Why guards control access in NestJS - Test Your Understanding

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the Guard interface from NestJS.

NestJS

import { [1] } from '@nestjs/common';

Drag options to blanks, or click blank then click option'

ACanActivate

BController

CGuard

DModule

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Guard' instead of 'CanActivate'.

Importing 'Controller' or 'Module' by mistake.

The CanActivate interface is used to create guards in NestJS.

2fill in blank

medium

Complete the code to implement the canActivate method in a guard.

NestJS

canActivate(context: ExecutionContext): [1] { return true; }

Drag options to blanks, or click blank then click option'

Astring

Bvoid

Cnumber

Dboolean

Attempts:

3 left

💡 Hint

Common Mistakes

Returning void or string instead of boolean.

Forgetting to return a value.

The canActivate method must return a boolean to allow or deny access.

3fill in blank

hard

Fix the error in the guard class declaration.

NestJS

export class AuthGuard implements [1] { }

Drag options to blanks, or click blank then click option'

AGuard

BInterceptor

CCanActivate

DPipeTransform

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'Guard' or 'Interceptor' instead of 'CanActivate'.

Confusing guards with pipes.

Guards must implement the CanActivate interface to control access.

4fill in blank

hard

Fill both blanks to create a guard that denies access when user is not authenticated.

NestJS

canActivate(context: ExecutionContext): [1] {
  const request = context.switchToHttp().getRequest();
  return request.user [2] undefined;
}

Drag options to blanks, or click blank then click option'

Aboolean

B!==

C===

Dvoid

Attempts:

3 left

💡 Hint

Common Mistakes

Returning void instead of boolean.

Using '===' instead of '!==' causing wrong access control.

The method returns a boolean. It denies access if request.user is not set.

5fill in blank

hard

Fill all three blanks to create a guard that allows access only if user role is 'admin'.

NestJS

canActivate(context: ExecutionContext): [1] {
  const request = context.switchToHttp().getRequest();
  const user = request.user;
  return user?.role [2] [3];
}

Drag options to blanks, or click blank then click option'

Aboolean

B===

C'admin'

D!==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '!==' instead of '===' causing wrong access control.

Returning non-boolean values.

The guard returns a boolean. It checks if user.role is exactly 'admin' to allow access.

Practice

1. What is the main purpose of guards in a NestJS application?

easy

A. To format the response data before sending it to the client

B. To control access by allowing or blocking requests based on conditions

C. To manage database connections and queries

D. To handle errors and exceptions globally

Solution

1. Step 1: Understand the role of guards

   Guards are designed to check conditions before a route handler runs, deciding if access should be granted.

2. Step 2: Compare with other responsibilities

   Formatting responses, managing database, and error handling are done by other parts of NestJS, not guards.

3. Final Answer:

   To control access by allowing or blocking requests based on conditions -> Option B

4. Quick Check:

   Guards control access = A [OK]

Hint: Guards decide if a request can proceed or not [OK]

Common Mistakes:

• Confusing guards with interceptors or filters
• Thinking guards handle data formatting
• Assuming guards manage database logic

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

easy

A. @UseGuards(AuthGuard) before the method declaration

B. @Guard(AuthGuard) inside the method body

C. UseGuards(AuthGuard) as a function call in the constructor

D. Apply guard logic directly inside the controller constructor

Solution

1. Step 1: Recall NestJS guard syntax

   Guards are applied using the @UseGuards() decorator placed before the method or class.

2. Step 2: Evaluate options

   Only @UseGuards(AuthGuard) before the method declaration uses the correct decorator syntax before the method. Others misuse syntax or placement.

3. Final Answer:

   @UseGuards(AuthGuard) before the method declaration -> Option A

4. Quick Check:

   @UseGuards decorator = A [OK]

Hint: Use @UseGuards decorator above methods or classes [OK]

Common Mistakes:

• Using @Guard instead of @UseGuards
• Calling UseGuards as a function inside constructor
• Placing guard logic inside constructor instead of decorator

3. Given this guard code snippet:

canActivate(context: ExecutionContext) {
  const request = context.switchToHttp().getRequest();
  return request.user && request.user.isAdmin;
}

What will happen if a user without isAdmin tries to access the guarded route?

medium

A. Access will be granted because the guard always returns true

B. The guard will ignore the user and allow access

C. An error will be thrown due to missing isAdmin property

D. Access will be denied because isAdmin is false or missing

Solution

1. Step 1: Analyze the guard condition

   The guard returns true only if request.user exists and request.user.isAdmin is truthy.

2. Step 2: Consider user without isAdmin

   If isAdmin is false or missing, the condition returns false, denying access.

3. Final Answer:

   Access will be denied because isAdmin is false or missing -> Option D

4. Quick Check:

   Guard returns false if not admin = B [OK]

Hint: Guard returns true only if user is admin [OK]

Common Mistakes:

• Assuming missing property causes error
• Thinking guard always returns true
• Ignoring the boolean check on isAdmin

4. Identify the error in this guard implementation:

canActivate(context: ExecutionContext) {
  const request = context.getRequest();
  if (!request.user) {
    throw new Error('User not found');
  }
  return true;
}

medium

A. Throwing an error inside a guard instead of returning false

B. Missing async keyword for canActivate method

C. Using context.getRequest() instead of context.switchToHttp().getRequest()

D. Returning true without checking user roles

Solution

1. Step 1: Check method usage for request

   While context.getRequest() is invalid, the main issue is guard behavior on failure.

2. Step 2: Understand guard error handling

   Guards should return false to deny access, not throw errors which cause server crashes.

3. Final Answer:

   Throwing an error inside a guard instead of returning false -> Option A

4. Quick Check:

   Guards return false, not throw errors [OK]

Hint: Return false to block access, don't throw errors in guards [OK]

Common Mistakes:

• Throwing errors instead of returning false
• Using incorrect context method for request
• Assuming async is required without async code

5. You want to restrict access to a route only to logged-in users with the role 'admin' or 'moderator'. Which guard logic correctly implements this?

hard

A. Return true if request.user does not exist

B. Return true if request.user exists and request.user.role === 'admin' && request.user.role === 'moderator'

C. Return true if request.user exists and request.user.role === 'admin' || request.user.role === 'moderator'

D. Return true if request.user exists and request.user.role !== 'admin' || request.user.role !== 'moderator'

Solution

1. Step 1: Understand role conditions

   The user must be logged in and have role 'admin' OR 'moderator' to access.

2. Step 2: Evaluate logical operators

   Return true if request.user exists and request.user.role === 'admin' || request.user.role === 'moderator' uses OR (||) correctly. Return true if request.user exists and request.user.role === 'admin' && request.user.role === 'moderator' uses AND (&&) which is impossible for one role. Return true if request.user exists and request.user.role !== 'admin' || request.user.role !== 'moderator' uses negations incorrectly. Return true if request.user does not exist allows access without user.

3. Final Answer:

   Return true if request.user exists and request.user.role === 'admin' || request.user.role === 'moderator' -> Option C

4. Quick Check:

   Use OR for multiple allowed roles = D [OK]

Hint: Use OR (||) to allow multiple roles [OK]

Common Mistakes:

• Using AND (&&) instead of OR (||) for roles
• Allowing access when user is missing
• Incorrect negation logic in conditions