Unit testing services in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the main goal of unit testing a service in NestJS?

To test the service's logic in isolation, ensuring each function works correctly without depending on other parts like databases or external APIs.

beginner

How do you create a testing module for a service in NestJS?

Use Test.createTestingModule({ providers: [YourService] }).compile() to set up a module that provides the service for testing.

intermediate

Why do we use mocks or spies in unit testing NestJS services?

Mocks replace real dependencies to isolate the service logic, while spies let us check if certain methods were called, helping us test behavior without side effects.

intermediate

What Jest function is commonly used to create a spy on a method in NestJS service tests?

jest.spyOn(object, 'methodName') is used to spy on a method to track calls and control its behavior during tests.

intermediate

How can you test asynchronous methods in a NestJS service?

Use async/await in your test functions and Jest's expect().resolves or expect().rejects to handle promises and verify async behavior.

Which NestJS testing utility helps create a module for unit testing a service?

A Test.createTestingModule()

B Module.forRoot()

C Service.register()

D AppModule.create()

Test.createTestingModule() sets up a testing module with providers for isolated unit tests.

What is the purpose of mocking dependencies in NestJS service tests?

A To isolate the service logic from external parts

B To add more features to the service

C To speed up the app in production

D To make the service dependent on real databases

Mocking isolates the service logic by replacing real dependencies with fake ones.

Which Jest function lets you check if a method was called during a test?

A jest.mock()

B jest.fn()

C jest.spyOn()

D jest.clearAllMocks()

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

How do you handle testing an async method in a NestJS service?

A Use setTimeout in tests

B Use callbacks only

C Ignore async behavior

D Use async/await and expect().resolves or expect().rejects

Async/await with Jest's resolves/rejects helps test promises properly.

What does the compile() method do in Test.createTestingModule()?

A Runs the tests

B Builds the testing module and resolves dependencies

C Deletes the module

D Starts the NestJS server

compile() finalizes the testing module setup and prepares it for use.

Explain how to set up and write a basic unit test for a NestJS service method.

Think about creating a testing module, getting the service, and writing a test function.

Describe why and how you would mock a dependency in a NestJS service unit test.

Focus on isolation and replacing real parts with fake ones.