Testing module setup in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the TestingModule from NestJS.

NestJS

import { [1] } from '@nestjs/testing';

Drag options to blanks, or click blank then click option'

ATestModule

BTestingModule

CModuleTesting

DNestTesting

Attempts:

3 left

💡 Hint

Common Mistakes

Using incorrect class names like TestModule or ModuleTesting.

Trying to import from other packages.

The TestingModule is imported from @nestjs/testing to create a test module.

2fill in blank

medium

Complete the code to create a testing module with a service provider.

NestJS

const moduleRef = await Test.createTestingModule({ providers: [[1]] }).compile();

Drag options to blanks, or click blank then click option'

AMyService

BMyController

CAppModule

DNestService

Attempts:

3 left

💡 Hint

Common Mistakes

Adding controllers or modules as providers.

Using incorrect service names.

Providers are services or classes to be tested, so MyService is the correct provider here.

3fill in blank

hard

Fix the error in retrieving the service instance from the testing module.

NestJS

const service = moduleRef.[1](MyService);

Drag options to blanks, or click blank then click option'

Aget

Bfetch

Cretrieve

Dfind

Attempts:

3 left

💡 Hint

Common Mistakes

Using methods like fetch or retrieve which do not exist on TestingModule.

Trying to access the service directly without using a method.

The get method is used to retrieve an instance of a provider from the testing module.

4fill in blank

hard

Fill both blanks to correctly import and create a testing module.

NestJS

import { [1] } from '@nestjs/testing';
const module = await [2].createTestingModule({}).compile();

Drag options to blanks, or click blank then click option'

ATest

BTestingModule

CTestModule

DNestTest

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the import with TestingModule which is a class, not the utility object.

Using incorrect names like TestModule or NestTest.

The Test object is imported and used to create the testing module.

5fill in blank

hard

Fill all three blanks to create a testing module, compile it, and retrieve a service.

NestJS

import { [1] } from '@nestjs/testing';

const moduleRef = await [2].createTestingModule({ providers: [MyService] }).compile();
const service = moduleRef.[3](MyService);

Drag options to blanks, or click blank then click option'

ATest

BTestingModule

Cget

DTestModule

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'TestingModule' as the utility object instead of 'Test'.

Using incorrect retrieval methods like 'fetch' or 'retrieve'.

Use Test to import and create the testing module, then use get to retrieve the service instance.

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