Protected routes with guards in NestJS

Introduction

Protected routes keep parts of your app safe by checking who can access them. Guards help decide if a user is allowed or not before showing the page.

When you want only logged-in users to see certain pages.

When you need to check user roles before allowing access.

When you want to block unauthorized users from API endpoints.

When you want to add extra checks before running route handlers.

Syntax

NestJS

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

@Injectable()
export class AuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean | Promise<boolean> | Observable<boolean> {
    // Your logic here
    return true; // or false
  }
}

The guard class must implement CanActivate interface.

The canActivate method returns true to allow access, false to block.

Examples

This guard checks if the request has an authorization header to allow access.

NestJS

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

@Injectable()
export class AuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    return !!request.headers.authorization;
  }
}

This guard allows access only if the user has the 'admin' role.

NestJS

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

@Injectable()
export class RolesGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    const user = request.user;
    return user?.role === 'admin';
  }
}

Apply the guard to protect the 'profile' route so only authorized users can access it.

NestJS

@UseGuards(AuthGuard)
@Get('profile')
getProfile() {
  return 'User profile data';
}

Sample Program

This example shows a simple guard that checks for an authorization header. The 'profile' route is protected and only accessible if the header is present.

NestJS

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

@Injectable()
class AuthGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    // Simple check: allow if header 'authorization' exists
    return !!request.headers.authorization;
  }
}

@Controller('users')
export class UserController {
  @UseGuards(AuthGuard)
  @Get('profile')
  getProfile() {
    return { message: 'This is a protected user profile' };
  }
}

Important Notes

Guards run before route handlers to decide access.

You can combine multiple guards using @UseGuards() with several classes.

Always test your guards to avoid accidentally blocking all users.

Summary

Guards protect routes by allowing or blocking access.

Implement CanActivate and use canActivate to add your logic.

Apply guards with @UseGuards() on controllers or routes.

Practice

1. What is the main purpose of a guard in NestJS when protecting routes?

easy

A. To decide if a request can access a route based on custom logic

B. To format the response data before sending it

C. To handle database connections automatically

D. To log every request made to the server

Solution

1. Step 1: Understand the role of guards

   Guards in NestJS are designed to control access to routes by running custom logic before the route handler executes.

2. Step 2: Identify the correct purpose

   Among the options, only deciding if a request can access a route matches the guard's role.

3. Final Answer:

   To decide if a request can access a route based on custom logic -> Option A

4. Quick Check:

   Guards control access = C [OK]

Hint: Guards allow or block access before route runs [OK]

Common Mistakes:

• Confusing guards with interceptors or middleware
• Thinking guards modify response data
• Assuming guards handle database tasks

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

easy

A. @UseGuards(AuthGuard) before the controller class

B. @UseGuards(AuthGuard) before the route handler method

C. @ApplyGuard(AuthGuard) inside the constructor

D. @Guard(AuthGuard) before the route handler

Solution

1. Step 1: Recall the decorator for guards

   The correct decorator to apply guards in NestJS is @UseGuards().

2. Step 2: Identify correct placement

   This decorator can be placed before a route handler method or a controller class. @UseGuards(AuthGuard) before the route handler method correctly shows it before a route handler method.

3. Final Answer:

   @UseGuards(AuthGuard) before the route handler method -> Option B

4. Quick Check:

   @UseGuards + route method = D [OK]

Hint: Use @UseGuards() decorator on methods or controllers [OK]

Common Mistakes:

• Using @Guard or @ApplyGuard which don't exist
• Placing decorators inside constructor
• Confusing middleware with guards

3. Given this guard implementation, what will happen when a request is made?

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

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

Applied to a route, what is the result?

medium

A. The guard throws an error causing a server crash

B. The route handler runs and returns its response

C. The request is blocked and the route handler does not run

D. The guard logs the request but allows it

Solution

1. Step 1: Analyze the canActivate method

   The guard's canActivate method always returns false, meaning it denies access.

2. Step 2: Understand guard effect on route

   When canActivate returns false, NestJS blocks the request and does not run the route handler.

3. Final Answer:

   The request is blocked and the route handler does not run -> Option C

4. Quick Check:

   canActivate false blocks request = A [OK]

Hint: canActivate false means block access [OK]

Common Mistakes:

• Thinking false lets the route run
• Assuming guard throws error on false
• Confusing logging with blocking

4. Identify the error in this guard code snippet:

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

export class MyGuard implements CanActivate {
  canActivate(context: ExecutionContext) {
    if (context.switchToHttp().getRequest().user) {
      return true;
    }
  }
}

medium

A. Incorrect import of ExecutionContext

B. Using switchToHttp() is not allowed in guards

C. canActivate should be async but is not

D. Missing return statement when user is not present

Solution

1. Step 1: Check canActivate return paths

   The method returns true if user exists, but returns nothing (undefined) otherwise.

2. Step 2: Understand guard return requirements

   canActivate must always return a boolean or Promise<boolean>. Missing return means undefined, which is invalid.

3. Final Answer:

   Missing return statement when user is not present -> Option D

4. Quick Check:

   All paths must return boolean = B [OK]

Hint: Always return true or false in canActivate [OK]

Common Mistakes:

• Forgetting else return false
• Assuming undefined is false
• Thinking async is always required

5. You want to protect a route so only users with role 'admin' can access it. Which guard implementation correctly enforces this?

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

export class RolesGuard implements CanActivate {
  canActivate(context: ExecutionContext): boolean {
    const request = context.switchToHttp().getRequest();
    const user = request.user;
    // Which condition correctly checks for admin role?
  }
}

hard

A. return user && user.role === 'admin';

B. return user && user.roles.includes('admin');

C. return user && user.roles === 'admin';

D. return user && user.role.includes('admin');

Solution

1. Step 1: Understand user role structure

   Assuming user has a single role string in property role, not an array.

2. Step 2: Check each condition

   return user && user.role === 'admin'; correctly compares user.role === 'admin'. return user && user.roles.includes('admin'); assumes roles is an array, which may be incorrect. Options C and D misuse property types.

3. Final Answer:

   return user && user.role === 'admin'; -> Option A

4. Quick Check:

   Check user.role string equals 'admin' = A [OK]

Hint: Match exact role property type to check admin [OK]

Common Mistakes:

• Confusing roles array with single role string
• Using includes on a string property
• Checking wrong property name