Testing module setup in NestJS - Step-by-Step Execution

Concept Flow - Testing module setup

Create TestingModuleBuilder

↓

Configure providers, controllers

↓

Compile TestingModule

↓

Retrieve instances with get()

↓

Run tests using instances

↓

END

This flow shows how NestJS testing modules are created, configured, compiled, and used to get instances for testing.

Execution Sample

NestJS

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

This code creates a testing module with MyService, compiles it, and retrieves the service instance for testing.

Execution Table

Step	Action	Input	Output	Notes
1	Call Test.createTestingModule	{ providers: [MyService] }	TestingModuleBuilder instance	Builder created with providers config
2	Call compile()	TestingModuleBuilder instance	TestingModule instance	Module compiled, dependencies resolved
3	Call get(MyService)	TestingModule instance	MyService instance	Service instance retrieved for tests
4	Use service in tests	MyService instance	Test results	Service methods can be called
5	End	-	-	Testing module setup complete

💡 Testing module setup ends after service instance is retrieved and ready for tests.

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	Final
module	undefined	TestingModuleBuilder	TestingModule	TestingModule	TestingModule
service	undefined	undefined	undefined	MyService instance	MyService instance

Key Moments - 2 Insights

Why do we need to call compile() after createTestingModule()?

compile() finalizes the module by resolving dependencies and preparing instances, as shown in step 2 of the execution_table.

What does module.get(MyService) return?

It returns the instance of MyService created inside the testing module, as shown in step 3 of the execution_table.

Visual Quiz - 3 Questions

Look at the execution_table, what is the output after calling compile()?

A TestingModule instance

B TestingModuleBuilder instance

C MyService instance

D Undefined

💡 Hint

Check step 2 in the execution_table where compile() is called.

At which step is the MyService instance first available?

A Step 1

B Step 3

C Step 2

D Step 4

💡 Hint

Look at when module.get(MyService) is called in the execution_table.

If you forget to call compile(), what will module.get(MyService) return?

A MyService instance

B TestingModuleBuilder instance

C Undefined or error

D Empty object

💡 Hint

Refer to the variable_tracker and the role of compile() in the key_moments.

Concept Snapshot

NestJS Testing Module Setup:
- Use Test.createTestingModule({ providers, controllers }) to start.
- Call compile() to finalize and resolve dependencies.
- Use module.get(Service) to retrieve instances.
- Instances are ready for testing after compile.
- Always compile before getting instances.

Full Transcript

In NestJS, testing modules are created using Test.createTestingModule with configuration for providers and controllers. This returns a TestingModuleBuilder. Calling compile() on this builder finalizes the module by resolving dependencies and preparing instances. After compilation, you can retrieve service instances using module.get(Service). These instances are then used in your tests. Skipping compile() will cause errors or undefined instances. This process ensures your tests run with properly configured and isolated modules.

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