Unit testing controllers in NestJS - Performance & Optimization

Performance: Unit testing controllers

LOW IMPACT

Unit testing controllers affects development speed and feedback loop but does not impact runtime page load or rendering performance.

Testing controller logic efficiently without unnecessary dependencies

NestJS

describe('UserController', () => {
  let controller: UserController;
  const mockUserService = { getUser: jest.fn().mockResolvedValue({ id: '1', name: 'Test' }) };
  beforeEach(() => {
    controller = new UserController(mockUserService as any);
  });
  it('should return user data', async () => {
    const result = await controller.getUser('1');
    expect(result).toEqual({ id: '1', name: 'Test' });
  });
});

Mocks dependencies to isolate controller logic, making tests faster and simpler.

📈 Performance Gainruns tests instantly, reduces memory and CPU usage

Testing controller logic efficiently without unnecessary dependencies

NestJS

describe('UserController', () => {
  let controller: UserController;
  beforeEach(async () => {
    const module = await Test.createTestingModule({
      controllers: [UserController],
      imports: [DatabaseModule, HttpModule],
      providers: [UserService],
    }).compile();
    controller = module.get<UserController>(UserController);
  });
  it('should return user data', async () => {
    const result = await controller.getUser('1');
    expect(result).toBeDefined();
  });
});

This loads full modules and dependencies, making tests slower and more resource-intensive.

📉 Performance Costblocks test execution longer, increases memory usage

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Full module loading in tests	0	0	0	[X] Bad
Mocking dependencies in tests	0	0	0	[OK] Good

Rendering Pipeline

Unit testing controllers runs outside the browser and does not affect the browser rendering pipeline.

⚠️ Bottlenecknone

Optimization Tips

1Mock dependencies to isolate controller logic and speed up tests.

2Avoid loading full modules in unit tests to reduce resource use.

3Unit tests do not affect browser rendering or Core Web Vitals.

Performance Quiz - 3 Questions

What is the main performance benefit of mocking dependencies in unit tests for controllers?

A Tests improve browser rendering speed

B Tests run faster by avoiding loading full modules

C Tests reduce network requests during page load

D Tests decrease CSS paint times

DevTools: Performance

How to check: Run tests with a profiler or watch test execution time in terminal or IDE test runner.

What to look for: Look for test duration and CPU usage to identify slow tests caused by heavy dependencies.

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