Question 1

What are Nest pipes used for?

Accepted Answer

Transform and/or validate input before it reaches the handler — often with class-transformer/validator. Built-ins: ValidationPipe, ParseIntPipe, etc. Can be param-scoped or global.

Question 2

ValidationPipe with whitelist/forbidNonWhitelisted — why?

Accepted Answer

Strips or rejects unknown properties to prevent mass assignment and unexpected fields. Always enable in APIs accepting bodies. Pair with DTO classes. This is a security best practice interview staple.

Question 3

Transform pipes vs validate-only — examples?

Accepted Answer

ParseUUIDPipe transforms/validates path ids; custom pipes can normalize phone numbers then validate. Keep pipes focused. Heavy business validation may belong in domain services.

Question 4

Where should pipes be registered: global vs param?

Accepted Answer

Global for consistent DTO validation; param-level for specific parsers; avoid duplicate conflicting pipes. Global ValidationPipe is common. Param pipes for IDs keep handlers clean.

Question 5

Debugging: ValidationPipe not running on a nested DTO.

Accepted Answer

Missing @ValidateNested/@Type(() => NestedDto) or transform:false. class-transformer needs type metadata for nested objects. Add tests for nested payloads.

Question 6

Lead: standardize input validation across many Nest services?

Accepted Answer

Shared ValidationPipe config, DTO lint rules, OpenAPI from DTOs, and reject unknown fields org-wide. Document error response shape. Version DTOs carefully. Prefer shared lib for common constraints.

Pipes NestJS Interview Questions

Other NestJS interview topics