<\/span><\/h3>\n\n\n\nMiddleware is code that runs before the request reaches the controller. In NestJS, it can be used for logging, authentication, or modifying the request object. You add middleware inside a module using the configure() method with MiddlewareConsumer.<\/p>\n\n\n\n
@Injectable()\nexport class LoggerMiddleware {\n use(req: Request, res: Response, next: Function) {\n console.log(`Request...`);\n next();\n }<\/code><\/pre>\n\n\n\n<\/span>3. What steps would you take to set up a simple server with NestJS?<\/span><\/h3>\n\n\n\nFirst, install the CLI using npm i -g @nestjs\/cli.<\/p>\n\n\n\n
Then run nest new project-name to generate the starter app. This creates the folder structure with modules, controllers, and services.<\/p>\n\n\n\n
Finally, start the server using npm run start or npm run start:dev.<\/p>\n\n\n\n
\/\/ main.ts\nasync function bootstrap() {\n const app = await NestFactory.create(AppModule);\n await app.listen(3000);\n}\nbootstrap();<\/code><\/pre>\n\n\n\n<\/span>4. How are HTTP requests typically managed inside a NestJS app?<\/span><\/h3>\n\n\n\nHTTP requests are handled by controllers. Each controller method is linked to an HTTP route using decorators like @Get(), @Post(), or @Delete(). Services are then used to keep the business logic separate from the request handling.<\/p>\n\n\n\n
@Controller('users')\nexport class UsersController {\n @Get()\n getUsers() {\n return 'All users';\n }\n}<\/code><\/pre>\n\n\n\n<\/span>5. Why are environment variables important in NestJS and how do you use them?<\/span><\/h3>\n\n\n\nEnvironment variables store sensitive or environment-specific data such as API keys or database URLs. In NestJS, you can use the @nestjs\/config package to load them. Values are placed in a .env file and accessed with ConfigService.<\/p>\n\n\n\n
\/\/ .env\nDB_HOST=localhost<\/code><\/pre>\n\n\n\n\/\/ app.module.ts\nConfigModule.forRoot();<\/code><\/pre>\n\n\n\n<\/span>6. What role do controllers play in a NestJS project?<\/span><\/h3>\n\n\n\nControllers are the entry points for client requests. They define routes and return responses. A controller delegates the actual logic to services, keeping the code clean and structured.<\/p>\n\n\n\n
<\/span>7. How do you structure a NestJS application?<\/span><\/h3>\n\n\n\nA common structure includes modules for features, controllers for handling routes, and services for business logic. The main.ts file starts the app, and shared utilities can be placed in a common module.<\/p>\n\n\n\n
This modular structure keeps the code easy to maintain as the app grows.<\/p>\n\n\n\n
<\/span>Advanced NestJS Interview Questions<\/span><\/h2>\n\n\n\nThese are advanced NestJS interview questions and answers designed for experienced professionals.<\/p>\n\n\n\n
<\/span>8. What are interceptors in NestJS, and could you share an example of when you might create your own?<\/span><\/h3>\n\n\n\nInterceptors are classes that can intercept requests before they reach the controller and responses before they are sent back to the client. They are powerful because they can transform data, extend behavior, and implement cross-cutting concerns like logging, caching, and error mapping.<\/p>\n\n\n\n
For example, a logging interceptor can measure how long a request took:<\/p>\n\n\n\n
@Injectable()\nexport class LoggingInterceptor implements NestInterceptor {\n intercept(context: ExecutionContext, next: CallHandler) {\n const now = Date.now();\n return next.handle().pipe(\n tap(() =>\n console.log(`Request handled in ${Date.now() - now}ms`),\n );\n );\n }\n}<\/code><\/pre>\n\n\n\nYou would use this interceptor when you need performance insights or centralized logging across multiple routes.<\/p>\n\n\n\n
<\/span>9. Could you explain what dynamic modules are in NestJS and give a scenario where they are useful?<\/span><\/h3>\n\n\n\nDynamic modules allow you to configure and customize a module at runtime. Unlike static modules, they can take arguments and adjust their providers based on input.<\/p>\n\n\n\n
This is useful in cases like multi-tenant applications, where each tenant might require a different database connection.<\/p>\n\n\n\n
@Module({})\nexport class ConfigModule {\n static register(apiKey: string): DynamicModule {\n return {\n module: ConfigModule,\n providers: [{ provide: 'API_KEY', useValue: apiKey }],\n exports: ['API_KEY'],\n }\n }\n}<\/code><\/pre>\n\n\n\nIn practice, you might call ConfigModule.register(‘my-secret-key’) inside AppModule to pass a unique configuration.<\/p>\n\n\n\n
<\/span>10. What benefits and challenges come with using NestJS for microservices?<\/span><\/h3>\n\n\n\nNestJS has built-in support for microservices through transport layers like TCP, Redis, Kafka, and NATS. The benefits include:<\/p>\n\n\n\n
\u25cf Independent scaling of each service.<\/p>\n\n\n\n
\u25cf Fault isolation, so one failing service does not break the whole system.<\/p>\n\n\n\n
\u25cf Technology independence, since each service can use its own stack.<\/p>\n\n\n\n
Challenges include:<\/p>\n\n\n\n
\u25cf Increased debugging complexity because logs and errors are spread across services.<\/p>\n\n\n\n
\u25cf Maintaining data consistency, which often requires patterns like Saga or Outbox.<\/p>\n\n\n\n
\u25cf Added latency from network communication.<\/p>\n\n\n\n
Many companies solve tracing issues by adding OpenTelemetry or Jaeger for distributed monitoring.<\/p>\n\n\n\n
<\/span>11. How does Dependency Injection function in NestJS, and what is the process to register a custom provider?<\/span><\/h3>\n\n\n\nDependency Injection (DI) in NestJS is built on top of TypeScript decorators and the IoC (Inversion of Control) principle. When you decorate a class with @Injectable(), NestJS can create and inject it wherever needed.<\/p>\n\n\n\n
For a custom provider, you can use different strategies:<\/p>\n\n\n\n
const CustomProvider = {\n provide: 'CUSTOM_CONFIG',\n useFactory: () => ({ url: 'http:\/\/api.example.com' }),\n};\n\n@Module({\n providers: [CustomProvider],\n exports: ['CUSTOM_CONFIG'],\n})\nexport class AppModule {}<\/code><\/pre>\n\n\n\nNow you can inject this config into any service:<\/p>\n\n\n\n
@Injectable()\nexport class MyService {\n constructor(@Inject('CUSTOM_CONFIG') private config: any) {}\n}<\/code><\/pre>\n\n\n\nThis approach is common when you rely on external APIs or configurations.<\/p>\n\n\n\n
<\/span>12. How do Guards work in NestJS, and how could you use them to control role-based access?<\/span><\/h3>\n\n\n\nGuards are classes that decide whether a request can proceed to the route handler. They are often used for authentication and authorization. For role-based access control (RBAC), you define required roles as metadata and then check them inside a guard.<\/p>\n\n\n\n
@Injectable()\nexport class RolesGuard implements CanActivate {\n constructor(private reflector: Reflector) {}\n canActivate(context: ExecutionContext): boolean {\n const roles = this.reflector.get<string[]>('roles', context.getHandler());\n if (!roles) return true;\n const { user } = context.switchToHttp().getRequest();\n return roles.some((role) => user.roles.includes(role));\n }\n}<\/code><\/pre>\n\n\n\nThen apply it to routes:<\/p>\n\n\n\n
@UseGuards(RolesGuard)\n@SetMetadata('roles', ['admin'])\n@Get('all') findAll() {}<\/code><\/pre>\n\n\n\nThis lets only users with the admin role access the endpoint.<\/p>\n\n\n\n
<\/span>13. How would you apply the CQRS (Command Query Responsibility Segregation) pattern in a NestJS application?<\/span><\/h3>\n\n\n\nCQRS separates commands (write operations) from queries (read operations). Instead of mixing both in a single service, you create dedicated handlers. NestJS supports this through the @nestjs\/cqrs package.<\/p>\n\n\n\n
@CommandHandler(CreateUserCommand)\nexport class CreateUserHandler {\n async execute(command: CreateUserCommand) {\n \/\/ create user logic\n }\n}<\/code><\/pre>\n\n\n\n@QueryHandler(GetUserQuery)\nexport class GetUserHandler {\n async execute(query: GetUserQuery) {\n \/\/ read user logic\n }\n}<\/code><\/pre>\n\n\n\nThis pattern improves scalability and clarity in large applications. It also works well with event sourcing, where events are stored and replayed to rebuild state.<\/p>\n\n\n\n
<\/span>14. What issues can circular dependencies cause in NestJS, and how do you resolve them?<\/span><\/h3>\n\n\n\nCircular dependencies happen when two modules or providers depend on each other. This can cause runtime errors because NestJS cannot resolve the order of injection.<\/p>\n\n\n\n
For example, if UserService depends on AuthService, and AuthService also depends on UserService, you will hit a circular dependency.<\/p>\n\n\n\n
You can fix this by:<\/p>\n\n\n\n
\u25cf Using forwardRef() in module imports.<\/p>\n\n\n\n
\u25cf Splitting shared logic into a separate service or module.<\/p>\n\n\n\n
\u25cf Rethinking the design so that one service emits events instead of directly calling the other.<\/p>\n\n\n\n
@Module({\n imports: [forwardRef(() => AuthModule)],\n})\nexport class UserModule {}<\/code><\/pre>\n\n\n\nThis tells NestJS to resolve the dependency later, breaking the circular reference.<\/p>\n\n\n\n