Unit testing controllers in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main purpose of unit testing controllers in NestJS?

To verify that the controller methods handle requests correctly and interact properly with services, without involving external dependencies.

beginner

Which NestJS testing utility helps create a testing module for controllers?

The Test.createTestingModule() method is used to build a testing module that can include controllers and mocked providers.

intermediate

Why do we mock services when unit testing controllers?

Mocking services isolates the controller logic by replacing real service calls with fake ones, ensuring tests focus only on controller behavior.

intermediate

How do you test a controller method that returns a promise in NestJS?

Use async/await in the test function to wait for the promise to resolve, then assert the returned value or behavior.

intermediate

What is a common pattern to verify that a controller calls a service method correctly?

Use Jest spies or mocks to check if the service method was called with expected arguments when the controller method runs.

What does Test.createTestingModule() do in NestJS unit tests?

A Creates a module for testing with controllers and providers

B Runs the entire application

C Mocks HTTP requests automatically

D Generates database schemas

It builds a testing module that can include controllers and mocked providers for isolated testing.

Why should services be mocked when unit testing controllers?

A To speed up database queries

B To test the service logic instead

C To isolate controller logic and avoid external dependencies

D To automatically generate test data

Mocking services isolates the controller so tests focus only on controller behavior.

Which Jest feature helps verify if a service method was called by a controller?

A jest.resetModules()

B jest.mock()

C jest.clearAllMocks()

D jest.spyOn()

jest.spyOn() creates a spy to track calls to a method.

How do you handle asynchronous controller methods in tests?

A Ignore the promise and test synchronously

B Use async/await to wait for the promise

C Use setTimeout to delay assertions

D Convert promises to callbacks

Using async/await ensures the test waits for the promise to resolve before asserting.

What is the best practice for testing controller methods that depend on services?

A Mock the services to isolate controller logic

B Use real services with a test database

C Skip testing those methods

D Test only the services, not the controllers

Mocking services isolates the controller and makes tests faster and more focused.

Explain how to set up a unit test for a NestJS controller method that calls a service.

Think about isolating the controller by mocking dependencies.

Describe why mocking is important in unit testing NestJS controllers and how it improves test quality.

Consider what happens if you don’t mock services in unit tests.