NestJS Quality Control & Assurance 3

Question 1

Which NestJS testing utility lets you replace an interceptor with a no-op version for a specific test?

Accepted Answer

overrideInterceptor()

Answer

overrideInterceptor() in the TestingModule builder accepts a class and swaps it with a test double using .useValue() or .useClass().

Question 2

What Jest configuration option should you set to automatically clear mock call history between each test in a NestJS project?

Accepted Answer

clearMocks: true

Answer

clearMocks: true in jest.config.ts resets mock.calls and mock.instances before every test without restoring the original implementation.

Question 3

When using class-validator in NestJS, which test approach correctly validates a DTO without starting the full application?

Accepted Answer

Call validate() from class-validator on a class instance created with plainToInstance()

Answer

plainToInstance + validate() from class-transformer/class-validator lets you unit-test DTO validation rules in isolation.

Question 4

Which tool integrates with NestJS to generate an OpenAPI specification that can be used as a contract test artifact?

Accepted Answer

@nestjs/swagger

Answer

@nestjs/swagger's SwaggerModule.createDocument() produces an OpenAPI 3 document that can be diffed across versions for breaking-change detection.

Question 5

What is the effect of passing `{ passthrough: true }` to `jest.spyOn()` in a NestJS test?

Accepted Answer

It is not a valid jest.spyOn option; passthrough is used with mockReturnValue

Answer

jest.spyOn() does not accept a passthrough option; to call through to the original, you simply don't add a mock implementation.

NestJS Practice Test