NestJS Case Studies & Practical Application 3

Question 1

A NestJS app under load shows high memory usage. Profiling reveals many request-scoped providers are never destroyed. What is the most likely cause?

Accepted Answer

The REQUEST-scoped providers are injected into singleton-scoped providers, preventing their destruction

Answer

When a REQUEST-scoped provider is injected into a SINGLETON, NestJS keeps a singleton instance alive holding a reference, preventing cleanup.

Question 2

You need to validate nested objects in a DTO using class-validator. A CreateOrderDto has an items array of OrderItemDto. Which decorator combination ensures items are also validated?

Accepted Answer

@ValidateNested({ each: true }) combined with @Type(() => OrderItemDto)

Answer

@ValidateNested triggers recursive validation and @Type tells class-transformer which class to instantiate for nested objects.

Question 3

A NestJS GraphQL API must enforce field-level authorization so some fields are only visible to admins. What is the most scalable approach?

Accepted Answer

Use a custom FieldMiddleware from @nestjs/graphql that checks the user role per field

Answer

Field middleware in NestJS GraphQL runs per field with the execution context, enabling reusable declarative field-level access control.

Question 4

Your NestJS app connects to RabbitMQ via @nestjs/microservices. Under high load, messages accumulate faster than they are processed. What configuration change reduces back-pressure?

Accepted Answer

Set prefetchCount on the RMQ transport options to limit in-flight messages per consumer

Answer

prefetchCount tells RabbitMQ how many unacknowledged messages to deliver per channel, preventing the consumer from being overwhelmed.

Question 5

A NestJS app using TypeORM needs to run a database migration before tests in a CI pipeline. What is the correct approach?

Accepted Answer

Run typeorm migration:run against the test database connection before the test suite starts

Answer

Running actual migrations in CI ensures tests execute against a schema matching production, catching migration-related regressions early.

NestJS Practice Test

NestJS Practice Test

NestJS Case Studies & Practical Application 3