Unit testing controllers in NestJS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ component_behavior

intermediate

What is the output of this NestJS controller unit test?

Consider this simple NestJS controller and its unit test. What will the test output be when run?

NestJS

import { Test, TestingModule } from '@nestjs/testing';
import { CatsController } from './cats.controller';
import { CatsService } from './cats.service';

class MockCatsService {
  findAll() {
    return ['cat1', 'cat2'];
  }
}

describe('CatsController', () => {
  let catsController: CatsController;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      controllers: [CatsController],
      providers: [{ provide: CatsService, useClass: MockCatsService }],
    }).compile();

    catsController = module.get<CatsController>(CatsController);
  });

  it('should return an array of cats', () => {
    expect(catsController.findAll()).toEqual(['cat1', 'cat2']);
  });
});

A The test passes because the controller returns ['cat1', 'cat2'] as expected.

B The test fails because the controller returns an empty array.

C The test throws a runtime error because CatsService is not defined.

D The test fails due to a syntax error in the test setup.

💡 Hint

Look at how the service is mocked and injected into the controller.

The test uses a mock service that returns ['cat1', 'cat2']. The controller calls this service method, so the test expects and receives this array, passing successfully.

📝 Syntax

intermediate

Which option correctly mocks a service method in a NestJS controller test?

You want to mock the 'create' method of a service in your NestJS controller unit test. Which option correctly sets up the mock?

NestJS

const mockService = {
  create: jest.fn().mockReturnValue('created'),
};

A const mockService = { create: jest.fn().mockReturnValue('created') };

B const mockService = { create: jest.mockReturnValue('created') };

C const mockService = { create: () => 'created' };

D const mockService = { create: jest.fn().returns('created') };

💡 Hint

Check the Jest syntax for mocking functions.

Option A correctly uses jest.fn() with mockReturnValue to mock the 'create' method. Option A is a simple function but not a Jest mock. Option A uses incorrect syntax. Option A uses 'returns' which is not a Jest method.

🔧 Debug

advanced

Why does this NestJS controller test fail with 'TypeError: Cannot read property'?

Given this test code, why does it throw a TypeError when calling the controller method?

NestJS

describe('UsersController', () => {
  let usersController: UsersController;
  let usersService: UsersService;

  beforeEach(async () => {
    const module = await Test.createTestingModule({
      controllers: [UsersController],
      providers: [UsersService],
    }).compile();

    usersController = module.get<UsersController>(UsersController);
    // usersService is not assigned here
  });

  it('should call service method', () => {
    jest.spyOn(usersService, 'findAll').mockReturnValue(['user1']);
    expect(usersController.findAll()).toEqual(['user1']);
  });
});

A Because jest.spyOn is used incorrectly on a non-function.

B Because the controller method findAll does not exist.

C Because usersService is undefined, so jest.spyOn fails with TypeError.

D Because the test module did not compile correctly.

💡 Hint

Check if usersService is assigned before using it.

The test never assigns usersService from the module, so it remains undefined. Calling jest.spyOn on undefined causes a TypeError.

❓ state_output

advanced

What is the value of 'result' after this NestJS controller test runs?

In this test, what will be the value of 'result' after calling the controller method?

NestJS

class MockService {
  getData() {
    return { count: 5 };
  }
}

let result;

describe('DataController', () => {
  let controller;

  beforeEach(async () => {
    const module = await Test.createTestingModule({
      controllers: [DataController],
      providers: [{ provide: DataService, useClass: MockService }],
    }).compile();

    controller = module.get<DataController>(DataController);
  });

  it('should set result to service data', () => {
    result = controller.getData();
  });
});

A An empty object {}

B undefined

C null

D { count: 5 }

💡 Hint

Look at what the mock service returns and what the controller calls.

The mock service's getData method returns { count: 5 }, and the controller calls this method, so result is set to { count: 5 }.

🧠 Conceptual

expert

Which option best explains why mocking dependencies is important in NestJS controller unit tests?

Why do we mock service dependencies when unit testing NestJS controllers?

A To speed up tests by running the full application stack including databases.

B To isolate the controller logic and avoid testing external service implementations or side effects.

C To ensure the controller uses the real service methods for integration testing.

D To avoid writing any test code for services and focus only on controllers.

💡 Hint

Think about what unit testing means and why isolation matters.

Mocking dependencies isolates the controller's logic so tests focus only on the controller behavior, not on external services or side effects like database calls.

Practice

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

easy

A. To verify the controller handles requests correctly without depending on other parts

B. To test the database connection and queries

C. To check the UI rendering of the application

D. To measure the performance of the entire application

Solution

1. Step 1: Understand the role of a controller

   Controllers handle incoming requests and return responses, so testing them focuses on this behavior.

2. Step 2: Identify what unit testing isolates

   Unit testing isolates the controller from other parts like services or databases to test its logic alone.

3. Final Answer:

   To verify the controller handles requests correctly without depending on other parts -> Option A

4. Quick Check:

   Unit testing controller logic = To verify the controller handles requests correctly without depending on other parts [OK]

Hint: Controllers handle requests; test them isolated from services [OK]

Common Mistakes:

• Confusing unit tests with integration tests
• Testing database or UI in controller unit tests
• Ignoring mocking dependencies

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

easy

A. const module = Test.createModule({ providers: [MyController] });

B. const module = new Module({ controllers: [MyController] });

C. const module = await Test.createTestingModule({ controllers: [MyController] }).compile();

D. const module = await Test.createTestingModule({ services: [MyController] }).compile();

Solution

1. Step 1: Recall NestJS testing syntax

   Use Test.createTestingModule with a config object specifying controllers or providers, then compile it.

2. Step 2: Identify correct property for controllers

   The property to register controllers is 'controllers', not 'services' or 'providers' for controllers.

3. Final Answer:

   const module = await Test.createTestingModule({ controllers: [MyController] }).compile(); -> Option C

4. Quick Check:

   Use controllers array in createTestingModule = const module = await Test.createTestingModule({ controllers: [MyController] }).compile(); [OK]

Hint: Use controllers: [] inside Test.createTestingModule [OK]

Common Mistakes:

• Using 'services' instead of 'controllers' key
• Forgetting to await compile()
• Using new Module() instead of Test.createTestingModule

3. Given this controller method and test snippet, what will result be?

class MyController {
  getHello() {
    return 'Hello World!';
  }
}

// Test snippet
const controller = new MyController();
const result = controller.getHello();

medium

A. null

B. undefined

C. An error because getHello is async

D. 'Hello World!'

Solution

1. Step 1: Analyze the controller method

   The method getHello returns the string 'Hello World!' synchronously.

2. Step 2: Check the test call

   Calling getHello() directly returns the string without errors or async behavior.

3. Final Answer:

   'Hello World!' -> Option D

4. Quick Check:

   Direct method call returns string = 'Hello World!' [OK]

Hint: Direct call returns method's return value immediately [OK]

Common Mistakes:

• Assuming async behavior without async keyword
• Expecting undefined if not mocked
• Confusing method call with property access

4. What is wrong with this unit test for a NestJS controller?

describe('MyController', () => {
  let controller: MyController;

  beforeEach(() => {
    controller = new MyController();
  });

  it('should return greeting', () => {
    expect(controller.getHello()).toBe('Hello World!');
  });
});

medium

A. The controller is not created using Test.createTestingModule

B. The test does not mock the service dependency

C. The test is missing async keyword

D. The expect statement uses toBe instead of toEqual

Solution

1. Step 1: Check controller instantiation

   Creating controller directly is allowed but only if it has no dependencies.

2. Step 2: Identify missing mocking

   If the controller depends on a service, it must be mocked to isolate controller logic.

3. Final Answer:

   The test does not mock the service dependency -> Option B

4. Quick Check:

   Missing mocks for dependencies = The test does not mock the service dependency [OK]

Hint: Always mock services when testing controllers with dependencies [OK]

Common Mistakes:

• Assuming direct instantiation is always wrong
• Forgetting to mock injected services
• Confusing toBe and toEqual for strings

5. You want to unit test a NestJS controller method that calls a service method returning a promise. Which approach correctly mocks the service to test the controller method synchronously?

hard

A. Mock the service method with jest.fn().mockResolvedValue(value) and await the controller method in the test

B. Mock the service method with jest.fn().mockReturnValue(value) without async/await

C. Call the real service method without mocking to test actual behavior

D. Mock the service method with jest.fn().mockImplementation(() => { throw new Error(); })

Solution

1. Step 1: Understand async service method

   The service method returns a promise, so the mock must simulate a resolved promise.

2. Step 2: Use jest.fn().mockResolvedValue

   This creates a mock that returns a resolved promise with the given value, allowing the controller method to await it.

3. Step 3: Await controller method in test

   The test must await the controller method to handle the async call properly.

4. Final Answer:

   Mock the service method with jest.fn().mockResolvedValue(value) and await the controller method in the test -> Option A

5. Quick Check:

   Use mockResolvedValue and await async calls = Mock the service method with jest.fn().mockResolvedValue(value) and await the controller method in the test [OK]

Hint: Use mockResolvedValue for async mocks and await calls [OK]

Common Mistakes:

• Using mockReturnValue for async methods
• Not awaiting async controller methods
• Testing real service instead of mocking