Refresh token pattern in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What happens when a refresh token is expired in this NestJS service?

Consider this NestJS AuthService method that handles refresh tokens. What will be the result if the refresh token is expired?

NestJS

async refreshToken(userId: string, refreshToken: string) {
  const user = await this.userService.findById(userId);
  if (!user || user.refreshToken !== refreshToken) {
    throw new UnauthorizedException('Invalid refresh token');
  }
  try {
    const payload = this.jwtService.verify(refreshToken);
    const newAccessToken = this.jwtService.sign({ sub: userId });
    return { accessToken: newAccessToken };
  } catch (e) {
    throw new UnauthorizedException('Refresh token expired');
  }
}

A The method returns a new access token ignoring the expired refresh token.

B The method returns null without throwing an error.

C The method throws UnauthorizedException with message 'Refresh token expired'.

D The method throws a NotFoundException because the user is not found.

💡 Hint

Look at the try-catch block and what happens when jwtService.verify fails.

When the refresh token is expired, jwtService.verify throws an error caught by the catch block, which then throws UnauthorizedException with message 'Refresh token expired'.

📝 Syntax

intermediate

Which option correctly defines a NestJS guard to check refresh token validity?

You want to create a guard that checks if the refresh token in the request headers is valid. Which code snippet is syntactically correct and follows NestJS patterns?

A

import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common';

@Injectable()
export class RefreshTokenGuard {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    const token = request.headers['refresh-token'];
    return token === 'valid-token';
  }
}

B

import { CanActivate, ExecutionContext } from '@nestjs/common';

export class RefreshTokenGuard implements CanActivate {
  canActivate(context: ExecutionContext) {
    const request = context.getRequest();
    const token = request.headers['refresh-token'];
    return token === 'valid-token';
  }
}

C

import { Injectable, CanActivate, ExecutionContext } from '@nestjs/common';

@Injectable()
export class RefreshTokenGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    const token = request.headers['refresh-token'];
    return token === 'valid-token';
  }
}

D

import { CanActivate, ExecutionContext, Injectable } from '@nestjs/common';

@Injectable()
export class RefreshTokenGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    const token = request.headers['refresh-token'];
    return token === 'valid-token';
  }
}

💡 Hint

Check for correct decorator usage and method signatures.

Option D correctly imports Injectable and CanActivate, uses @Injectable() decorator, and implements canActivate with proper context usage.

❓ state_output

advanced

What is the output of this refresh token service method after multiple calls?

Given this simplified NestJS AuthService snippet, what will be the value of user.refreshToken after calling refreshToken twice with the same initial refresh token?

NestJS

class AuthService {
  constructor(private userService) {}

  async refreshToken(userId, refreshToken) {
    const user = await this.userService.findById(userId);
    if (!user || user.refreshToken !== refreshToken) {
      throw new Error('Invalid refresh token');
    }
    const newRefreshToken = 'new-token-' + Math.random();
    user.refreshToken = newRefreshToken;
    await this.userService.update(userId, { refreshToken: newRefreshToken });
    return { accessToken: 'access-token', refreshToken: newRefreshToken };
  }
}

// Initial user.refreshToken = 'initial-token'

// Call 1: refreshToken(userId, 'initial-token')
// Call 2: refreshToken(userId, 'initial-token')

A The second call throws an error because the refresh token no longer matches the stored one.

B The second call returns a new refresh token and updates user.refreshToken again.

C The second call returns the same refresh token as the first call without error.

D The second call returns null because the refresh token is invalid.

💡 Hint

Think about how the stored refresh token changes after the first call.

After the first call, user.refreshToken is updated to a new random token. The second call uses the old token, which no longer matches, causing an error.

🔧 Debug

advanced

Why does this NestJS refresh token validation always fail?

Look at this code snippet from a NestJS AuthService. The refresh token validation always fails even with a correct token. What is the bug?

NestJS

async validateRefreshToken(userId: string, token: string) {
  const user = await this.userService.findById(userId);
  if (!user) return false;
  try {
    this.jwtService.verify(token, { secret: process.env.JWT_SECRET });
  } catch {
    return false;
  }
  return user.refreshToken === token;
}

A The secret used in verify is incorrect, so tokens never validate.

B jwtService.verify throws an error on invalid token but is used as a boolean, causing unhandled exceptions.

C The method should compare user.refreshToken before calling jwtService.verify.

D The method returns false because user.refreshToken is undefined.

💡 Hint

Check how jwtService.verify behaves on invalid tokens.

jwtService.verify throws an exception if the token is invalid. Using it as a boolean without try-catch causes the method to fail unexpectedly.

🧠 Conceptual

expert

Which statement best describes the security benefit of rotating refresh tokens in NestJS?

In a NestJS app implementing refresh token rotation, what is the main security advantage of issuing a new refresh token on each refresh request?

A It limits the window for token theft by invalidating old refresh tokens after use.

B It allows multiple devices to share the same refresh token safely.

C It eliminates the need for access tokens entirely.

D It reduces server load by caching refresh tokens longer.

💡 Hint

Think about what happens if a refresh token is stolen and rotation is used.

Rotating refresh tokens means each token can only be used once, so if a token is stolen, it quickly becomes useless, improving security.