World

Refresh tokens are used in real web apps to keep users logged in securely without asking for passwords repeatedly.

💼 Career

Understanding refresh token patterns is essential for backend developers working on authentication and security in modern web applications.

1

Create refresh token storage

Create a NestJS service class called AuthService with a private dictionary called refreshTokens that maps string user IDs to string refresh tokens. Initialize refreshTokens as an empty object.

NestJS

export class AuthService {
  // Your code here
}

Hint

Use private refreshTokens: Record<string, string> = {}; inside the class.

2

Add refresh token expiration config

Inside the AuthService class, add a private readonly variable called refreshTokenExpiry and set it to 7 * 24 * 60 * 60 (seconds in 7 days).

NestJS

export class AuthService {
  private refreshTokens: Record = {};
  // Your code here
}

Hint

This variable holds the refresh token expiration time in seconds.

3

Implement refresh token validation and access token generation

Add a method called refreshAccessToken in AuthService that takes userId: string and refreshToken: string. Inside, check if this.refreshTokens[userId] equals the given refreshToken. If not, return null. If valid, return a new access token string in the format `access-token-for-${userId}`.

NestJS

export class AuthService {
  private refreshTokens: Record = {};
  private readonly refreshTokenExpiry = 7 * 24 * 60 * 60;

  // Your code here
}

Hint

Check the stored token matches the input, then return a new access token string.

4

Add method to save refresh token

Add a method called saveRefreshToken in AuthService that takes userId: string and refreshToken: string. It should store the refresh token in this.refreshTokens using the user ID as the key.

NestJS

export class AuthService {
  private refreshTokens: Record = {};
  private readonly refreshTokenExpiry = 7 * 24 * 60 * 60;

  refreshAccessToken(userId: string, refreshToken: string): string | null {
    if (this.refreshTokens[userId] !== refreshToken) {
      return null;
    }
    return `access-token-for-${userId}`;
  }

  // Your code here
}

Hint

Store the refresh token in the dictionary using the user ID as key.

Practice

1. What is the main purpose of using a refresh token in a NestJS authentication system?

easy

A. To store user credentials securely on the client side

B. To replace the access token permanently

C. To issue a new access token without requiring the user to log in again

D. To encrypt the user's password

Solution

1. Step 1: Understand token roles in authentication

   Access tokens allow access but expire quickly; refresh tokens help get new access tokens without re-login.

2. Step 2: Identify refresh token purpose

   Refresh tokens are used to request new access tokens after expiration, keeping the user logged in safely.

3. Final Answer:

   To issue a new access token without requiring the user to log in again -> Option C

4. Quick Check:

   Refresh token = new access token without re-login [OK]

Hint: Refresh tokens renew access tokens without user login [OK]

Common Mistakes:

• Confusing refresh token with access token
• Thinking refresh token stores user credentials
• Assuming refresh token replaces access token permanently

2. Which of the following is the correct way to verify a refresh token in a NestJS service using JWT?

easy

A. jwtService.verify(refreshToken)

B. jwtService.encrypt(refreshToken)

C. jwtService.decode(accessToken)

D. jwtService.sign(payload)

Solution

1. Step 1: Identify JWT methods for token verification

   In NestJS, jwtService.verify() checks token validity and signature.

2. Step 2: Match method to refresh token verification

   To verify a refresh token, use jwtService.verify(refreshToken) to confirm it is valid and not expired.

3. Final Answer:

   jwtService.verify(refreshToken) -> Option A

4. Quick Check:

   Verify token = jwtService.verify() [OK]

Hint: Use jwtService.verify() to check token validity [OK]

Common Mistakes:

• Using sign() instead of verify() to check tokens
• Using decode() which does not verify signature
• Trying to encrypt tokens with jwtService

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

async refreshToken(token: string) {
  try {
    const payload = this.jwtService.verify(token);
    const newAccessToken = this.jwtService.sign({ userId: payload.sub });
    return { accessToken: newAccessToken };
  } catch (e) {
    throw new UnauthorizedException('Invalid refresh token');
  }
}

medium

A. Throws UnauthorizedException with message 'Invalid refresh token'

B. { accessToken: 'newly_generated_token_string' }

C. Returns the original refresh token

D. Returns null

Solution

1. Step 1: Analyze token verification

   The code verifies the refresh token; if valid, it extracts the payload.

2. Step 2: Generate new access token

   Using the payload's user ID, it signs a new access token and returns it in an object.

3. Final Answer:

   { accessToken: 'newly_generated_token_string' } -> Option B

4. Quick Check:

   Valid refresh token returns new access token [OK]

Hint: Valid token returns new access token object [OK]

Common Mistakes:

• Assuming it returns the refresh token again
• Thinking it throws error on valid token
• Expecting null instead of token object

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

async refreshToken(token: string) {
  const payload = this.jwtService.verify(token);
  const newAccessToken = this.jwtService.sign({ userId: payload.userId });
  return { accessToken: newAccessToken };
}

medium

A. Missing try-catch block to handle invalid tokens

B. Using sign() instead of verify() for token validation

C. Returning access token as a string instead of an object

D. Incorrect property name 'userId' instead of 'sub' in payload

Solution

1. Step 1: Check payload property usage

   JWT payload usually stores user ID in 'sub' property, not 'userId'.

2. Step 2: Identify mismatch causing error

   Accessing 'payload.userId' will be undefined, causing incorrect token signing.

3. Final Answer:

   Incorrect property name 'userId' instead of 'sub' in payload -> Option D

4. Quick Check:

   JWT user ID = payload.sub, not payload.userId [OK]

Hint: Check JWT payload keys carefully (usually 'sub') [OK]

Common Mistakes:

• Ignoring missing try-catch for errors
• Assuming payload uses 'userId' key
• Confusing sign() and verify() methods

5. You want to implement a refresh token system in NestJS that invalidates old refresh tokens after use. Which approach correctly achieves this?

hard

A. Store refresh tokens in a database and delete the old token when issuing a new one

B. Generate refresh tokens without storing them and rely on JWT expiration only

C. Use the same refresh token indefinitely without expiration

D. Send the access token as the refresh token

Solution

1. Step 1: Understand token invalidation

   To invalidate old refresh tokens, you must track them server-side, usually in a database.

2. Step 2: Implement token replacement logic

   When issuing a new refresh token, delete or mark the old one invalid in the database to prevent reuse.

3. Final Answer:

   Store refresh tokens in a database and delete the old token when issuing a new one -> Option A

4. Quick Check:

   Invalidate old tokens by storing and deleting them [OK]

Hint: Track refresh tokens server-side to invalidate old ones [OK]

Common Mistakes:

• Not storing tokens, so can't invalidate old ones
• Using refresh tokens without expiration
• Confusing access token with refresh token