JWT authentication guard in NestJS - Mini Project: Build & Apply

JWT Authentication Guard in NestJS

📖 Scenario: You are building a secure API using NestJS. You want to protect certain routes so only users with a valid JWT token can access them.

🎯 Goal: Create a JWT authentication guard that checks if the incoming request has a valid JWT token and allows access only if the token is valid.

📋 What You'll Learn

Create a JWT payload interface

Create a JWT authentication guard class

Use the CanActivate interface and ExecutionContext

Validate the JWT token from the request headers

Allow access only if the token is valid

💡 Why This Matters

🌍 Real World

JWT authentication guards are used in real APIs to protect sensitive routes and ensure only authorized users can access them.

💼 Career

Understanding how to implement JWT guards is essential for backend developers working with NestJS or similar frameworks to build secure applications.

1

Create the JWT payload interface

Create an interface called JwtPayload with a single string property userId.

NestJS

// Your code here

Hint

Interfaces define the shape of data. Here, JwtPayload will represent the data inside the JWT token.

2

Create the JWT authentication guard class

Create a class called JwtAuthGuard that implements CanActivate from @nestjs/common. Import CanActivate and ExecutionContext from @nestjs/common.

NestJS

export interface JwtPayload {
  userId: string;
}

// Your code here

Hint

The guard class must implement CanActivate and have a canActivate method that returns a boolean.

3

Extract and validate the JWT token

Inside the canActivate method, get the request object from context.switchToHttp().getRequest(). Extract the authorization header from request.headers. Return false if the header is missing or does not start with 'Bearer '. Otherwise, extract the token string after 'Bearer '.

NestJS

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

export interface JwtPayload {
  userId: string;
}

@Injectable()
export class JwtAuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    // Your code here
  }
}

Hint

The JWT token is usually sent in the Authorization header as Bearer <token>. We check this format before extracting the token.

4

Validate the JWT token and complete the guard

Import verify from jsonwebtoken. Inside canActivate, use verify(token, 'secretKey') to decode the token. If verification throws an error, return false. If successful, attach the decoded payload to request.user and return true. Use a try-catch block for error handling.

NestJS

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

export interface JwtPayload {
  userId: string;
}

@Injectable()
export class JwtAuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    const authHeader = request.headers['authorization'];
    if (!authHeader || !authHeader.startsWith('Bearer ')) {
      return false;
    }
    const token = authHeader.slice(7);
    // Your code here
  }
}

Hint

Use verify to check the token. If it fails, deny access. If it passes, save the user info for later use.

Practice

1. What is the main purpose of a JWT authentication guard in a NestJS application?

easy

A. To verify the JWT token and protect routes from unauthorized access

B. To generate new JWT tokens for users

C. To store user data in the database

D. To handle HTTP request logging

Solution

1. Step 1: Understand the role of JWT guard

   The JWT authentication guard checks if the incoming request has a valid JWT token.

2. Step 2: Identify what the guard protects

   The guard protects routes by allowing access only if the token is valid, preventing unauthorized users.

3. Final Answer:

   To verify the JWT token and protect routes from unauthorized access -> Option A

4. Quick Check:

   JWT guard = verify token and protect routes [OK]

Hint: JWT guard checks token validity to protect routes [OK]

Common Mistakes:

• Confusing guard with token generation
• Thinking guard stores user data
• Assuming guard handles logging

2. Which of the following is the correct way to apply a JWT authentication guard to a controller route in NestJS?

easy

A. Use @Get('jwt') decorator on the route

B. @Controller('jwt') before the class

C. Inject JwtService directly in the route handler

D. @UseGuards(AuthGuard('jwt')) before the route handler method

Solution

1. Step 1: Recall how guards are applied in NestJS

   Guards are applied using the @UseGuards() decorator before route handlers or controllers.

2. Step 2: Identify the correct guard usage for JWT

   The JWT guard is used as AuthGuard('jwt') inside @UseGuards decorator.

3. Final Answer:

   @UseGuards(AuthGuard('jwt')) before the route handler method -> Option D

4. Quick Check:

   @UseGuards(AuthGuard('jwt')) = correct syntax [OK]

Hint: Use @UseGuards(AuthGuard('jwt')) to protect routes [OK]

Common Mistakes:

• Using @Controller('jwt') instead of guard
• Injecting JwtService instead of using guard
• Confusing route decorators with guards

3. Given the following NestJS JWT guard code snippet, what will happen if the request has no Authorization header?

canActivate(context: ExecutionContext) {
  const request = context.switchToHttp().getRequest();
  const authHeader = request.headers['authorization'];
  if (!authHeader) {
    throw new UnauthorizedException('No token provided');
  }
  // further token validation...
  return true;
}

medium

A. The guard allows the request to proceed

B. The guard throws an UnauthorizedException with message 'No token provided'

C. The guard returns false silently

D. The guard logs a warning but allows access

Solution

1. Step 1: Analyze the condition for missing Authorization header

   If the header is missing, the code throws UnauthorizedException with a specific message.

2. Step 2: Understand the effect of throwing UnauthorizedException

   Throwing this exception stops the request and returns an HTTP 401 error to the client.

3. Final Answer:

   The guard throws an UnauthorizedException with message 'No token provided' -> Option B

4. Quick Check:

   No Authorization header = UnauthorizedException thrown [OK]

Hint: Missing token causes UnauthorizedException throw [OK]

Common Mistakes:

• Assuming guard returns false instead of throwing
• Thinking request proceeds without token
• Confusing logging with exception throwing

4. Identify the error in this JWT guard snippet that causes it to always allow access even with invalid tokens:

async canActivate(context: ExecutionContext) {
  const request = context.switchToHttp().getRequest();
  const token = request.headers['authorization']?.split(' ')[1];
  if (!token) return false;
  try {
    const payload = await this.jwtService.verifyAsync(token);
    request.user = payload;
  } catch {
    return true;
  }
  return true;
}

medium

A. The method should not be async

B. The token extraction splits incorrectly

C. The catch block returns true, allowing access on token errors

D. The user is not attached to the request

Solution

1. Step 1: Review the catch block behavior

   The catch block returns true, which means even if token verification fails, access is allowed.

2. Step 2: Understand correct error handling in guards

   On verification failure, the guard should deny access by returning false or throwing an exception.

3. Final Answer:

   The catch block returns true, allowing access on token errors -> Option C

4. Quick Check:

   Catch returning true = always allow access [OK]

Hint: Catch block must deny access, not allow it [OK]

Common Mistakes:

• Returning true in catch block
• Ignoring token verification errors
• Not attaching user to request

5. You want to create a custom JWT authentication guard that extracts the user from the token and adds a custom property isAdmin based on the user's role. Which approach correctly extends the default JWT guard to achieve this?

hard

A. Extend AuthGuard('jwt'), override handleRequest to add isAdmin to user, then return user

B. Create a new guard without extending AuthGuard and manually decode token

C. Use a middleware instead of a guard to add isAdmin property

D. Override canActivate to return true always and add isAdmin in controller

Solution

1. Step 1: Understand extending AuthGuard('jwt')

   Extending the default JWT guard allows reuse of token validation logic.

2. Step 2: Override handleRequest to customize user object

   handleRequest receives the validated user; adding isAdmin here modifies the user before passing it on.

3. Step 3: Return the modified user to allow access

   Returning the user with isAdmin set ensures the controller can access this property safely.

4. Final Answer:

   Extend AuthGuard('jwt'), override handleRequest to add isAdmin to user, then return user -> Option A

5. Quick Check:

   Extend guard + override handleRequest = customize user [OK]

Hint: Extend AuthGuard and override handleRequest to customize user [OK]

Common Mistakes:

• Not extending AuthGuard and duplicating logic
• Using middleware instead of guard for auth
• Overriding canActivate incorrectly