Unit testing controllers in NestJS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to import the testing module for a NestJS controller test.

NestJS

import { Test, TestingModule } from '@nestjs/[1]';

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

Acore

Bcommon

Ctesting

Dplatform-express

Attempts:

3 left

💡 Hint

Common Mistakes

Importing from '@nestjs/common' instead of '@nestjs/testing'.

Using '@nestjs/core' which is for core framework features.

In NestJS, the testing utilities are imported from '@nestjs/testing'. This module helps create a testing environment for controllers and services.

2fill in blank

medium

Complete the code to create a testing module for the controller named 'CatsController'.

NestJS

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

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

ACatsController

BDogsController

CBirdsController

DFishController

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different controller name than the one being tested.

Forgetting to include the controller in the testing module.

When testing a controller, you include it in the 'controllers' array of the testing module. Here, we want to test 'CatsController'.

3fill in blank

hard

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

NestJS

const controller = moduleRef.[1](CatsController);

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

Aget

BgetController

Cretrieve

Dfetch

Attempts:

3 left

💡 Hint

Common Mistakes

Using non-existent methods like 'getController' or 'fetch'.

Trying to access the controller directly without using 'get'.

To get an instance of a provider or controller from the testing module, use the 'get' method.

4fill in blank

hard

Fill both blanks to mock a service method and test the controller's response.

NestJS

jest.spyOn(service, '[1]').mockImplementation(() => [2]);

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

AfindAll

B[{ id: 1, name: 'Fluffy' }]

Creturn 'Hello'

Dcreate

Attempts:

3 left

💡 Hint

Common Mistakes

Mocking the wrong method name.

Returning a wrong type like a string instead of an array.

We spy on the 'findAll' method of the service and mock it to return a list of cats. This helps test the controller without calling the real service.

5fill in blank

hard

Fill all three blanks to write a test that checks if the controller's getAll method returns the mocked data.

NestJS

it('should return an array of cats', async () => {
  const result = await controller.[1]();
  expect(result).toEqual([2]);
  expect(service.[3]).toHaveBeenCalled();
});

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

AgetAll

B[{ id: 1, name: 'Fluffy' }]

CfindAll

Dcreate

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong method names in controller or service.

Not matching the expected data with the mocked return.

The test calls the controller's 'getAll' method, expects the mocked array, and checks that the service's 'findAll' method was called.

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