Testing module setup in NestJS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of the TestingModule in NestJS?

The TestingModule in NestJS creates a test environment that mimics the real module setup. It allows you to isolate and test components like services and controllers with their dependencies.

beginner

Which NestJS function is used to create a TestingModule?

The function Test.createTestingModule() is used to create a TestingModule. It accepts a module metadata object similar to a regular module.

beginner

How do you compile a TestingModule before using it in tests?

You call await module.compile() after creating the TestingModule. This prepares the module and its dependencies for injection and use in tests.

beginner

Why do we use module.get() in NestJS testing?

We use module.get() to retrieve an instance of a provider (like a service) from the TestingModule. This lets us test the provider directly.

intermediate

What is the benefit of isolating dependencies in a TestingModule?

Isolating dependencies lets you test components without relying on real external services or databases. This makes tests faster, more reliable, and easier to understand.

Which method creates a TestingModule in NestJS?

A Test.createTestingModule()

B Module.createTest()

C NestFactory.createTestingModule()

D TestingModule.new()

The correct method is Test.createTestingModule(), which sets up the test module.

What must you call after creating a TestingModule to prepare it for use?

A module.compile()

B module.build()

C module.initialize()

D module.start()

You call module.compile() to finalize the TestingModule setup before using it.

How do you get a service instance from a TestingModule?

A module.retrieve(ServiceClass)

B module.find(ServiceClass)

C module.get(ServiceClass)

D module.fetch(ServiceClass)

Use module.get(ServiceClass) to retrieve the service instance for testing.

Why is it good to mock dependencies in a TestingModule?

A To make tests slower

B To isolate the unit being tested

C To increase external calls

D To avoid writing tests

Mocking dependencies isolates the unit under test, making tests more reliable and faster.

Which of these is NOT part of TestingModule setup?

A Defining providers

B Retrieving instances with get()

C Compiling the module

D Starting the HTTP server

Starting the HTTP server is not part of TestingModule setup; tests run without starting servers.

Explain the steps to set up a TestingModule in NestJS for a service test.

Think about creating, compiling, and getting the service.

Why is isolating dependencies important in NestJS TestingModule setup?

Consider how tests behave with and without real dependencies.

Practice

1. What is the main purpose of using Test.createTestingModule() in NestJS testing?

easy

A. To create a small test version of your app module with needed parts

B. To start the full application server for integration testing

C. To generate database seed data automatically

D. To deploy the app to production environment

Solution

1. Step 1: Understand the role of Test.createTestingModule()

   This method creates a lightweight module for testing, including only what you add (controllers, providers, imports).

2. Step 2: Compare with other options

   Starting the full app server or deploying is not the purpose of this method. It is only for isolated testing setup.

3. Final Answer:

   To create a small test version of your app module with needed parts -> Option A

4. Quick Check:

   Testing module setup = small test module [OK]

Hint: Testing module creates mini app for safe isolated tests [OK]

Common Mistakes:

• Thinking it starts the full app server
• Confusing with deployment or seeding
• Assuming it runs tests automatically

2. Which of the following is the correct way to compile a testing module in NestJS?

easy

A. await compileTestingModule(Test.createTestingModule({}));

B. Test.createTestingModule({}).compile();

C. Test.compileTestingModule({});

D. await Test.createTestingModule({}).compile();

Solution

1. Step 1: Recall the async nature of compile()

   The compile() method returns a Promise, so it must be awaited.

2. Step 2: Check syntax correctness

   Only await Test.createTestingModule({}).compile(); correctly uses await with Test.createTestingModule({}).compile(). Others are either missing await or use invalid methods.

3. Final Answer:

   await Test.createTestingModule({}).compile(); -> Option D

4. Quick Check:

   Compile returns Promise, so use await [OK]

Hint: Always await compile() on testing module [OK]

Common Mistakes:

• Forgetting to await compile()
• Using non-existent compileTestingModule method
• Calling compile without parentheses

3. Given this test setup code snippet, what will service.getData() return?

const moduleRef = await Test.createTestingModule({
  providers: [{
    provide: 'DataService',
    useValue: { getData: () => 'mocked data' },
  }],
}).compile();
const service = moduleRef.get('DataService');
return service.getData();

medium

A. Throws runtime error

B. undefined

C. 'mocked data'

D. 'real data'

Solution

1. Step 1: Identify the provider mock

   The provider 'DataService' is mocked with an object whose getData method returns 'mocked data'.

2. Step 2: Understand the get call

   Calling moduleRef.get('DataService') returns the mocked object, so service.getData() returns 'mocked data'.

3. Final Answer:

   'mocked data' -> Option C

4. Quick Check:

   Mock provider method returns 'mocked data' [OK]

Hint: Mocked provider returns its defined value [OK]

Common Mistakes:

• Expecting real data instead of mock
• Assuming get returns undefined without provider
• Confusing provider token with class

4. What is wrong with this NestJS testing module setup?

const moduleRef = await Test.createTestingModule({
  controllers: [AppController],
  providers: [AppService],
});
const controller = moduleRef.get(AppController);

medium

A. Controllers should not be added in testing modules

B. Missing call to compile() before getting controller

C. Providers array should be empty

D. Cannot get controller by class reference

Solution

1. Step 1: Check testing module lifecycle

   After creating the testing module, you must call compile() to finalize it before retrieving instances.

2. Step 2: Identify missing compile call

   The code does not call await moduleRef.compile(), so moduleRef.get() will fail or behave unexpectedly.

3. Final Answer:

   Missing call to compile() before getting controller -> Option B

4. Quick Check:

   Always compile testing module before get() [OK]

Hint: Always await compile() before accessing module instances [OK]

Common Mistakes:

• Forgetting to compile testing module
• Thinking controllers can't be added
• Trying to get by string token instead of class

5. You want to test a service that depends on another service. How do you set up the testing module to replace the dependency with a mock implementation?

hard

A. Add the service and provide a mock using useValue for the dependency in providers array

B. Only add the main service, NestJS auto-mocks dependencies

C. Add both services without mocks, real implementations are always used

D. Use imports to load the dependency module instead of mocking

Solution

1. Step 1: Understand dependency injection in testing

   To replace a dependency, you provide a mock object using useValue in the providers array.

2. Step 2: Setup providers correctly

   Add the main service normally and add the dependency with provide and useValue to inject the mock.

3. Final Answer:

   Add the service and provide a mock using useValue for the dependency in providers array -> Option A

4. Quick Check:

   Mock dependencies with useValue in providers [OK]

Hint: Mock dependencies by providing useValue in providers [OK]

Common Mistakes:

• Assuming NestJS auto-mocks dependencies
• Not providing mock for dependency
• Using imports instead of mocking for isolated tests