Comprehensive Overview of NestJS Architecture and Core Components

NestJS is a progressive Node.js framework that provides a modular architecture inspired by Angular, enabling developers to build scalable, testable, and maintainable server‑side applications using TypeScript or JavaScript.

The core building blocks are Controllers (handling incoming requests), Providers (services, repositories, etc.), and Modules (grouping related components), each decorated with @Controller , @Injectable , and @Module respectively.

Typical code examples illustrate a CatsController with @Get and @Post routes, a CatsService implementing CRUD logic, and a CatsModule that registers the controller and provider:

@Controller('cats')
export class CatsController {
  constructor(private readonly catsService: CatsService) {}

  @Post()
  @UsePipes(new ValidationPipe())
  async create(@Body() createCatDto: CreateCatDto) {
    return this.catsService.create(createCatDto);
  }

  @Get()
  async findAll(): Promise
{
    return this.catsService.findAll();
  }
}

Additional features such as Middleware, Pipes, Guards, Interceptors, and Exception Filters are supported; each is implemented as an @Injectable class and can be applied globally or per route using decorators like @UseGuards , @UsePipes , @UseInterceptors , and @UseFilters .

The article shows how to configure global middleware, asynchronous middleware, functional middleware, and how to set up validation pipes with class-validator , as well as role‑based guards using a custom @Roles decorator and the Reflector API:

export const Roles = (...roles: string[]) => ReflectMetadata('roles', roles);

@Injectable()
export class RolesGuard implements CanActivate {
  constructor(private readonly reflector: Reflector) {}
  canActivate(context: ExecutionContext): boolean {
    const roles = this.reflector.get
('roles', context.getHandler());
    if (!roles) return true;
    const request = context.switchToHttp().getRequest();
    const user = request.user;
    return user && user.roles && roles.some(role => user.roles.includes(role));
  }
}

A visual response order diagram and a recommended project directory layout are provided to help developers organize their NestJS applications efficiently.