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.

Practice

1. What is the main purpose of unit testing a service in NestJS?

easy

A. To verify the service works correctly on its own without dependencies

B. To test the entire application including all modules

C. To check the UI components connected to the service

D. To deploy the service to production automatically

Solution

1. Step 1: Understand unit testing scope

   Unit testing focuses on testing a single piece of code in isolation, here the service alone.

2. Step 2: Identify what unit testing excludes

   It excludes integration with other modules or UI, focusing only on the service logic.

3. Final Answer:

   To verify the service works correctly on its own without dependencies -> Option A

4. Quick Check:

   Unit testing = isolated service check [OK]

Hint: Unit test means testing one piece alone, no dependencies [OK]

Common Mistakes:

• Confusing unit test with integration test
• Thinking unit test covers UI or deployment
• Assuming unit test runs the whole app

2. Which of the following is the correct way to get a service instance in a NestJS unit test?

easy

A. const service = require('MyService');

B. const service = new MyService();

C. const service = module.get<MyService>(MyService);

D. const service = app.getService(MyService);

Solution

1. Step 1: Use NestJS TestingModule

   In unit tests, you create a TestingModule and get the service instance via module.get.

2. Step 2: Identify correct syntax

   The syntax module.get<MyService>(MyService) is the official way to retrieve the service from the test module.

3. Final Answer:

   const service = module.get<MyService>(MyService); -> Option C

4. Quick Check:

   Use module.get to get service instance [OK]

Hint: Always get service from TestingModule with module.get<Type>(Type) [OK]

Common Mistakes:

• Instantiating service directly without module
• Using app.getService which is not a NestJS method
• Using require which is not for NestJS DI

3. Given this test code snippet, what will be the output of service.getData()?

const mockData = ['a', 'b'];
const mockRepo = { findAll: () => mockData };

const module = await Test.createTestingModule({
  providers: [
    MyService,
    { provide: MyRepository, useValue: mockRepo },
  ],
}).compile();

const service = module.get<MyService>(MyService);

console.log(service.getData());

medium

A. An error because MyRepository is not mocked

B. undefined

C. [] (empty array)

D. ['a', 'b']

Solution

1. Step 1: Understand mocking of MyRepository

   The mockRepo object replaces MyRepository with a findAll method returning ['a', 'b'].

2. Step 2: Analyze service.getData()

   Assuming getData calls repository.findAll(), it will return ['a', 'b'] from the mock.

3. Final Answer:

   ['a', 'b'] -> Option D

4. Quick Check:

   Mocked repo returns ['a', 'b'] so service returns same [OK]

Hint: Mocked methods return mock values used by service [OK]

Common Mistakes:

• Forgetting to mock repository causing undefined
• Expecting real database call instead of mock
• Assuming empty array without mock data

4. What is the main issue in this test setup code?

const module = await Test.createTestingModule({
  providers: [MyService],
}).compile();

const service = module.get<MyService>(MyService);

jest.spyOn(service, 'findAll').mockReturnValue(['x', 'y']);

medium

A. You cannot spy on service methods before mocking dependencies

B. The service method findAll should be mocked on the dependency, not service

C. jest.spyOn syntax is incorrect here

D. Missing async keyword on test function

Solution

1. Step 1: Identify what to mock in unit tests

   In unit tests, you mock dependencies of the service, not the service's own methods.

2. Step 2: Understand why mocking service method is wrong

   Mocking service.findAll replaces the method under test, defeating the purpose of testing it.

3. Final Answer:

   The service method findAll should be mocked on the dependency, not service -> Option B

4. Quick Check:

   Mock dependencies, not the service itself [OK]

Hint: Mock dependencies, never the service method you test [OK]

Common Mistakes:

• Mocking the service method instead of its dependencies
• Confusing spyOn usage with mocking dependencies
• Ignoring async test function requirements

5. You want to test a service method that calls two different repository methods internally. How should you set up your unit test to isolate the service logic correctly?

hard

A. Mock both repository methods with expected return values and inject them into the test module

B. Call the real repository methods to test the full flow

C. Mock only one repository method and leave the other real

D. Do not mock anything and test the service directly

Solution

1. Step 1: Identify dependencies to mock

   Since the service calls two repository methods, both should be mocked to isolate service logic.

2. Step 2: Setup test module with mocks

   Provide mock implementations for both methods in the test module providers to control their outputs.

3. Step 3: Avoid real calls in unit tests

   Calling real repository methods would make it an integration test, not a unit test.

4. Final Answer:

   Mock both repository methods with expected return values and inject them into the test module -> Option A

5. Quick Check:

   Mock all dependencies to isolate service logic [OK]

Hint: Mock all called dependencies to isolate service logic [OK]

Common Mistakes:

• Mocking only some dependencies causing partial isolation
• Calling real methods making it integration test
• Not injecting mocks properly into test module