NestJS Interview Questions & Answers

What is a NestJS module and why does it exist?

imports vs exports vs providers in @Module — differences?

What are global modules and when are they appropriate?

Dynamic modules (forRoot/forRootAsync) — why?

Feature module vs shared module vs core module — how do you split?

Lead: structure Nest modules for a multi-team monorepo API.

What is a Nest controller responsible for?

How do @Param, @Query, @Body, and @Headers work together?

Versioning APIs in Nest — approaches?

When should logic leave the controller?

Scenario: controller returns entities with password hashes. Fix?

Lead: conventions for public vs internal Nest controllers?

What counts as a provider in Nest?

useFactory vs useClass vs useValue — when?

What is a custom injection token and why use string/symbol tokens?

Transient vs request vs singleton scope — trade-offs?

Debugging: Nest says provider not found at runtime.

Lead: provider design for ports-and-adapters in Nest?

How does Nest's DI container resolve dependencies?

Why forwardRef exists and when it's a smell?

Optional dependencies with @Optional() — use cases?

Constructor injection vs @Inject() property injection?

Scenario: two implementations of the same interface for A/B payments.

Lead: how do you keep DI graphs understandable at 200+ providers?

What is Nest middleware vs Express middleware?

Where do you apply middleware selectively?

Middleware vs Guard vs Interceptor — which for auth?

Debugging: middleware not running for a route.

Lead: correlation ID middleware standards across services.

What does a Nest Guard do?

How do you build role-based guards with custom decorators?

Guard order and multiple guards — behavior?

Guards vs middleware for API keys?

Scenario: public health route blocked by global AuthGuard.

Lead: authorization model beyond role strings in Nest?

What are Nest pipes used for?

ValidationPipe with whitelist/forbidNonWhitelisted — why?

Transform pipes vs validate-only — examples?

Where should pipes be registered: global vs param?

Debugging: ValidationPipe not running on a nested DTO.

Lead: standardize input validation across many Nest services?

What are Nest interceptors?

Interceptor vs middleware vs filter — response mapping?

How do you implement a timeout interceptor?

Caching interceptor pitfalls?

Scenario: need to measure handler duration and status.

Lead: when interceptors become a dumping ground — redesign?

What does an exception filter do?

HttpException vs domain errors — mapping strategy?

Built-in vs custom filters — when custom?

Order of filters and @Catch specificity?

Debugging: errors return Nest's default JSON in production unexpectedly.

Lead: unify error contracts across Nest and non-Nest services?

What is a DTO in Nest and why separate from entities?

class-validator decorators on DTOs — essentials?

PartialType, PickType, OmitType, IntersectionType — when?

Scenario: client sends extra fields that overwrite isAdmin.

Lead: DTO versioning and OpenAPI discipline?

How does ValidationPipe + class-validator validate bodies?

Validate nested arrays of objects — requirements?

Custom validators — when?

Zod/io-ts vs class-validator in Nest — trade-offs?

Scenario: validation passes but business rule fails later.

Lead: org-wide validation standards for Nest APIs?

Passport in Nest — typical pattern?

JWT AuthGuard flow in Nest?

How do you implement refresh tokens with Nest?

Global AuthGuard with public routes — pattern?

Session-based auth with Nest — considerations?

Lead: integrate Nest services with corporate OIDC/SSO.

Authentication vs authorization in Nest?

RBAC with roles metadata — limitations?

How do you authorize user can edit own post?

CASL or policy-based auth in Nest — why?

Debugging: authorized in UI but 403 from API.

Lead: multi-tenant authorization architecture for Nest?

TypeORM vs Prisma with Nest — practical trade-offs?

Where should repositories live in Nest modules?

How do you handle transactions in Nest services?

N+1 queries in Nest resolvers/services — prevention?

Migrations in production Nest apps — best practices?

Lead: multi-DB or CQRS read models with Nest?

How do Nest microservices differ from HTTP controllers?

Request-response vs event patterns in Nest microservices?

Why is idempotency critical for Kafka/Rabbit consumers?

gRPC with Nest — when prefer it?

Debugging: Nest microservice hangs waiting for response.

Lead: when NOT to split a Nest monolith into microservices?

What is TestingModule in Nest?

Unit test a service with mocked repo — pattern?

e2e testing Nest HTTP apps — approach?

How do you test guards/pipes/interceptors in isolation?

Scenario: flaky e2e due to module singletons holding state.

Lead: testing strategy pyramid for a large Nest codebase?

Is Nest only for microservices? What default architecture fits most teams?

Clean/hexagonal architecture mapping onto Nest?

Config management best practices in Nest?

Scenario: circular dependency between OrdersModule and UsersModule.

How do you approach background jobs in Nest apps?

Lead: evolve a Nest monolith toward service extraction.