Refresh token pattern in NestJS

Introduction

The refresh token pattern helps keep users logged in safely without asking for their password again and again.

When you want users to stay logged in for a long time without re-entering credentials.

When you want to improve security by limiting how long access tokens last.

When you want to allow users to get new access tokens without logging in again.

When you want to protect your app from stolen tokens by using short-lived access tokens.

When you want to separate short-term access from long-term authentication.

Syntax

NestJS

async refreshToken(oldRefreshToken: string): Promise<{ accessToken: string }> {
  // 1. Verify the old refresh token
  // 2. Check if it is valid and not expired
  // 3. Generate a new access token
  // 4. Optionally generate a new refresh token
  // 5. Return the new tokens
}

The refresh token is usually stored securely and sent only when requesting a new access token.

Access tokens are short-lived; refresh tokens last longer and are used to get new access tokens.

Examples

Basic example: verify old refresh token, then create a new access token.

NestJS

async refreshToken(oldRefreshToken: string) {
  const payload = this.jwtService.verify(oldRefreshToken);
  if (!payload) throw new UnauthorizedException();
  const newAccessToken = this.jwtService.sign({ userId: payload.userId });
  return { accessToken: newAccessToken };
}

More complete example: verifies with a secret, returns both new access and refresh tokens.

NestJS

async refreshToken(oldRefreshToken: string) {
  const payload = this.jwtService.verify(oldRefreshToken, { secret: this.configService.get('REFRESH_TOKEN_SECRET') });
  if (!payload) throw new UnauthorizedException();
  const newAccessToken = this.jwtService.sign({ userId: payload.userId });
  const newRefreshToken = this.jwtService.sign({ userId: payload.userId }, { expiresIn: '7d' });
  return { accessToken: newAccessToken, refreshToken: newRefreshToken };
}

Sample Program

This NestJS service method checks the old refresh token. If valid, it creates new access and refresh tokens with different expiration times.

NestJS

import { Injectable, UnauthorizedException } from '@nestjs/common';
import { JwtService } from '@nestjs/jwt';

@Injectable()
export class AuthService {
  constructor(private jwtService: JwtService) {}

  async refreshToken(oldRefreshToken: string): Promise<{ accessToken: string; refreshToken: string }> {
    try {
      const payload = this.jwtService.verify(oldRefreshToken, { secret: 'refreshSecret' });
      const newAccessToken = this.jwtService.sign({ userId: payload.userId }, { expiresIn: '15m' });
      const newRefreshToken = this.jwtService.sign({ userId: payload.userId }, { expiresIn: '7d', secret: 'refreshSecret' });
      return { accessToken: newAccessToken, refreshToken: newRefreshToken };
    } catch (e) {
      throw new UnauthorizedException('Invalid refresh token');
    }
  }
}

Important Notes

Always keep refresh tokens secret and store them securely, like in HttpOnly cookies.

Set short expiration for access tokens to reduce risk if stolen.

Handle errors carefully to avoid giving clues about token validity.

Summary

The refresh token pattern helps keep users logged in safely.

Use short-lived access tokens and longer-lived refresh tokens.

Verify refresh tokens before issuing new access tokens.

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