Combining multiple guards in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of guards in NestJS?

Guards in NestJS control whether a request can proceed to the route handler by checking conditions like authentication or roles.

beginner

How do you apply multiple guards to a single route in NestJS?

You apply multiple guards by passing them as arguments to the @UseGuards() decorator, like @UseGuards(Guard1, Guard2).

intermediate

What happens if one guard in a combined set returns false?

If any guard returns false, NestJS stops and denies access to the route, so all guards must pass for the request to proceed.

intermediate

Can you combine guards globally and locally in NestJS?

Yes, you can apply guards globally for all routes and also locally on specific controllers or routes. Local guards run after global ones.

beginner

Example: How to combine AuthGuard and RolesGuard on a route?

Use @UseGuards(AuthGuard, RolesGuard) above the route handler. This checks authentication first, then user roles before allowing access.

How do you apply multiple guards to a NestJS route?

A NestJS does not support multiple guards

B Pass them as separate @UseGuards() decorators

C Use @ApplyGuards() decorator

D Pass them as an array to a single @UseGuards() decorator

You pass multiple guards as arguments to a single @UseGuards() decorator, like @UseGuards(Guard1, Guard2).

What happens if one guard returns false in a combined guard set?

A Access is denied and request stops

B Only that guard is ignored

C The request proceeds anyway

D NestJS throws an error

If any guard returns false, NestJS denies access and stops processing the request.

Can you combine global and local guards in NestJS?

A No, only one type can be used

B Yes, global guards run before local guards

C Yes, local guards run before global guards

D Only global guards are supported

Global guards run first, then local guards run on specific controllers or routes.

Which decorator is used to apply guards in NestJS?

A @UseFilters()

B @UseInterceptors()

C @UseGuards()

D @UsePipes()

@UseGuards() is the decorator to apply guards to routes or controllers.

What is a common use case for combining AuthGuard and RolesGuard?

A To check user authentication and authorization

B To validate request body format

C To log requests

D To handle exceptions

AuthGuard checks if the user is logged in, RolesGuard checks if the user has the right permissions.

Explain how NestJS processes multiple guards applied to a route.

Think about a security checkpoint where multiple checks must pass.

Describe how to apply guards globally and locally in NestJS and how they interact.

Consider a building with a main gate (global) and room doors (local).

Practice

1.

What happens when you use @UseGuards(AuthGuard, RolesGuard) on a NestJS route handler?

Choose the best description.

easy

A. Only AuthGuard is checked; RolesGuard is ignored.

B. Both AuthGuard and RolesGuard must allow access for the request to proceed.

C. Either AuthGuard or RolesGuard allowing access is enough to proceed.

D. Guards run in parallel but only the first to approve grants access.

Solution

1. Step 1: Understand how multiple guards work in NestJS

   When multiple guards are used with @UseGuards(), NestJS runs them in sequence and requires all to approve access.

2. Step 2: Apply this to the given guards

   Both AuthGuard and RolesGuard must return true for the request to proceed.

3. Final Answer:

   Both AuthGuard and RolesGuard must allow access for the request to proceed. -> Option B

4. Quick Check:

   All guards must approve = A [OK]

Hint: All guards must pass for access, not just one [OK]

Common Mistakes:

• Thinking only one guard needs to pass
• Assuming guards run in parallel and race
• Believing guards are ignored if one fails

2.

Which of the following is the correct syntax to apply two guards AuthGuard and RolesGuard on a controller method?

?

easy

A. @UseGuards(AuthGuard && RolesGuard)
handleRequest() {}

B. @UseGuards(AuthGuard RolesGuard)
handleRequest() {}

C. @UseGuards(AuthGuard || RolesGuard)
handleRequest() {}

D. @UseGuards(AuthGuard, RolesGuard)
handleRequest() {}

Solution

1. Step 1: Recall the correct syntax for multiple guards

   In NestJS, multiple guards are passed as separate arguments to @UseGuards(), separated by commas.

2. Step 2: Check each option

   @UseGuards(AuthGuard, RolesGuard) handleRequest() {} correctly uses comma-separated guards. @UseGuards(AuthGuard RolesGuard) handleRequest() {} misses the comma between guards, causing a syntax error. Using && or || evaluates to only one guard.

3. Final Answer:

   @UseGuards(AuthGuard, RolesGuard) handleRequest() {} -> Option D

4. Quick Check:

   Comma-separated guards = C [OK]

Hint: Use commas to separate guards, avoid logical operators or missing commas in @UseGuards() [OK]

Common Mistakes:

• Omitting comma between guards
• Using && or || inside @UseGuards()
• Forgetting to separate guards by commas

3.

Given the following code snippet, what will be the output when a request is made?

@UseGuards(AuthGuard, RolesGuard)
@Get('profile')
getProfile() {
  return 'User Profile';
}

// AuthGuard returns true
// RolesGuard returns false

medium

A. Throws an UnauthorizedException

B. Returns 'User Profile'

C. Returns null

D. Returns an empty string

Solution

1. Step 1: Understand guard return values effect

   Guards must all return true to allow access. If any guard returns false, NestJS throws an UnauthorizedException by default.

2. Step 2: Apply given guard results

   Here, AuthGuard returns true but RolesGuard returns false, so access is denied and an exception is thrown.

3. Final Answer:

   Throws an UnauthorizedException -> Option A

4. Quick Check:

   Any guard false = UnauthorizedException [OK]

Hint: All guards must return true; one false blocks access [OK]

Common Mistakes:

• Assuming one guard false still allows access
• Expecting null or empty string instead of exception
• Confusing guard return values with response data

4.

Identify the error in this NestJS controller snippet using multiple guards:

@UseGuards(AuthGuard RolesGuard)
@Get('data')
getData() {
  return 'Data';
}

medium

A. The @Get decorator is misplaced

B. Guards must be passed as an array

C. Missing comma between guards in @UseGuards()

D. Guards cannot be combined on the same method

Solution

1. Step 1: Check syntax of @UseGuards()

   The guards inside @UseGuards() must be separated by commas.

2. Step 2: Identify the error in the snippet

   The snippet misses a comma between AuthGuard and RolesGuard, causing a syntax error.

3. Final Answer:

   Missing comma between guards in @UseGuards() -> Option C

4. Quick Check:

   Comma missing between guards = D [OK]

Hint: Check commas between guards inside @UseGuards() [OK]

Common Mistakes:

• Forgetting commas between guards
• Passing guards as an array incorrectly
• Misplacing decorators order

5.

You want to protect a route so that only authenticated users with the role 'admin' can access it. You have AuthGuard and RolesGuard available. How should you combine them in your controller method?

hard

A. @UseGuards(AuthGuard, RolesGuard) // RolesGuard checks for 'admin' role

B. @UseGuards(RolesGuard, AuthGuard) // Order does not matter

C. @UseGuards(AuthGuard || RolesGuard) // Either guard allows access

D. @UseGuards([RolesGuard, AuthGuard]) // Guards passed as array

Solution

1. Step 1: Understand guard order and combination

   Guards are combined by passing them comma-separated in @UseGuards(). Order matters: AuthGuard first authenticates and sets req.user, then RolesGuard checks the role.

2. Step 2: Choose correct syntax and logic

   @UseGuards(AuthGuard, RolesGuard) // RolesGuard checks for 'admin' role uses correct syntax, order, and notes the role check. Reversing to RolesGuard first fails as no user is set, and claims order irrelevant which is misleading. Array with reversed order fails similarly. Logical operators pass only one guard.

3. Final Answer:

   @UseGuards(AuthGuard, RolesGuard) // RolesGuard checks for 'admin' role -> Option A

4. Quick Check:

   Comma-separated guards with role check = B [OK]

Hint: Use comma-separated guards; RolesGuard checks roles [OK]

Common Mistakes:

• Using logical operators inside @UseGuards()
• Passing guards as an array
• Assuming guard order always matters