Unit testing controllers in NestJS - Mini Project: Build & Apply

Unit Testing Controllers in NestJS

📖 Scenario: You are building a simple NestJS application that manages books. You want to make sure your controller works correctly by writing unit tests for it.

🎯 Goal: Create a unit test for a NestJS controller called BooksController that has a method findAll returning a list of books.

📋 What You'll Learn

Create a BooksController class with a findAll method returning an array of book objects

Create a unit test file for BooksController

Set up a testing module with BooksController in the test

Write a test case that checks findAll returns the expected books array

💡 Why This Matters

🌍 Real World

Unit testing controllers ensures your API endpoints behave as expected before deploying your NestJS application.

💼 Career

Writing unit tests for controllers is a key skill for backend developers working with NestJS to maintain reliable and maintainable code.

1

Create the BooksController with findAll method

Create a class called BooksController with a method findAll that returns this exact array: [{ id: 1, title: '1984' }, { id: 2, title: 'Brave New World' }].

NestJS

# Your code here

Hint

Define a class with a method that returns the exact array of book objects.

2

Set up the testing module with BooksController

In a test file, import Test and TestingModule from @nestjs/testing. Create a variable booksController. Use beforeEach to create a testing module with BooksController as a controller and assign the controller instance to booksController.

NestJS

export class BooksController {
  findAll() {
    return [
      { id: 1, title: '1984' },
      { id: 2, title: 'Brave New World' }
    ];
  }
}

// Your test setup here

Hint

Use NestJS testing utilities to create a module and get the controller instance.

3

Write a test case for findAll method

Inside the describe block, add a test case with it named 'should return an array of books'. Inside it, call booksController.findAll() and expect the result to equal the exact array [{ id: 1, title: '1984' }, { id: 2, title: 'Brave New World' }].

NestJS

import { Test, TestingModule } from '@nestjs/testing';

export class BooksController {
  findAll() {
    return [
      { id: 1, title: '1984' },
      { id: 2, title: 'Brave New World' }
    ];
  }
}

describe('BooksController', () => {
  let booksController: BooksController;

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

    booksController = module.get(BooksController);
  });

  // Add your test case below
  
  // Your code here
});

Hint

Write a test that calls findAll and checks the returned array matches exactly.

4

Complete the test file with proper imports and structure

Add the import statement for describe, it, and expect from @jest/globals at the top. Ensure the test file exports nothing and contains the full test suite for BooksController with the test case for findAll.

NestJS

// Add import for describe, it, expect from '@jest/globals'

import { Test, TestingModule } from '@nestjs/testing';

export class BooksController {
  findAll() {
    return [
      { id: 1, title: '1984' },
      { id: 2, title: 'Brave New World' }
    ];
  }
}

// Your test suite below

Hint

Jest globals like describe, it, and expect should be imported explicitly.

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