Combining multiple guards in NestJS - Step-by-Step Execution

Concept Flow - Combining multiple guards

Request comes in

↓

Guard 1 runs

↓

Guard 2 runs

The request passes through Guard 1 first. If it passes, Guard 2 runs. If all guards pass, access is allowed; if any guard fails, the request is rejected.

Execution Sample

NestJS

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

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

This code applies two guards, AuthGuard and RolesGuard, to protect the 'profile' route. Both must pass for access.

Execution Table

Step	Guard	Guard Result	Action	Request Outcome
1	AuthGuard	Pass	Continue to next guard	Pending
2	RolesGuard	Pass	Allow access	Access granted
3	End	-	-	Request handled successfully

💡 Execution stops after all guards pass and access is granted.

Variable Tracker

Variable	Start	After AuthGuard	After RolesGuard	Final
requestAllowed	false	true	true	true

Key Moments - 2 Insights

Why does the request get rejected if the first guard fails?

Because guards run in order, and if any guard returns false (see Step 1 in execution_table), the request is immediately rejected without running later guards.

Can the second guard run if the first guard fails?

No. The second guard only runs if the first guard passes, as shown in the flow and execution_table steps 1 and 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the requestAllowed value after AuthGuard passes?

A undefined

B false

C true

D null

💡 Hint

Check variable_tracker row for requestAllowed after AuthGuard

At which step does the request get access granted?

A Step 2

B Step 1

C Step 3

D None

💡 Hint

See execution_table 'Request Outcome' column for Step 2

If AuthGuard fails, what happens to RolesGuard?

A It runs after a delay

B It does not run

C It runs anyway

D It runs twice

💡 Hint

Refer to concept_flow and key_moments about guard order

Concept Snapshot

Use @UseGuards(Guard1, Guard2) to combine guards.
Guards run in order; if any guard returns false, request is denied.
All guards must pass to allow access.
Useful for layered security checks.
Easy to add or remove guards as needed.

Full Transcript

In NestJS, you can protect routes by combining multiple guards using the @UseGuards decorator. When a request comes in, the first guard runs. If it passes, the next guard runs. This continues until all guards pass, then the request is allowed. If any guard fails, the request is rejected immediately. For example, using @UseGuards(AuthGuard, RolesGuard) means the request must pass authentication and role checks. The execution table shows each guard's result and the final outcome. The variable tracker shows the requestAllowed state changing from false to true after guards pass. Remember, guards run in order and stop on failure. This helps build secure and layered access control in your NestJS apps.

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