Unit and E2E Testing
Write effective unit tests for individual components and end-to-end tests for complete API flows using Jest and Supertest.
Unit and E2E Testing is a free NestJS Enterprise Backend APIs lesson on CoddyKit — lesson 1 of 6. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the NestJS Enterprise Backend APIs learning path, one of 6 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Why Test Your NestJS App?
Writing tests is crucial for building robust applications. It helps ensure your code works as expected and prevents regressions when you make changes.
- Reliability: Confirms features work.
- Refactoring Confidence: Allows changes without fear.
- Documentation: Tests show how code should be used.
In NestJS, we often focus on two main types: Unit Tests and End-to-End (E2E) Tests.
Unit Testing: Isolated Checks
Unit tests focus on the smallest testable parts of your application, called 'units'. Think of individual functions, methods, or classes (like a NestJS service).
The goal is to test each unit in isolation, meaning without its external dependencies (like databases, external APIs, or other services). This makes them fast and easy to pinpoint issues.
Jest: Your Testing Buddy
NestJS projects come pre-configured with Jest, a popular JavaScript testing framework. Jest makes writing and running tests straightforward.
You can run your tests directly from your terminal. Let's see a simple example of a Jest test file.
// my-first.spec.ts
describe('Basic Jest Test', () => {
it('should be true', () => {
// The 'expect' function is how you assert things
expect(true).toBe(true);
});
it('should compare numbers', () => {
expect(5).toBeGreaterThan(3);
});
});Unit Testing a NestJS Service
Let's test a simple NestJS service that handles user data. We'll use @nestjs/testing to create a test module, allowing us to inject our service and its mocked dependencies.
Here, we're testing a UsersService's findAll() method, ensuring it returns users and interacts correctly with its (mocked) repository.
// users.service.spec.ts
import { Test } from '@nestjs/testing';
import { UsersService } from './users.service'; // Assume this file exists
// Mock the repository to isolate the service
const mockUsersRepo = {
find: jest.fn(() => Promise.resolve([{ id: 1, name: 'Alice' }])),
};
describe('UsersService Unit Test', () => {
let service: UsersService;
beforeEach(async () => {
const module = await Test.createTestingModule({
providers: [
UsersService,
{ provide: 'UserRepository', useValue: mockUsersRepo },
],
}).compile();
service = module.get<UsersService>(UsersService);
});
it('should return an array of users', async () => {
const result = await service.findAll();
expect(result).toEqual([{ id: 1, name: 'Alice' }]);
expect(mockUsersRepo.find).toHaveBeenCalledTimes(1);
});
});Mocking for Isolation
Mocking is key in unit testing. It means replacing real dependencies (like a database connection or another service) with controlled, fake versions.
This ensures your test only focuses on the unit itself, not on potential issues in its dependencies. In Jest, jest.fn() creates a mock function you can track and control.
// mock-dependency.spec.ts
const mockLogger = {
log: jest.fn(), // A mock function
};
class MyComponent {
constructor(private logger: typeof mockLogger) {}
doSomething(message: string) {
this.logger.log(`Doing: ${message}`);
return `Done: ${message}`;
}
}
describe('Mocking Example', () => {
it('should call the mock logger', () => {
const component = new MyComponent(mockLogger);
const result = component.doSomething('Hello');
expect(result).toBe('Done: Hello');
// Verify the mock function was called with specific arguments
expect(mockLogger.log).toHaveBeenCalledWith('Doing: Hello');
});
});End-to-End (E2E) Testing: Full Flow
While unit tests check small parts, End-to-End (E2E) tests verify the entire system, from the user's perspective. For an API, this means making actual HTTP requests to your running NestJS application.
E2E tests ensure that all components (controllers, services, database, authentication, etc.) work together correctly as a complete flow. They are slower but provide high confidence.
Supertest: API Request Helper
For E2E tests, we need a way to make HTTP requests to our NestJS application. Supertest is a popular library that makes this easy, allowing you to simulate requests and assert on responses.
NestJS's testing utilities integrate well with Supertest, enabling you to spin up a test instance of your application.
E2E Testing a NestJS Controller
Let's write an E2E test for a simple AppController that exposes a GET / endpoint. We'll use @nestjs/testing to create an application instance and supertest to send an HTTP request to it.
This test simulates a client hitting your API and checks the HTTP status code and response body.
// app.e2e-spec.ts
import * as request from 'supertest';
import { Test } from '@nestjs/testing';
import { INestApplication } from '@nestjs/common';
import { AppModule } from './../src/app.module'; // Your main app module
describe('AppController (e2e)', () => {
let app: INestApplication;
beforeAll(async () => {
const moduleFixture = await Test.createTestingModule({
imports: [AppModule],
}).compile();
app = moduleFixture.createNestApplication();
await app.init(); // Initialize the NestJS app
});
it('/ (GET) should return "Hello World!"', () => {
return request(app.getHttpServer())
.get('/')
.expect(200)
.expect('Hello World!');
});
afterAll(async () => {
await app.close(); // Close the app after tests
});
});E2E & Database Management
When running E2E tests, especially those involving databases, you need a strategy for managing test data. This often includes:
- Dedicated Test Database: Use a separate database instance for tests.
- Seeding Data: Populate the database with known test data before each test or test suite.
- Cleaning Up: Clear or reset the database after tests to ensure isolation and prevent test pollution.
This ensures tests are consistent and repeatable.
Quick Check: Testing Types
Consider the following statements about Unit and End-to-End (E2E) testing. Which one is TRUE?
Recap: Unit & E2E Testing
Great job! You've explored the foundations of testing in NestJS:
- Unit Tests: Focus on isolated components (services, functions), using Jest for fast, precise checks. Mocking is key here.
- E2E Tests: Validate the entire application flow, from request to response, often using Supertest to simulate HTTP calls. They are slower but provide high confidence in the integrated system.
Both testing types are vital for building reliable and maintainable NestJS applications.
Frequently asked questions
Is the “Unit and E2E Testing” lesson free?
Yes — the full text of “Unit and E2E Testing” is free to read here on the web, and the NestJS Enterprise Backend APIs course includes 6 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the NestJS Enterprise Backend APIs course, upgrade to CoddyKit PRO.
What will I learn in “Unit and E2E Testing”?
Write effective unit tests for individual components and end-to-end tests for complete API flows using Jest and Supertest. You practise NestJS Enterprise Backend APIs with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.
Do I need any experience to start NestJS Enterprise Backend APIs?
No prior experience is required. NestJS Enterprise Backend APIs on CoddyKit is structured for beginners through advanced learners; this is — lesson 1 of 6, so you can start here or from the beginning and move at your own pace.
How long does the “Unit and E2E Testing” lesson take?
Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.
Can I write and run code in this NestJS Enterprise Backend APIs lesson?
Yes. Every NestJS Enterprise Backend APIs lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.