Guard interface (canActivate) in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

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

NestJS

canActivate(context: ExecutionContext): boolean | Promise<boolean> | Observable<boolean> {
  return [1];
}

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

Atrue

Bfalse

Cnull

Dundefined

Attempts:

3 left

💡 Hint

Common Mistakes

Returning false blocks access.

Returning null or undefined causes errors.

The canActivate method must return true to allow access.

2fill in blank

medium

Complete the code to extract the request object inside canActivate using ExecutionContext.

NestJS

const request = context.switchToHttp().[1]();

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

AgetResponse

BgetNext

CgetRequest

DgetHandler

Attempts:

3 left

💡 Hint

Common Mistakes

Using getResponse instead of getRequest.

Trying to access request directly without switchToHttp.

Use getRequest() to access the HTTP request object inside the guard.

3fill in blank

hard

Fix the error in the guard method to correctly check if user is authenticated.

NestJS

const user = request.user;
if (!user) {
  throw new [1]('Unauthorized');
}
return true;

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

AForbiddenException

BUnauthorizedException

CBadRequestException

DNotFoundException

Attempts:

3 left

💡 Hint

Common Mistakes

Throwing ForbiddenException instead of UnauthorizedException.

Not throwing any exception and returning false.

Throw UnauthorizedException when the user is not authenticated.

4fill in blank

hard

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

NestJS

const user = request.user;
return user?.role [1] [2];

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

A===

B!==

C'admin'

D'user'

Attempts:

3 left

💡 Hint

Common Mistakes

Using !== instead of ===.

Comparing to 'user' instead of 'admin'.

The guard returns true only if user.role === 'admin'.

5fill in blank

hard

Fill all three blanks to implement a guard that checks if request method is POST and user is authenticated.

NestJS

const request = context.switchToHttp().getRequest();
if (request.method [1] 'POST' && request.user [2] null) {
  return [3];
}
return false;

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

A===

B!==

Ctrue

Dfalse

Attempts:

3 left

💡 Hint

Common Mistakes

Using == instead of ===.

Checking user === null instead of !== null.

Returning false instead of true when conditions met.

The guard checks if method is 'POST' and user is not null, then returns true to allow access.

Practice

1. What is the main purpose of the canActivate method in a NestJS guard?

easy

A. To format the response data

B. To handle database connections

C. To decide if a request should be allowed or denied access

D. To log user activity

Solution

1. Step 1: Understand the role of guards in NestJS

   Guards are used to control access to routes by allowing or denying requests.

2. Step 2: Identify the function of canActivate

   The canActivate method returns true or false to allow or deny access.

3. Final Answer:

   To decide if a request should be allowed or denied access -> Option C

4. Quick Check:

   Guard access control = canActivate returns true/false [OK]

Hint: Remember: canActivate returns true to allow access [OK]

Common Mistakes:

• Confusing canActivate with response formatting
• Thinking canActivate handles database logic
• Assuming canActivate logs user actions

2. Which of the following is the correct syntax to implement a guard class with canActivate in NestJS?

easy

A. class MyGuard { canActivate(context) { return true; } }

B. function MyGuard() { canActivate() { return true; } }

C. const MyGuard = () => { canActivate() { return true; } }

D. class MyGuard implements CanActivate { canActivate(context: ExecutionContext): boolean { return true; } }

Solution

1. Step 1: Recognize the NestJS guard pattern

   Guards are classes that implement the CanActivate interface.

2. Step 2: Check the method signature and class syntax

   The method must be named canActivate and accept an ExecutionContext parameter, returning a boolean.

3. Final Answer:

   class MyGuard implements CanActivate { canActivate(context: ExecutionContext): boolean { return true; } } -> Option D

4. Quick Check:

   Guard class implements CanActivate with canActivate method [OK]

Hint: Use class with implements CanActivate and proper method signature [OK]

Common Mistakes:

• Using function instead of class
• Missing implements CanActivate
• Incorrect method parameters or return type

3. Given this guard code, what will be the result when a request is made?

class AlwaysDenyGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    return false;
  }
}

medium

A. The request is denied access

B. The request is allowed to proceed

C. A runtime error occurs

D. The guard is ignored and request proceeds

Solution

1. Step 1: Analyze the canActivate return value

   The method always returns false, meaning access is denied.

2. Step 2: Understand guard effect on request

   When canActivate returns false, NestJS blocks the request from continuing.

3. Final Answer:

   The request is denied access -> Option A

4. Quick Check:

   canActivate false = deny request [OK]

Hint: False return in canActivate means deny access [OK]

Common Mistakes:

• Assuming false allows access
• Thinking guard causes error on false
• Believing guard is skipped if false

4. Identify the error in this guard implementation:

class MyGuard implements CanActivate {
  canActivate() {
    return true;
  }
}

medium

A. Incorrect return type for canActivate

B. Missing parameter in canActivate method

C. Class does not implement CanActivate interface

D. canActivate method should be async

Solution

1. Step 1: Check method signature requirements

   The canActivate method must accept a parameter of type ExecutionContext.

2. Step 2: Compare with given code

   The method has no parameters, which breaks the interface contract.

3. Final Answer:

   Missing parameter in canActivate method -> Option B

4. Quick Check:

   canActivate needs ExecutionContext parameter [OK]

Hint: canActivate always needs ExecutionContext parameter [OK]

Common Mistakes:

• Omitting method parameters
• Assuming async is required
• Ignoring interface implementation rules

5. You want to create a guard that only allows access if the user role is 'admin'. Which of these canActivate implementations correctly checks the role from the request object?

hard

A. canActivate(context: ExecutionContext): boolean {
const request = context.switchToHttp().getRequest();
return request.user?.role === 'admin';
}

B. canActivate(context: ExecutionContext): boolean {
const user = context.getUser();
return user.role === 'admin';
}

C. canActivate(context: ExecutionContext): boolean {
const request = context.getRequest();
return request.user.role === 'admin';
}

D. canActivate(context: ExecutionContext): boolean {
const req = context.switchToHttp().getRequest();
return req.user.role == 'admin';
}

Solution

1. Step 1: Access the HTTP request correctly

   Use context.switchToHttp().getRequest() to get the request object.

2. Step 2: Safely check the user role

   Use optional chaining request.user?.role to avoid errors if user is undefined.

3. Final Answer:

   canActivate(context: ExecutionContext): boolean { const request = context.switchToHttp().getRequest(); return request.user?.role === 'admin'; } -> Option A

4. Quick Check:

   Use switchToHttp and optional chaining for safe role check [OK]

Hint: Use switchToHttp().getRequest() and optional chaining for safety [OK]

Common Mistakes:

• Using context.getUser() which does not exist
• Calling context.getRequest() directly without switchToHttp()
• Not using optional chaining causing runtime errors