Question 1

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

Accepted Answer

Data Transfer Objects define API shapes; entities model persistence — decoupling prevents leaking DB schema. Request and response DTOs can differ. Validation decorators live on DTOs. Never expose entities blindly.

Question 2

class-validator decorators on DTOs — essentials?

Accepted Answer

IsString/IsEmail/IsUUID/IsOptional/ValidateNested, plus whitelist via ValidationPipe. Keep messages clear. Share base DTO classes carefully. Prefer composition over deep inheritance.

Question 3

PartialType, PickType, OmitType, IntersectionType — when?

Accepted Answer

Mapped types to derive update/create DTOs without duplication while staying explicit. Update often uses PartialType(CreateDto). Don't invent one mega DTO for all operations.

Question 4

Scenario: client sends extra fields that overwrite isAdmin.

Accepted Answer

Mass assignment — whitelist DTOs and never bind directly to entities with privileged fields. forbidNonWhitelisted helps. Separate admin-only DTOs/routes.

Question 5

Lead: DTO versioning and OpenAPI discipline?

Accepted Answer

Version DTOs with API versions, generate Swagger from decorators, and break changes via new versions. Contract tests against OpenAPI. Review DTO changes like public API.

DTOs NestJS Interview Questions

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