Question 1

How does ValidationPipe + class-validator validate bodies?

Accepted Answer

Transforms plain objects to DTO classes then runs decorators; throws BadRequest on failures. Requires transform for nested types. Customize exceptionFactory for error shape.

Question 2

Validate nested arrays of objects — requirements?

Accepted Answer

@ValidateNested({ each: true }) and @Type(() => ItemDto) with transform enabled. Missing @Type is the most common bug. Add unit tests for nested payloads.

Question 3

Custom validators — when?

Accepted Answer

Cross-field rules, DB uniqueness checks (carefully), or domain formats not covered by built-ins. Prefer sync validators in pipes; async uniqueness often better in services to keep pipes pure.

Question 4

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

Accepted Answer

Zod offers TS-first inference; class-validator is decorator-native to Nest examples and Swagger plugins. Either works — consistency matters. Nest pipes can adapt Zod schemas.

Question 5

Scenario: validation passes but business rule fails later.

Accepted Answer

Input validation is not domain invariants — enforce invariants in domain services and map to 422/409. Don't overload DTOs with all business rules. Keep boundaries clear.

Question 6

Lead: org-wide validation standards for Nest APIs?

Accepted Answer

Global pipe config, shared error format, security defaults (whitelist), and linters for DTO coverage. Security review for file uploads and string lengths. Fuzz critical parsers.

Validation NestJS Interview Questions

Other NestJS interview topics