NestJS Quality Control & Assurance 4

Question 1

Which NestJS-specific Jest preset should be configured in jest.config.ts for a standard NestJS project?

Accepted Answer

ts-jest

Answer

ts-jest transpiles TypeScript and is the standard preset used by the NestJS CLI-generated jest.config.js.

Question 2

What is a primary advantage of using `jest.mock()` at the module level instead of `jest.spyOn()` inside individual tests?

Accepted Answer

It hoists the mock before imports, ensuring all imports receive the mocked version

Answer

jest.mock() is hoisted to the top of the file by Babel/ts-jest, so even top-level import statements receive the mock.

Question 3

When testing a NestJS GraphQL resolver, which method of the TestingModule allows you to invoke the resolver method directly without going through the GraphQL layer?

Accepted Answer

module.get(Resolver).methodName()

Answer

Resolvers are plain providers; retrieving them with module.get() and calling the method directly is the simplest unit-test approach.

Question 4

Which approach correctly tests a NestJS ExceptionFilter without starting the full HTTP server?

Accepted Answer

Call filter.catch() directly with mocked ArgumentsHost and HttpException

Answer

ExceptionFilters are plain classes; instantiating the filter and calling catch() with mock ArgumentsHost objects lets you unit-test the error response logic.

Question 5

What does the `--coverage` flag in `jest` produce that is useful for NestJS CI pipelines?

Accepted Answer

A code coverage report showing statement, branch, function, and line percentages

Answer

--coverage instruments the code with Istanbul and outputs HTML/LCOV reports with line-level coverage data.

NestJS Practice Test

NestJS Practice Test

NestJS Quality Control & Assurance 4