Role-based guards in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a role-based guard in NestJS?

A role-based guard in NestJS is a piece of code that checks if a user has the right role to access a certain part of the application. It helps protect routes by allowing only users with specific roles to proceed.

beginner

How do you apply a role-based guard to a route in NestJS?

You apply a role-based guard by using the @UseGuards() decorator on a controller or route handler and passing the guard class that checks user roles.

intermediate

What is the purpose of the CanActivate interface in NestJS guards?

The CanActivate interface defines a method called canActivate() that returns true or false to allow or deny access. Role-based guards implement this interface to check user roles before allowing access.

intermediate

How can you pass roles to a role-based guard in NestJS?

You can pass roles using a custom decorator that stores roles as metadata on the route. The guard then reads this metadata to check if the user has one of the required roles.

beginner

Why is role-based guarding important in web applications?

Role-based guarding helps keep parts of an app safe by making sure only users with the right permissions can access sensitive data or actions. It supports security and proper user management.

Which NestJS interface must a role-based guard implement?

A PipeTransform

B OnModuleInit

C ExceptionFilter

D CanActivate

Role-based guards implement the CanActivate interface to decide if a request can proceed.

How do you specify which roles can access a route?

A By setting roles in the main.ts file

B By hardcoding roles inside the guard only

C By using a custom decorator that sets roles metadata

D By using the @Injectable decorator

A custom decorator sets roles as metadata on routes, which the guard reads to check access.

What does the canActivate() method return to allow access?

A true

B null

C undefined

D false

Returning true from canActivate() means access is allowed.

Where do you apply a role-based guard in NestJS?

A In the database schema

B On controllers or route handlers using @UseGuards()

C Inside the service classes

D Only in the main application bootstrap

Guards are applied on controllers or routes with the @UseGuards() decorator.

What is the main benefit of using role-based guards?

A To restrict access based on user roles

B To speed up database queries

C To style the user interface

D To log user activity

Role-based guards restrict access to routes depending on user roles.

Explain how role-based guards work in NestJS and how you implement them.

Think about how NestJS checks if a user can access a route based on their role.

Describe why role-based guards are important for application security.

Consider what happens if anyone could access all parts of an app.

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