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NestJS Interview Questions

NestJS interview-এ decorator বা CLI command মুখস্থ করার চেয়ে request lifecycle এবং dependency graph বোঝা বেশি গুরুত্বপূর্ণ। Module boundary, dependency injection, validation, guards, interceptors, persistence, testing ও deployment কীভাবে একসঙ্গে কাজ করে—এই question bank সেই reasoning practice করায়।

How to use this question bank​

  1. Core concepts, modules, controllers এবং dependency injection আগে শেষ করুন।
  2. একটি request middleware থেকে pipe, guard, interceptor, handler ও exception filter পর্যন্ত trace করুন।
  3. Authentication, database ও microservice প্রশ্নে security এবং failure behavior উল্লেখ করুন।
  4. প্রতিটি design answer testability, coupling এবং operational cost দিয়ে evaluate করুন।

150+ core questions · 420+ follow-up questions · 20 topics Fixed all bugs, added 7 missing topics, expanded all thin sections, 2–3 follow-ups per question.

1. Core Concepts and Architecture​

  1. What is NestJS, and what problems does it solve?

    • How does NestJS's architecture compare to Angular's (modules, DI, decorators)?
    • What pain points of raw Express does NestJS address (structure, testability, scalability)?
    • Who are the typical teams that benefit most from adopting NestJS?

  2. How does NestJS leverage TypeScript?

    • What specific TypeScript features (decorators, metadata reflection, generics) improve NestJS development?
    • How does emitDecoratorMetadata in tsconfig.json enable NestJS's dependency injection?
    • What happens if you try to use NestJS with plain JavaScript?

  3. What is the architectural style of NestJS?

    • How does NestJS combine OOP, functional programming, and reactive programming?
    • How does NestJS enforce a layered architecture (controllers, services, repositories)?
    • What is the NestJS request pipeline from incoming HTTP request to outgoing response?

  4. What are the main components of a NestJS application?

    • What is the role of each building block: modules, controllers, providers, guards, interceptors, pipes, and filters?
    • Which component is most critical for scalability and why?
    • How do these components map to the responsibilities in a clean architecture?

  5. How does NestJS differ from Express or Fastify?

    • When would you choose Fastify as the underlying HTTP adapter over Express in NestJS?
    • What is the NestJS platform abstraction and how do you swap the underlying HTTP driver?
    • What does NestJS add on top of Express that Express does not provide out of the box?

  6. What is the role of the @nestjs/core package?

    • What key classes does @nestjs/core export (NestFactory, Reflector, ContextIdFactory)?
    • Can you name a scenario where you interact directly with @nestjs/core beyond bootstrapping?
    • What is NestFactory.createMicroservice and how does it differ from NestFactory.create?

  7. What is the difference between monolithic and microservices architecture in NestJS?

    • What challenges arise when transitioning a NestJS monolith to microservices?
    • How does NestJS support both patterns without requiring a full rewrite?
    • What is a hybrid NestJS application and when would you use one?

  8. How does NestJS support both REST and GraphQL APIs?

    • How do you run both a REST controller and a GraphQL resolver in the same NestJS application?
    • What are the trade-offs between REST and GraphQL for large-scale NestJS backends?
    • How do you share DTOs and validation logic between REST and GraphQL in NestJS?

  9. What is the purpose of the Nest CLI?

    • How does the CLI improve development workflows (generating resources, running schematics)?
    • What is a NestJS schematic and how do you create a custom one?
    • How do you generate a complete CRUD resource with a single CLI command?

  10. What are the benefits of using NestJS for enterprise applications?

    • How does NestJS ensure maintainability in large teams (enforced structure, testability)?
    • What is the role of the NestJS monorepo support for large codebases?
    • How does NestJS's opinionated structure reduce decision fatigue compared to raw Express?

2. Modules​

  1. What is a module in NestJS, and how is it defined?

    • How do you decide what components (providers, controllers) belong in a given module?
    • What is the difference between a feature module and a shared module?
    • What is the root module (AppModule) and what is its special role in bootstrapping?

  2. What are the imports, exports, providers, and controllers arrays in a module decorator?

    • What happens if you add a provider to providers but forget to add it to exports?
    • When should you put a provider in exports vs keeping it private to the module?
    • What is the difference between importing a module and importing a provider directly?

  3. What is a dynamic module, and when would you use it?

    • What is the difference between forRoot and forFeature in dynamic modules?
    • Can you give a real-world example of a dynamic module (e.g., ConfigModule.forRoot, TypeOrmModule.forFeature)?
    • How do you pass configuration options into a dynamic module?

  4. How do you create a global module in NestJS?

    • What is the @Global() decorator and what does it do to the module's providers?
    • What are the risks of overusing global modules (hidden dependencies, testability issues)?
    • When is it appropriate to make a module global (e.g., ConfigModule, LoggerModule)?

  5. What happens if you forget to export a provider from a module?

    • How do you debug a Nest can't resolve dependencies error?
    • How does the NestJS dependency graph help you trace missing providers?
    • How do you avoid this error by structuring your modules correctly?

  6. What is a shared module in NestJS?

    • How do you create a shared module that exposes reusable providers to the rest of the app?
    • What is the difference between a shared module and a global module?
    • How do you avoid circular imports when building shared modules?

  7. How does NestJS handle lazy-loaded modules?

    • What is LazyModuleLoader and when is it useful?
    • How do lazy-loaded modules improve cold start time in serverless deployments?
    • What are the limitations of lazy-loaded modules (e.g., guards, middleware not auto-applied)?

3. Controllers​

  1. What is a controller in NestJS, and how does it handle requests?

    • How do you structure controllers for large applications (one controller per resource)?
    • What is the difference between @Controller('users') path prefix and individual @Get, @Post route paths?
    • How does NestJS route matching work when multiple routes could match a request?

  2. How do you define a route with parameters in NestJS?

    • How do you validate and transform route parameters using pipes?
    • What is the difference between @Param('id') and @Param() (all params as object)?
    • How do you define optional route segments in NestJS?

  3. What is the difference between @Body, @Query, and @Param?

    • When would you use @Query over @Param for resource filtering?
    • How do you bind a DTO to @Body for automatic validation?
    • What is @Headers() and how do you access specific request headers?

  4. How do you handle file uploads in a controller?

    • How do you use @UseInterceptors(FileInterceptor) and @UploadedFile together?
    • How do you validate file type and size within the controller or a pipe?
    • What is the difference between FileInterceptor (single file) and FilesInterceptor (multiple files)?

  5. How do you version APIs in NestJS?

    • What are the four versioning strategies in NestJS (URI, Header, Media type, Custom)?
    • What are the pros and cons of URI versioning (/v1/users) vs header versioning?
    • How do you set a default version for all routes and override it per controller?

  6. How do you send custom HTTP responses in NestJS?

    • What is the difference between using @Res() directly and letting NestJS handle the response?
    • How do you set custom status codes using @HttpCode() decorator?
    • When does using @Res() break NestJS interceptors and how do you avoid that?

  7. What is response serialization in NestJS?

    • How does ClassSerializerInterceptor work with @Exclude() and @Expose() from class-transformer?
    • How do you apply @SerializeOptions({ strategy: 'excludeAll' }) to a controller?
    • How do you serialize nested objects in a response DTO?

  8. How do you handle streaming responses in NestJS?

    • How do you pipe a Node.js ReadableStream to an HTTP response in a NestJS controller?
    • What is StreamableFile in NestJS and when do you use it?
    • How do you set the correct Content-Type and Content-Disposition headers for file downloads?

4. Dependency Injection​

  1. What is Dependency Injection in NestJS, and why is it important?

    • How does DI improve testability by allowing you to swap real implementations for mocks?
    • How does NestJS's DI container differ from manually instantiating classes?
    • What is the difference between constructor injection and property injection in NestJS?

  2. How do you inject a service into a controller?

    • What happens at runtime if the service is not marked with @Injectable?
    • What is the role of the module's providers array in making a service injectable?
    • How does TypeScript's emitDecoratorMetadata allow NestJS to infer the injection token from the type?

  3. What is the @Injectable decorator, and when is it used?

    • Can a class without @Injectable be injected — and what error do you get if you try?
    • Why do you need @Injectable even on classes that don't inject anything themselves?
    • What is the scope option on @Injectable and what are its three values?

  4. How do you solve a circular dependency in NestJS?

    • How does forwardRef(() => ServiceName) break a circular dependency?
    • What are alternative architectural approaches to eliminate circular dependencies (event emitter, shared module)?
    • How do circular dependencies affect application startup performance?

  5. What is a custom provider, and how do you create one?

    • What is the difference between useClass, useValue, useFactory, and useExisting?
    • When would you use useFactory over useValue (async initialization, conditional logic)?
    • How do you inject other providers into a useFactory function using the inject array?

  6. How do you handle provider scopes in NestJS DI?

    • What are the three provider scopes: DEFAULT (singleton), REQUEST, and TRANSIENT?
    • What are the performance implications of request-scoped providers (new instance per request)?
    • How does a request-scoped provider propagate up to its consumers (scope bubbling)?

  7. What is the @Inject decorator, and when is it needed?

    • When do you need @Inject(TOKEN) instead of relying on TypeScript type inference?
    • How do you use @Inject with string or Symbol injection tokens?
    • Can you use @Inject with built-in NestJS providers like HttpService?

  8. How do you test DI in NestJS?

    • How do you use Test.createTestingModule to build an isolated DI container for unit tests?
    • How do you mock a provider using { provide: ServiceName, useValue: mockObject }?
    • How do you override a provider only for a specific test without affecting others?

  9. How do you create an asynchronous provider in NestJS?

    • How do you use useFactory with async/await to initialize a provider asynchronously?
    • What is registerAsync on dynamic modules (e.g., TypeOrmModule.forRootAsync) and how does it work?
    • What are the use cases for async providers (database connection, remote config fetch)?

5. Providers and Services​

  1. What is a provider in NestJS?

    • What types of values can be a NestJS provider (class, value, factory, alias)?
    • How is a provider different from a plain TypeScript class?
    • What is an injection token and how do you define a custom one using InjectionToken?

  2. What is the difference between a service and a repository in NestJS?

    • How do you structure services and repositories in a clean architecture (service calls repo, controller calls service)?
    • What is the repository pattern and how does TypeORM's @InjectRepository implement it?
    • When should business logic live in a service vs a domain entity?

  3. How do you create a factory provider in NestJS?

    • How do you inject other providers into a factory using the inject array?
    • How do you conditionally return different implementations from a factory?
    • What is the difference between a factory provider and a useClass provider?

  4. What is the role of the @Optional decorator?

    • What happens at runtime if an optional dependency is not registered in the module?
    • When is @Optional appropriate vs making the dependency mandatory?
    • How do you combine @Optional with @Inject for non-class tokens?

  5. How do you share a provider across multiple modules in NestJS?

    • What is the correct pattern: export from a shared module and import that module?
    • What are the risks of sharing stateful providers across modules?
    • How does NestJS ensure that a singleton provider is the same instance across all modules?

  6. What is the difference between singleton and transient providers?

    • How do you declare a transient provider and what does NestJS do differently for it?
    • When is a transient provider preferable to a singleton?
    • How do you verify in a test that a provider is being instantiated fresh each time?

6. Middleware, Pipes, Guards, and Interceptors​

  1. What is middleware in NestJS, and how is it applied?

    • How does NestJS middleware differ from Express middleware in terms of application?
    • How do you apply middleware to specific routes using configure in a module?
    • Can middleware access NestJS's DI container — and if so, how?

  2. What are pipes in NestJS, and how do they work?

    • What are the two roles of pipes: transformation and validation?
    • How do you chain multiple pipes on a single parameter?
    • What are the NestJS built-in pipes and when would you use each (ParseIntPipe, ValidationPipe, ParseUUIDPipe)?

  3. How do you create a custom pipe in NestJS?

    • What interface does a custom pipe implement and what method must it define?
    • When would you need a custom pipe over the built-in ValidationPipe?
    • How do you throw a BadRequestException from inside a custom pipe?

  4. What are guards in NestJS, and how are they used?

    • How do guards interact with the ExecutionContext to access request data?
    • How do you apply a guard at the method, controller, and global level?
    • What is the return value of a guard's canActivate method and what happens on false?

  5. What are interceptors in NestJS, and when are they useful?

    • How do interceptors use RxJS Observable to wrap the request/response cycle?
    • Can interceptors modify both the request before it reaches the handler and the response after?
    • What are common use cases for interceptors (logging, caching, response transformation, timing)?

  6. What is the difference between middleware, pipes, guards, and interceptors?

    • What is the exact execution order: middleware → guard → interceptor → pipe → handler → interceptor → filter?
    • When would you choose an interceptor over middleware for response transformation?
    • How do you decide between a guard and middleware for authentication?

  7. How do you apply multiple guards to a route in NestJS?

    • In what order does NestJS evaluate multiple guards — and does evaluation short-circuit on the first failure?
    • How do you combine an authentication guard and an authorization guard on the same route?
    • How do you write a guard that checks custom metadata set by a decorator?

  8. What is the ExecutionContext in NestJS?

    • How do you switch context using switchToHttp(), switchToRpc(), and switchToWs()?
    • How do you access the underlying request object from ExecutionContext?
    • How do you use ExecutionContext to read custom metadata in a guard?

7. Exception Handling​

  1. How do you handle exceptions in NestJS?

    • What is the built-in global exception filter and what exceptions does it handle by default?
    • How do you throw HTTP exceptions from a service or controller (NotFoundException, BadRequestException)?
    • How do you customize the shape of the error response body?

  2. What is an exception filter, and how do you create one?

    • What interface does a custom exception filter implement and what method must it define?
    • How do you handle non-HTTP exceptions (e.g., database errors) in a custom exception filter?
    • How do you access the request and response objects inside an exception filter?

  3. How do you apply an exception filter at different levels in NestJS?

    • What is the difference between applying a filter with @UseFilters on a method, controller, and globally with app.useGlobalFilters?
    • What are the limitations of globally registered exception filters (no DI access) and how do you work around them?
    • How do you apply a filter only to specific HTTP exception types?

  4. What is the difference between HttpException and its subclasses?

    • What is the difference between HttpException (base), BadRequestException (400), UnauthorizedException (401), NotFoundException (404), and InternalServerErrorException (500)?
    • When would you create a custom exception class that extends HttpException?
    • How do you pass a custom error object (not just a string message) to HttpException?

  5. How do you build a custom exception hierarchy in NestJS?

    • How do you create a base domain exception class and extend it for specific errors?
    • How do you map domain exceptions to HTTP status codes in an exception filter?
    • What is the benefit of domain-specific exceptions vs throwing HttpException directly from a service?

  6. How do you log exceptions in NestJS?

    • How do you use the NestJS built-in Logger inside an exception filter?
    • How do you integrate a third-party logger (Winston, Pino) with NestJS exception handling?
    • What information should you always include in an exception log (stack trace, request ID, timestamp)?

8. Validation with class-validator and class-transformer​

  1. What is ValidationPipe in NestJS and how do you enable it globally?

    • What is the difference between enabling ValidationPipe globally via app.useGlobalPipes vs APP_PIPE token?
    • What does the whitelist: true option do and why is it a security best practice?
    • What does forbidNonWhitelisted: true do and how does it differ from whitelist: true?

  2. How do you define a DTO with class-validator decorators?

    • What are the most commonly used class-validator decorators (@IsString, @IsEmail, @IsInt, @IsOptional, @IsEnum, @MinLength)?
    • How do you validate nested objects inside a DTO using @ValidateNested and @Type?
    • How do you validate arrays of objects inside a DTO?

  3. What is class-transformer and how does it work with NestJS?

    • What is the difference between @Exclude() and @Expose() from class-transformer?
    • How do you use @Transform() to normalize or sanitize incoming data (e.g., trim strings, convert to lowercase)?
    • What does plainToInstance do and when would you call it manually?

  4. How do you create custom validation decorators with class-validator?

    • How do you use registerDecorator to build a fully custom validation rule?
    • How do you create a ValidatorConstraint class that performs async validation (e.g., check if email exists in DB)?
    • How do you inject NestJS services into a custom async validator?

  5. What is the transform: true option on ValidationPipe and why is it important?

    • How does transform: true automatically convert query string numbers from string to number type?
    • How do you use @Type(() => Number) from class-transformer with transform: true?
    • What are the risks of enabling transform: true without strict DTO definitions?

  6. How do you validate query parameters and route params in NestJS?

    • How do you apply a DTO to @Query() parameters using ValidationPipe?
    • How do you use ParseIntPipe or ParseUUIDPipe for individual route params?
    • How do you handle partial validation for optional query parameters?

9. Database Integration​

  1. How do you integrate TypeORM with NestJS?

    • How do you configure TypeOrmModule.forRoot with database connection options?
    • How do you handle database transactions in TypeORM within a NestJS service?
    • What is the difference between TypeOrmModule.forRoot and TypeOrmModule.forRootAsync?

  2. What is the difference between @nestjs/typeorm and @nestjs/mongoose?

    • When would you choose MongoDB (Mongoose) over a SQL database (TypeORM) in NestJS?
    • How does schema definition differ between TypeORM entities and Mongoose schemas?
    • How do you handle relations in TypeORM (@OneToMany, @ManyToOne) vs Mongoose (populate)?

  3. How do you use repositories in NestJS with TypeORM?

    • How do you inject a TypeORM repository using @InjectRepository(Entity)?
    • What are the benefits of custom repositories (extending Repository<Entity>) over the default one?
    • How do you use the QueryBuilder for complex queries in a NestJS service?

  4. How do you handle database migrations in NestJS with TypeORM?

    • How do you generate and run migrations using the TypeORM CLI?
    • How do you automate migrations in a CI/CD pipeline?
    • What is the risk of using synchronize: true in production and why should it be disabled?

  5. How do you configure multiple database connections in NestJS?

    • How do you use name on TypeOrmModule.forRoot to define a named connection?
    • How do you inject a repository from a named connection using @InjectRepository(Entity, 'connectionName')?
    • When would you need multiple database connections in a single NestJS application?

  6. How do you implement soft deletes in TypeORM with NestJS?

    • What is the @DeleteDateColumn decorator and how does TypeORM use it for soft deletes?
    • How does softDelete differ from delete in a TypeORM repository?
    • How do you query and restore soft-deleted records?

  7. How do you handle eager vs lazy loading in TypeORM?

    • What is the difference between eager: true on a relation and using relations in a find call?
    • What are the performance trade-offs of eager loading vs explicit join queries?
    • How do you avoid N+1 query problems in TypeORM with NestJS?

  8. How do you integrate Mongoose with NestJS?

    • How do you define a Mongoose schema and model using @Schema and @Prop decorators?
    • How do you inject a Mongoose model into a service using @InjectModel?
    • How do you handle Mongoose middleware (pre/post hooks) in NestJS?

10. Prisma with NestJS​

  1. What is Prisma and why is it popular in the NestJS ecosystem?

    • How does Prisma's type safety compare to TypeORM or Mongoose in a TypeScript project?
    • What are the components of Prisma (Prisma Client, Prisma Migrate, Prisma Studio)?
    • What are the trade-offs of Prisma vs TypeORM for a NestJS project?

  2. How do you integrate Prisma into a NestJS application?

    • How do you create a PrismaService that extends PrismaClient and implements OnModuleInit?
    • Why should you call $connect() in onModuleInit and $disconnect() in onModuleDestroy?
    • How do you make PrismaService globally available across all modules?

  3. What is the Prisma schema file and how do you define models in it?

    • How do you define a model, fields, relations, and enums in schema.prisma?
    • What is the @id, @unique, @default, and @relation directive in Prisma schema?
    • How do you define a one-to-many and many-to-many relation in Prisma?

  4. How do you run Prisma migrations in NestJS?

    • What is the difference between prisma migrate dev (development) and prisma migrate deploy (production)?
    • How do you integrate prisma migrate deploy into a Docker or CI/CD deployment pipeline?
    • What does prisma generate do and when must you run it?

  5. How do you use Prisma Client for CRUD operations in a NestJS service?

    • How do you use findMany, findUnique, create, update, upsert, and delete in Prisma Client?
    • How do you filter, sort, and paginate results using Prisma's query API?
    • How do you handle Prisma's PrismaClientKnownRequestError (e.g., unique constraint violation)?

  6. How do you handle database transactions in Prisma with NestJS?

    • What is prisma.$transaction and how do you use it for atomic operations?
    • What is the difference between sequential transactions and interactive transactions in Prisma?
    • How do you roll back a Prisma transaction on error?

  7. How do you use Prisma with NestJS for relation queries?

    • How do you use include to eagerly load related records in a Prisma query?
    • How do you use select to return only specific fields and nested relations?
    • How do you create a record and its related records in a single Prisma create call?

11. Microservices​

  1. How does NestJS support microservices?

    • What is the difference between a NestJS HTTP application and a NestJS microservice application?
    • Which transport layer is best suited for high-throughput microservices (Kafka, NATS, Redis)?
    • How do you create a hybrid app that handles both HTTP and microservice messages?

  2. What is a message pattern in NestJS microservices?

    • What is the difference between @MessagePattern (request-response) and @EventPattern (fire-and-forget)?
    • How do you handle message versioning in a microservices architecture?
    • How do you test a microservice message handler in NestJS?

  3. What are the different transport layers supported by NestJS microservices?

    • What are the trade-offs between TCP, Redis, NATS, Kafka, and RabbitMQ transport layers?
    • How do you configure Kafka transport in a NestJS microservice?
    • What is the gRPC transport in NestJS and how do you define a .proto file for it?

  4. How do you handle errors in a NestJS microservices setup?

    • How do you propagate errors from a microservice back to the caller using RpcException?
    • How do you use an RPC exception filter to catch and format microservice errors?
    • What happens to unhandled errors in a NestJS microservice (are they swallowed silently)?

  5. How do you scale a NestJS microservices architecture?

    • How do you run multiple instances of a NestJS microservice for load balancing?
    • What is service discovery and how do you implement it with NestJS microservices?
    • How do you implement a circuit breaker between NestJS microservices?

  6. What is CQRS and how does @nestjs/cqrs implement it?

    • What is the difference between a Command, a Query, and an Event in CQRS?
    • How do you implement a CommandHandler and register it in a NestJS module?
    • How do EventBus and QueryBus work and when do you use each?

  7. How do you implement the SAGA pattern in NestJS?

    • What is a saga in the context of @nestjs/cqrs (reacting to events and dispatching commands)?
    • How do sagas manage distributed transactions without two-phase commit?
    • How do you handle saga failures and compensating transactions in NestJS?

12. Authentication and Security​

  1. How do you implement JWT authentication in NestJS?

    • How do you use @nestjs/jwt and @nestjs/passport together to implement JWT auth?
    • How do you configure JwtModule.registerAsync to load the secret from ConfigService?
    • How do you attach the authenticated user to the request object in a JWT guard?

  2. How do you implement a Passport local strategy in NestJS?

    • What is the LocalStrategy and how does it validate username and password?
    • How does AuthGuard('local') trigger the Passport local strategy?
    • How do you return a JWT after successful local authentication?

  3. What is role-based access control (RBAC) and how do you implement it in NestJS?

    • How do you create a @Roles custom decorator and a RolesGuard that reads it?
    • How do you use the Reflector class inside a guard to read custom metadata?
    • What is the difference between RBAC and ABAC (Attribute-Based Access Control)?

  4. How do you implement refresh token rotation in NestJS?

    • How do you store refresh tokens securely (hashed in the database, httpOnly cookie)?
    • How do you detect and invalidate a refresh token after it has been used once?
    • What endpoint pattern do you use for /auth/refresh in a NestJS REST API?

  5. How do you prevent CORS issues in NestJS?

    • How do you enable CORS globally using app.enableCors() with specific origins?
    • How do you configure different CORS settings per environment (dev vs prod)?
    • What are the security risks of using origin: '*' in production?

  6. What is CSRF protection and how do you implement it in NestJS?

    • How do you use the csurf middleware with NestJS and Express?
    • When is CSRF protection unnecessary (stateless JWT APIs with no cookies)?
    • What is the SameSite cookie attribute and how does it reduce CSRF risk?

  7. How do you secure WebSocket connections in NestJS?

    • How do you authenticate a WebSocket connection using a JWT in the handshake headers?
    • How do you apply a guard to a WebSocket gateway in NestJS?
    • How do you handle token expiry for long-lived WebSocket connections?

  8. How do you perform security hardening on a NestJS application?

    • How do you use helmet middleware to set secure HTTP headers in NestJS?
    • How do you implement request rate limiting using @nestjs/throttler?
    • How do you run npm audit and integrate dependency scanning into a CI/CD pipeline?

13. Testing​

  1. How do you write unit tests in NestJS?

    • How do you use Test.createTestingModule to set up an isolated module for testing?
    • How do you structure tests for maintainability (one test file per service/controller)?
    • What is the Arrange-Act-Assert pattern and how does it apply to NestJS unit tests?

  2. How do you mock dependencies in NestJS tests?

    • How do you provide a mock service using { provide: ServiceName, useValue: mockObject }?
    • How do you mock external HTTP calls using jest.spyOn or nock?
    • How do you test that a method was called with specific arguments using Jest matchers?

  3. What is the difference between unit tests and integration tests in NestJS?

    • How do you write integration tests using Test.createTestingModule with a real database?
    • How do you set up and tear down test data between integration tests?
    • What is an end-to-end (E2E) test in NestJS and how do you write one?

  4. How do you write E2E tests in NestJS?

    • How do you use supertest with NestJS to send real HTTP requests in E2E tests?
    • How do you bootstrap the full NestJS application in a test environment?
    • How do you seed a test database and clean it up after each E2E test suite?

  5. How do you test guards and interceptors in NestJS?

    • How do you test a guard by mocking ExecutionContext and calling canActivate directly?
    • How do you test an interceptor by providing a mock CallHandler with an observable?
    • How do you test that an interceptor transforms the response correctly?

  6. How do you test middleware in NestJS?

    • How do you simulate an HTTP request through middleware in a unit test?
    • How do you test middleware that calls next() vs middleware that blocks the request?
    • How do you test middleware that depends on NestJS-injected services?

  7. How do you test exception filters in NestJS?

    • How do you trigger an exception in an E2E test and assert the response shape?
    • How do you unit test the catch method of an exception filter?
    • How do you verify that the correct HTTP status code is returned by a custom filter?

  8. How do you measure and enforce test coverage in NestJS?

    • How do you configure Jest coverage thresholds in jest.config.ts or package.json?
    • What is a meaningful coverage target for a NestJS backend (line, branch, function)?
    • What are the risks of targeting 100% coverage in a NestJS project?

14. Performance and Optimization​

  1. How do you optimize a NestJS application for performance?

    • What tools do you use to profile NestJS performance (clinic.js, --prof, Datadog APM)?
    • How do you identify slow routes or database queries using request logging?
    • What is the impact of request-scoped providers on throughput and how do you minimize it?

  2. What is caching in NestJS and how do you implement it?

    • How do you use @nestjs/cache-manager to cache method results in a service?
    • How do you use Redis as the cache store with cache-manager?
    • How do you invalidate a specific cache key when data changes?

  3. How do you implement lazy loading in NestJS?

    • What is LazyModuleLoader and how do you use it to load a module on demand?
    • What are the performance benefits of lazy loading in a serverless or large modular app?
    • What are the limitations (guards, middleware, pipes not automatically applied)?

  4. What is the benefit of using Fastify over Express in NestJS?

    • What throughput improvements does Fastify provide over Express in benchmarks?
    • How do you switch the NestJS HTTP adapter from Express to Fastify?
    • What Express-specific middleware is incompatible with Fastify and how do you replace it?

  5. How do you use compression in NestJS?

    • How do you add gzip or Brotli compression middleware to a NestJS Express application?
    • What is the CPU cost of compression and when should you offload it to Nginx or a CDN?
    • How do you enable compression only for responses above a certain size threshold?

  6. How do you profile and debug a NestJS application?

    • How do you use the --inspect Node.js flag with a running NestJS app for Chrome DevTools profiling?
    • How do you identify memory leaks in a NestJS application?
    • What is the NestJS Logger and how do you replace it with a structured logger like Pino?

15. GraphQL​

  1. How do you set up GraphQL in NestJS?

    • What is the difference between GraphQLModule.forRoot with code-first and schema-first options?
    • How do you integrate @nestjs/graphql with Apollo Server vs Mercurius?
    • How do you enable GraphQL playground and introspection safely in production?

  2. What is a resolver in NestJS GraphQL?

    • What decorators define a query (@Query), mutation (@Mutation), and subscription (@Subscription) in a resolver?
    • How does the resolver chain work for nested types using @ResolveField?
    • How do you optimize resolver performance to avoid N+1 queries?

  3. What is the difference between code-first and schema-first GraphQL in NestJS?

    • How do you define a GraphQL type using @ObjectType and @Field in code-first?
    • What is the schema.gql auto-generated file and when is it updated?
    • Which approach is better for large teams — and what are the trade-offs?

  4. How do you handle authentication in NestJS GraphQL APIs?

    • How do you pass the authenticated user to the GraphQL context function?
    • How do you apply a guard to a specific GraphQL query or mutation?
    • How do you implement field-level authorization in a NestJS resolver?

  5. How do you implement DataLoader in NestJS GraphQL?

    • What problem does DataLoader solve in a GraphQL resolver (N+1 queries)?
    • How do you create a per-request DataLoader instance in NestJS using a factory provider?
    • How do you inject a DataLoader into a resolver and use it in @ResolveField?

  6. How do you handle GraphQL subscriptions in NestJS?

    • How do you set up subscriptions-transport-ws or graphql-ws with NestJS?
    • How do you use PubSub from graphql-subscriptions to publish and listen to events?
    • How do you scale GraphQL subscriptions across multiple NestJS instances using Redis PubSub?

  7. How do you optimize a NestJS GraphQL API?

    • How do you implement query complexity analysis to reject expensive queries?
    • How do you implement query depth limiting in NestJS GraphQL?
    • How do you cache GraphQL responses using persisted queries or response caching?

16. WebSockets​

  1. How do you implement WebSockets in NestJS?

    • What is a WebSocket gateway and how do you create one using @WebSocketGateway?
    • How do you handle WebSocket events using @SubscribeMessage?
    • How do you broadcast a message to all connected clients using @WebSocketServer?

  2. What is the difference between WebSockets and Server-Sent Events (SSE)?

    • When would you use SSE over WebSockets in a NestJS application?
    • How do you implement SSE in NestJS using @Sse() and EventEmitter?
    • What are the connection limits and browser support differences between SSE and WebSockets?

  3. How do you handle WebSocket disconnections in NestJS?

    • How do you use handleDisconnect and handleConnection lifecycle hooks in a gateway?
    • How do you track connected clients and clean up resources on disconnect?
    • How do you reconnect clients automatically on the frontend after a disconnection?

  4. How do you scale WebSocket applications in NestJS?

    • What is the @nestjs/platform-socket.io Redis adapter and how does it enable multi-node WebSocket scaling?
    • What is sticky session and when is it required for WebSocket servers behind a load balancer?
    • How do you test WebSocket gateways including connection, message, and disconnect events?

  5. What is a WebSocket namespace in NestJS?

    • How do you define a namespace on @WebSocketGateway({ namespace: '/chat' })?
    • How do namespaces improve organization and isolation in a WebSocket application?
    • How do you apply different guards or middleware to different namespaces?

17. Advanced Patterns​

  1. How do you implement a custom decorator in NestJS?

    • How do you create a parameter decorator that extracts data from the request?
    • How do you create a method decorator that sets metadata using SetMetadata?
    • How do you compose multiple decorators into a single custom decorator using applyDecorators?

  2. What is the Reflector class in NestJS, and how is it used?

    • How do you use Reflector.get and Reflector.getAllAndMerge inside a guard?
    • How do you read metadata set by SetMetadata at both method and class level?
    • What is Reflector.getAllAndOverride and when is it more appropriate than getAllAndMerge?

  3. What is event sourcing and how do you implement it in NestJS?

    • How does event sourcing differ from traditional CRUD (storing events vs current state)?
    • How do you use @nestjs/cqrs EventBus to publish and subscribe to domain events?
    • How do you handle event versioning as your domain evolves over time?

  4. What is the outbox pattern and how do you implement it in NestJS?

    • What problem does the outbox pattern solve (dual write, message loss on service crash)?
    • How do you implement an outbox table in Prisma or TypeORM with NestJS?
    • How do a background poller or CDC (change data capture) tool relay outbox events to a message broker?

  5. How do you implement multi-tenancy in NestJS?

    • What are the three multi-tenancy database strategies (shared schema, schema-per-tenant, DB-per-tenant)?
    • How do you use a request-scoped provider to inject the tenant context into a service?
    • How do you use Prisma or TypeORM with row-level tenant isolation?

18. Configuration and Environment​

  1. How do you manage environment variables in NestJS?

    • How do you use @nestjs/config with ConfigModule.forRoot to load .env files?
    • How do you access config values using ConfigService.get<string>('KEY')?
    • How do you secure sensitive environment variables in production (AWS Secrets Manager, Vault)?

  2. What is the difference between forRoot and forFeature in NestJS?

    • How do forRoot and forFeature enable the module registration pattern for database connection sharing?
    • Can forFeature be used in a module that has not called forRoot — and what happens?
    • How do you implement this pattern in your own dynamic modules?

  3. How do you handle different configurations for development and production?

    • How do you use process.env.NODE_ENV to load different .env files (.env.development, .env.production)?
    • How do you use the load option on ConfigModule to load typed configuration objects?
    • How do you test environment-specific configurations in unit tests?

  4. How do you validate environment variables in NestJS?

    • How do you use Joi schema validation in ConfigModule.forRoot({ validationSchema }) to validate env vars at startup?
    • How do you use class-validator with a config class and validate option on ConfigModule?
    • What happens if validation fails — does NestJS start or throw at bootstrap?

  5. What is the @nestjs/config ConfigService and how do you type it?

    • How do you create a typed config interface and use ConfigService.get<MyConfig>('database')?
    • How do you use namespaced config with registerAs('database', () => ({ ... })) and ConfigService.get?
    • How do you inject ConfigService into a TypeOrmModule.forRootAsync factory?

19. Ecosystem and Tooling​

  1. What is @nestjs/swagger and how do you use it?

    • How do you set up Swagger UI using SwaggerModule.setup and DocumentBuilder?
    • How do you use @ApiProperty, @ApiResponse, @ApiTags, and @ApiBearerAuth to document an API?
    • How do you generate a TypeScript client from a NestJS Swagger spec?

  2. How do you document DTOs with @nestjs/swagger?

    • How does @ApiProperty({ description, example, required }) document a DTO field?
    • How do you document enums, arrays, and nested objects in a Swagger DTO?
    • How do you use @ApiPropertyOptional to mark optional fields?

  3. What is @nestjs/terminus and how do you implement health checks?

    • How do you create a HealthController with @nestjs/terminus that exposes /health?
    • What health indicators does Terminus provide out of the box (database, Redis, HTTP, disk, memory)?
    • How do you configure a Kubernetes readiness probe and liveness probe using a Terminus health endpoint?

  4. How do you implement background job processing in NestJS with BullMQ?

    • How do you set up @nestjs/bullmq with a Redis connection and define a queue?
    • How do you create a @Processor class and handle jobs with @Process (or @OnWorkerEvent)?
    • How do you handle job retries, backoff, and failed jobs in BullMQ?

  5. How do you monitor BullMQ queues in a NestJS application?

    • What is Bull Board and how do you integrate it with a NestJS application?
    • What metrics should you track for a job queue (throughput, failure rate, queue depth)?
    • How do you set up alerts for stalled or failed jobs?

  6. What are NestJS lifecycle hooks and how do you use them?

    • What is the difference between OnModuleInit, OnApplicationBootstrap, OnModuleDestroy, and BeforeApplicationShutdown?
    • How do you use OnModuleInit to run setup logic (DB connection, cache warm-up) before the app accepts requests?
    • How do you use OnApplicationShutdown for graceful shutdown (draining queues, closing DB connections)?

  7. How do you implement graceful shutdown in NestJS?

    • How do you enable shutdown hooks using app.enableShutdownHooks()?
    • What happens when Kubernetes sends a SIGTERM to a NestJS pod — and how do you handle it?
    • How do you drain in-flight HTTP requests before the process exits?

  8. What is the NestJS Logger and how do you replace it?

    • How do you use the built-in Logger class from @nestjs/common with context?
    • How do you replace the NestJS default logger with Winston or Pino using app.useLogger?
    • How do you set the log level per environment (debug in dev, warn in production)?

  9. How do you implement internationalization (i18n) in NestJS?

    • How do you use nestjs-i18n to support multiple languages in a NestJS API?
    • How do you translate validation error messages from class-validator using i18n?
    • How do you detect the user's preferred language from Accept-Language headers?

20. Docker, Deployment, and Scenario-Based Questions​

  1. How do you write a Dockerfile for a NestJS application?

    • What is the recommended base image for a NestJS production container (node:20-alpine)?
    • How do you use a multi-stage build to compile TypeScript in one stage and run the output in another?
    • Why should you copy package.json and install dependencies before copying the rest of the source code?

  2. What is a multi-stage Docker build for NestJS and how does it reduce image size?

    • How do you exclude devDependencies from the production image using npm ci --only=production?
    • What is the difference between COPY --from=builder in multi-stage builds?
    • How do you run NestJS as a non-root user inside the container for security?

  3. How do you use Docker Compose with a NestJS application?

    • How do you define NestJS, PostgreSQL, and Redis services in a docker-compose.yml?
    • How do services in Docker Compose communicate with each other (service name as hostname)?
    • How do you use volumes to persist database data and hot-reload NestJS during development?

  4. How do you deploy a NestJS application to Kubernetes?

    • What Kubernetes resources do you need for a NestJS deployment (Deployment, Service, ConfigMap, Secret)?
    • How do you configure resource limits and requests for a NestJS container in Kubernetes?
    • How do you use a Kubernetes readinessProbe pointing to your NestJS Terminus /health endpoint?

  5. How do you implement CI/CD for a NestJS application?

    • What stages should a NestJS CI/CD pipeline include (lint, test, build, docker push, deploy)?
    • How do you run NestJS E2E tests in a CI pipeline with a real database (GitHub Actions service containers)?
    • What is a blue-green deployment for NestJS and how does it achieve zero downtime?

  6. Describe a challenging NestJS-specific problem you solved.

    • How did NestJS's DI system, module structure, or decorator pattern play a role in the challenge?
    • How did you debug a Nest can't resolve dependencies error in a large module graph?
    • What would you architect differently with your current knowledge of NestJS internals?

  7. How would you debug a NestJS application crashing in production?

    • How do you read and interpret NestJS startup logs to find a missing provider or bad config?
    • How do you use distributed tracing (OpenTelemetry) to find the failing request in a NestJS microservice?
    • What is your process for enabling DEBUG=* or verbose NestJS logging without redeploying?

  8. What would you do if a NestJS API route is returning 500 errors intermittently?

    • How do you use a global exception filter with structured logging to capture the error context?
    • How do you correlate a 500 error with a specific database query, guard, or interceptor using request IDs?
    • How do you use feature flags to disable a faulty endpoint without a full redeploy?

  9. How do you ensure code quality in a NestJS project?

    • How do you configure ESLint with the @typescript-eslint plugin and NestJS-specific rules?
    • How do you use husky and lint-staged to run linting and tests on pre-commit?
    • How do you enforce test coverage thresholds and type checks in a NestJS CI pipeline?

  10. How do you onboard a new developer to a NestJS project?

    • What NestJS-specific concepts do you prioritize explaining first (DI, module graph, request pipeline)?
    • How do you use Docker Compose and npm run start:dev to get a new developer running in one command?
    • How do you document module dependencies and the overall module graph for a large NestJS codebase?

  11. How do you design a NestJS application for a team of 10+ developers?

    • How do you use NestJS monorepo support (nest generate app, nest generate library) to split a large codebase?
    • How do you enforce module boundaries to prevent cross-domain coupling?
    • What shared libraries (auth, logging, database) do you extract into NestJS libraries for reuse across apps?

  12. How do you handle a performance regression in a NestJS API?

    • How do you use NestJS interceptors to add per-request timing logs for all routes?
    • How do you identify whether the regression is in a guard, interceptor, pipe, or service using execution time breakdowns?
    • What is your rollback strategy if a performance fix introduces a new bug?

  13. How would you migrate a large Express REST API to NestJS?

    • What is the strangler fig pattern and how do you apply it to incrementally replace Express routes with NestJS controllers?
    • How do you preserve existing Express middleware (auth, logging) during migration?
    • What is your strategy for migrating tests from a raw Express test suite to NestJS Test.createTestingModule?