Introduction:

NestJS, with its modular architecture and robust features, empowers developers to build scalable and maintainable applications. Two essential features that contribute significantly to NestJS's versatility and security are Guards and Custom Decorators. In this comprehensive guide, we'll delve into the world of Guards and Custom Decorators in NestJS, exploring their functionalities, use cases, and best practices for mastering them.

Understanding Guards:

Guards in NestJS act as middleware components that intercept incoming requests before they reach route handlers. They allow developers to implement authentication, authorization, rate limiting, and other custom logic to control access to routes and resources within an application.

Creating Custom Guards:

To create custom guards in NestJS, developers can implement classes that adhere to the CanActivate interface for synchronous guards or the CanActivate interface for asynchronous guards. Within the guard class, they implement the canActivate() method, where they write the logic to determine whether the request should be allowed or denied.

Example of an Authentication Guard:

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

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

Using Guards in Routes:

To apply guards to specific routes in NestJS, developers use the @UseGuards() decorator at the controller or method level. This decorator accepts one or more guard classes as arguments, which will be executed in the order they are provided.

Example of Using Guards in a Controller:

import { Controller, Get, UseGuards } from '@nestjs/common';
import { AuthGuard } from './auth.guard';

@Controller('users')
export class UsersController {
  @Get()
  @UseGuards(AuthGuard)
  getUsers() {
    // Route handler logic
  }
}

Understanding Custom Decorators:

Custom Decorators in NestJS allow developers to extend the framework's functionality by adding custom metadata and behavior to classes, methods, properties, or parameters. They enhance code readability, promote code reuse, and enable a declarative programming style.

Creating Custom Decorators:

To create a custom decorator in NestJS, developers define a function that returns a decorator function. This decorator function can then be applied to classes, methods, properties, or parameters using the @ syntax.

Example of a Custom Decorator:

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

export const CustomDecorator = createParamDecorator(
  (data: unknown, context: ExecutionContext) => {
    const request = context.switchToHttp().getRequest();
    // Custom logic here
    return request.customData;
  },
);

Conclusion:

Guards and Custom Decorators are powerful features in NestJS that enable developers to implement authentication, authorization, and custom behavior with ease. By mastering guards and custom decorators, developers can enhance the security, readability, and extensibility of their NestJS applications. Experiment with these features in your projects, and unlock their full potential to build scalable, secure, and maintainable applications with NestJS.