Role-based guards in NestJS - Mini Project: Build & Apply

Role-based Guards in NestJS

📖 Scenario: You are building a simple NestJS backend for a company. Different users have different roles like admin and user. You want to protect certain routes so only users with the right role can access them.

🎯 Goal: Create a role-based guard in NestJS that checks if a user has the required role before allowing access to a route.

📋 What You'll Learn

Create a roles array to hold user roles

Create a constant for the required role

Implement a guard class that checks user roles

Apply the guard to a controller route

💡 Why This Matters

🌍 Real World

Role-based guards are used in real applications to restrict access to certain parts of a backend API based on user permissions.

💼 Career

Understanding role-based guards is essential for backend developers working with NestJS to build secure and maintainable applications.

1

Create a roles array

Create a constant array called roles with the exact values 'admin' and 'user'.

NestJS

# Create a constant array called roles with 'admin' and 'user'
# Your code here

Hint

Use const roles = ['admin', 'user']; to create the array.

2

Create a required role constant

Create a constant called requiredRole and set it to the string 'admin'.

NestJS

const roles = ['admin', 'user'];
# Create a constant called requiredRole and set it to 'admin'
# Your code here

Hint

Use const requiredRole = 'admin'; to set the required role.

3

Implement the role guard class

Create a class called RolesGuard that implements CanActivate. Inside, write a canActivate method that takes context and returns true only if requiredRole is included in roles.

NestJS

const roles = ['admin', 'user'];
const requiredRole = 'admin';

# Create RolesGuard class implementing CanActivate
# Add canActivate method checking if requiredRole is in roles
# Your code here

Hint

Use roles.includes(requiredRole) inside canActivate to check the role.

4

Apply the guard to a controller route

In a controller class called AppController, create a method getAdminData decorated with @Get('admin') and @UseGuards(RolesGuard). The method should return the string 'Admin content'.

NestJS

const roles = ['admin', 'user'];
const requiredRole = 'admin';

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

@Injectable()
export class RolesGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    return roles.includes(requiredRole);
  }
}

# Create AppController class
# Add getAdminData method with @Get('admin') and @UseGuards(RolesGuard)
# Return 'Admin content' from getAdminData
# Your code here

Hint

Use @UseGuards(RolesGuard) above the method to protect the route.

Practice

1. What is the main purpose of a role-based guard in NestJS?

easy

A. To manage application configuration

B. To handle database connections

C. To format HTTP responses

D. To restrict access to routes based on user roles

Solution

1. Step 1: Understand role-based guard function

   Role-based guards check user roles before allowing access to routes.

2. Step 2: Identify the correct purpose

   Restricting access based on roles matches the guard's role in security.

3. Final Answer:

   To restrict access to routes based on user roles -> Option D

4. Quick Check:

   Role-based guard = restrict access by roles [OK]

Hint: Guards control access; roles limit who can enter [OK]

Common Mistakes:

• Confusing guards with database or config management
• Thinking guards format responses
• Assuming guards handle unrelated tasks

2. Which of the following is the correct way to apply a role-based guard to a route in NestJS?

easy

A. @UseGuards(RolesGuard)\n@Get('profile')

B. @Controller('profile')\n@RolesGuard()

C. @Roles('admin')\n@UseGuards(AuthGuard)

D. @UseGuards(AuthGuard)\n@Get('profile')

Solution

1. Step 1: Identify correct guard usage syntax

   Use @UseGuards with the guard class to apply it to routes.

2. Step 2: Match with role-based guard usage

   @UseGuards(RolesGuard) is correct; @Roles decorator marks roles separately.

3. Final Answer:

   @UseGuards(RolesGuard)\n@Get('profile') -> Option A

4. Quick Check:

   @UseGuards(RolesGuard) applies guard [OK]

Hint: Use @UseGuards with guard class to protect routes [OK]

Common Mistakes:

• Using @RolesGuard() instead of @UseGuards
• Mixing @Roles and @UseGuards incorrectly
• Applying guards without @UseGuards decorator

3. Given this guard code snippet, what will be the result if the user role is 'user' and the route requires 'admin' role?

canActivate(context: ExecutionContext) {
  const roles = this.reflector.get('roles', context.getHandler());
  const request = context.switchToHttp().getRequest();
  const user = request.user;
  return roles.includes(user.role);
}

medium

A. Access granted

B. Access denied

C. Runtime error due to missing roles

D. Always returns true

Solution

1. Step 1: Understand role check logic

   The guard checks if the user's role is included in the allowed roles array.

2. Step 2: Apply given roles and user role

   User role is 'user', route requires 'admin', so roles.includes('user') is false.

3. Final Answer:

   Access denied -> Option B

4. Quick Check:

   User role not in allowed roles = deny access [OK]

Hint: Check if user role is in allowed roles array [OK]

Common Mistakes:

• Assuming access is granted if roles differ
• Thinking roles includes user role always
• Ignoring roles array presence

4. Identify the error in this role-based guard implementation:

canActivate(context: ExecutionContext) {
  const roles = this.reflector.get('roles', context.getHandler());
  const request = context.switchToHttp().getRequest();
  const user = request.user;
  if (!roles) {
    return false;
  }
  return roles.includes(user.role);
}

medium

A. Returns false if no roles metadata, blocking all access

B. Missing async keyword causes runtime error

C. Incorrect use of context.getHandler()

D. Should check user permissions instead of roles

Solution

1. Step 1: Analyze roles metadata check

   If roles metadata is missing, the guard returns false blocking access.

2. Step 2: Understand typical behavior for missing roles

   Usually, missing roles means no restriction, so returning false is too strict.

3. Final Answer:

   Returns false if no roles metadata, blocking all access -> Option A

4. Quick Check:

   No roles metadata should allow access, not block [OK]

Hint: Missing roles metadata should not block access by default [OK]

Common Mistakes:

• Assuming missing roles means deny access
• Forgetting to handle undefined roles gracefully
• Confusing async necessity in this context

5. You want to create a role-based guard that allows access if the user has any one of multiple roles specified on a route. Which approach correctly implements this behavior?

hard

A. Use multiple guards each checking one role and combine them with @UseGuards

B. Check only the first role in roles array against user.role in canActivate

C. Use @Roles('admin', 'editor') decorator and check if user.role is included in roles array inside canActivate

D. Ignore roles and allow access to all authenticated users

Solution

1. Step 1: Understand multiple roles in @Roles decorator

   @Roles can accept multiple roles as arguments to specify allowed roles.

2. Step 2: Implement canActivate to check inclusion

   Checking if user.role is in roles array allows access if user has any allowed role.

3. Final Answer:

   Use @Roles('admin', 'editor') decorator and check if user.role is included in roles array inside canActivate -> Option C

4. Quick Check:

   Check user role in roles array for multiple allowed roles [OK]

Hint: Check if user role is in roles array for multiple roles [OK]

Common Mistakes:

• Checking only first role ignores others
• Using multiple guards unnecessarily
• Allowing all authenticated users ignores roles