NestJS Quiz ( Advance ) - All Questions

This advanced NestJS quiz is designed for senior backend developers and architects preparing for high-level interviews. It focuses on NestJS internals, dependency injection mechanics, request lifecycle, performance optimization, microservices, GraphQL, security, testing strategies, and real-world production scenarios.

Question 1: Why is dependency injection central to NestJS architecture?

It improves routing speed
It enables loose coupling and testability
It replaces middleware
It simplifies HTTP adapters

Explanation: DI promotes loose coupling and easier testing.

Question 2: What happens internally when a request-scoped provider is used?

A single instance is reused
A new instance is created per request context
It becomes transient
It is cached globally

Explanation: Request-scoped providers are created per request.

Question 3: What is the biggest performance drawback of request-scoped providers?

Memory leaks
Increased instantiation overhead
Broken dependency graph
Lost singleton behavior

Explanation: They increase object creation cost per request.

Question 4: Why are transient providers rarely recommended?

They break routing
They create new instances on every injection
They disable DI
They prevent async operations

Explanation: Transient providers create many instances.

Question 5: What does the NestJS injector resolve first?

Controllers
Providers dependencies
Modules
Middleware

Explanation: Dependencies are resolved before instantiation.

Question 6: What problem does forwardRef() truly solve?

Async loading
Circular dependency resolution
Lazy loading modules
Provider scope conflicts

Explanation: forwardRef resolves circular dependencies.

Question 7: Which layer executes first in the NestJS request lifecycle?

Guards
Middleware
Interceptors
Pipes

Explanation: Middleware executes before guards.

Question 8: Which layer can short-circuit a request before reaching the controller?

Pipes
Interceptors
Guards
Filters

Explanation: Guards can block execution.

Question 9: Why should business logic not be placed inside controllers?

Performance issues
Testing complexity and poor separation of concerns
Routing limitations
Decorator conflicts

Explanation: Logic belongs in services for testability.

Question 10: Which NestJS feature is best for cross-cutting concerns?

Controllers
Interceptors
Providers
Modules

Explanation: Interceptors handle cross-cutting logic.

Question 11: What is the key difference between middleware and interceptors?

Execution order
Interceptors support RxJS streams
Middleware supports DI
Interceptors run only once

Explanation: Interceptors can manipulate streams.

Question 12: Why are pipes preferred over manual validation?

Better performance
Centralized validation and transformation
Less code
Async support

Explanation: Pipes centralize validation logic.

Question 13: What risk exists when disabling ValidationPipe whitelist?

Slower requests
Unexpected payload properties
Broken routing
Serialization issues

Explanation: Extra properties may pass through.

Question 14: Why should @Res() be avoided in most cases?

Breaks routing
Bypasses NestJS response lifecycle
Disables guards
Blocks async pipes

Explanation: It bypasses interceptors and filters.

Question 15: Which scenario justifies using @Res({ passthrough: true })?

Manual routing
Partial response control with Nest lifecycle
Faster responses
Disabling interceptors

Explanation: It allows partial control safely.

Question 16: Why are global interceptors powerful but dangerous?

Performance cost
They affect every request
Hard to test
They break DI

Explanation: They impact the entire application.

Question 17: What is the main purpose of ExecutionContext?

Routing
Access request metadata across contexts
Error handling
Dependency injection

Explanation: ExecutionContext abstracts request context.

Question 18: Why is Passport integration implemented via guards?

Performance
Authorization fits guard responsibility
Simpler syntax
DI limitation

Explanation: Guards handle authorization decisions.

Question 19: What is the biggest JWT authentication pitfall?

Token size
Token revocation difficulty
Encryption cost
Header parsing

Explanation: JWTs are hard to revoke.

Question 20: Why should refresh tokens be stored securely?

Performance
Prevent token theft
Reduce DB calls
Improve routing

Explanation: Stolen refresh tokens are dangerous.

Question 21: Which NestJS feature enables CQRS pattern?

@nestjs/graphql
@nestjs/cqrs
@nestjs/typeorm
@nestjs/config

Explanation: nestjs/cqrs supports CQRS.

Question 22: What is the main benefit of CQRS?

Faster routing
Separation of read and write models
Simpler code
Reduced memory

Explanation: CQRS separates read/write concerns.

Question 23: Why is EventEmitter not ideal for large-scale systems?

Slow performance
Tight coupling and lack of durability
Memory leaks
Limited syntax

Explanation: Events are not durable.

Question 24: Which transport is best for high-throughput microservices?

HTTP
TCP
Redis
NATS

Explanation: NATS is optimized for messaging.

Question 25: Why should DTOs be immutable?

Performance
Predictable data flow
Smaller bundles
Serialization

Explanation: Immutability prevents side effects.

Question 26: What is the main risk of sharing entities across layers?

Type conflicts
Leaky abstractions
Slow queries
Circular imports

Explanation: Entities leaking cause tight coupling.

Question 27: Why is Repository pattern preferred in NestJS?

Less code
Abstracts data access logic
Improves routing
Enhances decorators

Explanation: It decouples persistence logic.

Question 28: What is the purpose of custom providers?

Override framework behavior
Control instantiation logic
Create controllers
Register routes

Explanation: Custom providers control DI behavior.

Question 29: Which provider type enables dynamic configuration?

useClass
useValue
useFactory
useExisting

Explanation: Factories allow dynamic creation.

Question 30: Why should heavy logic not run in onModuleInit?

DI not ready
Blocks application startup
Causes memory leaks
Breaks lifecycle

Explanation: It delays app readiness.

Question 31: What is the biggest challenge in NestJS microservices debugging?

Syntax errors
Distributed tracing
DTO validation
Transport selection

Explanation: Tracing across services is complex.

Question 32: Why is correlation ID important?

Security
Tracking requests across services
Caching
Routing

Explanation: It helps trace distributed requests.

Question 33: Which feature enables graceful shutdown?

enableShutdownHooks()
app.close()
process.exit()
beforeExit()

Explanation: Shutdown hooks allow cleanup.

Question 34: Why is app.enableShutdownHooks() critical in production?

Performance
Resource cleanup on termination
Routing safety
Async handling

Explanation: It ensures graceful shutdown.

Question 35: Which testing strategy best suits large NestJS apps?

Only unit tests
Only e2e tests
Combination of unit, integration, and e2e
Manual testing

Explanation: Multiple test layers ensure reliability.

Question 36: Why should e2e tests avoid real external services?

Speed
Flakiness and instability
Cost
Security

Explanation: External services cause flaky tests.

Question 37: What is the main benefit of using TestContainers?

Faster tests
Production-like dependencies
Simpler setup
Mock-free tests

Explanation: They provide realistic environments.

Question 38: Why should configuration be validated at startup?

Performance
Fail fast on misconfiguration
Security
Ease of use

Explanation: Failing fast avoids runtime issues.

Question 39: Which NestJS feature helps version APIs cleanly?

Guards
URI versioning
Interceptors
Middleware

Explanation: URI or header versioning is supported.

Question 40: What is the main downside of global modules?

Slower startup
Hidden dependencies
Memory usage
DI conflicts

Explanation: They obscure dependency boundaries.

Question 41: Why is statelessness important for scalability?

Security
Horizontal scaling support
Simpler code
Better logging

Explanation: Stateless apps scale horizontally.

Question 42: Which architectural mistake most often hurts NestJS performance?

Using DTOs
Overusing request-scoped providers
Using modules
Using guards

Explanation: Request scope adds overhead.

Question 43: Why should background jobs be separated from HTTP servers?

Security
Isolation and scalability
Code reuse
Simpler DI

Explanation: Isolation improves reliability.

Question 44: Which NestJS feature supports background job processing?

@nestjs/schedule
@nestjs/bull
@nestjs/queue
@nestjs/tasks

Explanation: Bull integrates job queues.

Question 45: What is the ultimate architectural goal of NestJS?

Minimal APIs
Scalable, maintainable server-side systems
Rapid prototyping
Microservices only

Explanation: NestJS targets scalable backend systems.

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