Combining multiple guards in NestJS - Mini Project: Build & Apply

Combining Multiple Guards in NestJS

📖 Scenario: You are building a simple NestJS API that needs to protect a route with two guards: one to check if the user is authenticated, and another to check if the user has admin rights.This is like having two security guards at a door: one checks your ID, the other checks if you have a special pass.

🎯 Goal: Create a NestJS controller with a route protected by two guards combined using the @UseGuards() decorator.The route should only be accessible if both guards allow access.

📋 What You'll Learn

Create two guard classes: AuthGuard and AdminGuard

Create a controller called AppController with a route GET /protected

Apply both guards together on the /protected route using @UseGuards()

The guards should implement the CanActivate interface and return true or false accordingly

💡 Why This Matters

🌍 Real World

Combining multiple guards is common in real NestJS apps to layer security checks, like authentication and authorization.

💼 Career

Understanding how to combine guards helps you build secure APIs and is a key skill for backend developers working with NestJS.

1

Create the two guard classes

Create two guard classes called AuthGuard and AdminGuard. Each should implement the CanActivate interface from @nestjs/common and have a canActivate() method that returns true.

NestJS

import { CanActivate, ExecutionContext } from '@nestjs/common';

// Your code here

Hint

Remember to import CanActivate and ExecutionContext from @nestjs/common.

2

Create the controller with a protected route

Create a controller class called AppController with a method getProtected() decorated with @Get('protected') from @nestjs/common. The method should return the string 'Protected content'.

NestJS

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

// Your code here

Hint

Import Controller and Get from @nestjs/common and decorate the class and method properly.

3

Apply both guards to the protected route

Use the @UseGuards() decorator from @nestjs/common on the getProtected() method. Pass both AuthGuard and AdminGuard inside @UseGuards() to combine them.

NestJS

import { UseGuards } from '@nestjs/common';

// Add @UseGuards(AuthGuard, AdminGuard) above getProtected()
// Your code here

Hint

Import UseGuards and add @UseGuards(AuthGuard, AdminGuard) above the getProtected() method.

4

Export the controller and guards for use

Add export keywords to the AppController, AuthGuard, and AdminGuard classes so they can be used in other parts of the application.

NestJS

// Your code here

Hint

Make sure all classes have the export keyword before class.

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