Integration testing in NestJS - Mini Project: Build & Apply

Integration Testing with NestJS

📖 Scenario: You are building a simple NestJS service that manages a list of books. You want to make sure your service works correctly by writing integration tests that check the full flow of your application.

🎯 Goal: Build a NestJS integration test that sets up the testing module, injects the service, and verifies that the service returns the expected list of books.

📋 What You'll Learn

Create a basic array of book objects as initial data

Set up a NestJS testing module with the service

Write an integration test that calls the service method

Assert the returned data matches the initial book list

💡 Why This Matters

🌍 Real World

Integration tests help ensure that different parts of your NestJS application work together correctly before deploying to users.

💼 Career

Writing integration tests is a key skill for backend developers to maintain reliable and bug-free applications in professional environments.

1

DATA SETUP: Create initial book data

Create a constant array called books with these exact objects: { id: 1, title: 'NestJS Basics' } and { id: 2, title: 'Testing with Jest' }.

NestJS

// Create a constant array called books with two book objects
// Your code here

Hint

Use const books = [ ... ] with two objects inside.

2

CONFIGURATION: Set up NestJS testing module

Create a variable called moduleRef and assign it the result of await Test.createTestingModule({ providers: [BooksService] }).compile() inside an async function.

NestJS

const books = [
  { id: 1, title: 'NestJS Basics' },
  { id: 2, title: 'Testing with Jest' }
];

// Create moduleRef with Test.createTestingModule and compile
// Your code here

Hint

Use await Test.createTestingModule({ providers: [BooksService] }).compile() and assign it to moduleRef.

3

CORE LOGIC: Retrieve service and call method

Create a variable called booksService and assign it the result of moduleRef.get(BooksService). Then call booksService.findAll() and assign the result to result.

NestJS

const books = [
  { id: 1, title: 'NestJS Basics' },
  { id: 2, title: 'Testing with Jest' }
];

const moduleRef = await Test.createTestingModule({
  providers: [BooksService],
}).compile();

// Get booksService from moduleRef and call findAll
// Your code here

Hint

Use moduleRef.get(BooksService) to get the service, then call findAll().

4

COMPLETION: Assert the returned books match initial data

Write an assertion using expect(result).toEqual(books) to check that the service returns the correct book list.

NestJS

const books = [
  { id: 1, title: 'NestJS Basics' },
  { id: 2, title: 'Testing with Jest' }
];

const moduleRef = await Test.createTestingModule({
  providers: [BooksService],
}).compile();

const booksService = moduleRef.get(BooksService);
const result = booksService.findAll();

// Assert that result equals books
// Your code here

Hint

Use expect(result).toEqual(books) to check the returned data.

Practice

1. What is the main purpose of integration testing in NestJS?

easy

A. To test individual functions in isolation

B. To check how different parts of the application work together

C. To measure application performance under load

D. To check the syntax of TypeScript code

Solution

1. Step 1: Understand integration testing goal

   Integration testing focuses on verifying that multiple parts of the app interact correctly.

2. Step 2: Differentiate from other test types

   Unit tests check single functions; performance tests measure speed; syntax checks are done by compilers.

3. Final Answer:

   To check how different parts of the application work together -> Option B

4. Quick Check:

   Integration testing = check parts working together [OK]

Hint: Integration tests check combined parts, not single functions [OK]

Common Mistakes:

• Confusing integration with unit testing
• Thinking integration tests check performance
• Believing integration tests check code syntax

2. Which of the following is the correct way to initialize a NestJS application for integration testing?

easy

A. const app = await Test.createTestingModule({ imports: [AppModule] }).start();

B. const app = new AppModule(); app.start();

C. const app = await Test.createTestingModule({ controllers: [AppController] }).init();

D. const app = (await Test.createTestingModule({ imports: [AppModule] }).compile()).createNestApplication(); await app.init();

Solution

1. Step 1: Recognize correct testing module setup

   Use Test.createTestingModule with imports and then compile() to build the module.

2. Step 2: Create app instance and initialize

   After compiling, create the app with createNestApplication() then call app.init() to start the NestJS app for testing.

3. Final Answer:

   const app = (await Test.createTestingModule({ imports: [AppModule] }).compile()).createNestApplication(); await app.init(); -> Option D

4. Quick Check:

   compile() -> createNestApplication() -> init() [OK]

Hint: Always compile() -> createNestApplication() -> init() [OK]

Common Mistakes:

• Calling init() directly on createTestingModule
• Using start() method which does not exist
• Not compiling the testing module before init

3. Given this test snippet, what will be the HTTP status code returned?

const response = await request(app.getHttpServer()).get('/users');
expect(response.status).toBe(200);

medium

A. 200

B. 401

C. 500

D. 404

Solution

1. Step 1: Understand the test request

   The test sends a GET request to '/users' route using supertest on the initialized app.

2. Step 2: Check expected status code

   The test expects the response status to be 200, meaning success and route exists.

3. Final Answer:

   200 -> Option A

4. Quick Check:

   GET /users returns 200 success [OK]

Hint: 200 means route responded successfully [OK]

Common Mistakes:

• Confusing 404 (not found) with 200
• Assuming 500 error without error in code
• Mixing authentication errors with route success

4. You wrote this integration test but it fails with "Cannot read property 'getHttpServer' of undefined". What is the likely cause?

let app;
beforeAll(async () => {
  app = Test.createTestingModule({ imports: [AppModule] });
  await app.init();
});

medium

A. Forgot to call compile() before init()

B. Used beforeAll instead of beforeEach

C. Did not import supertest library

D. AppModule is missing from imports

Solution

1. Step 1: Identify missing compile() call

   Test.createTestingModule returns a builder; you must call compile() to get the module instance.

2. Step 2: Understand why getHttpServer is undefined

   Without compile(), app is not a proper module instance, so app.getHttpServer is undefined causing the error.

3. Final Answer:

   Forgot to call compile() before init() -> Option A

4. Quick Check:

   compile() needed before init() [OK]

Hint: Always compile() before init() to get app instance [OK]

Common Mistakes:

• Skipping compile() step
• Confusing beforeAll with beforeEach for setup
• Assuming missing imports cause this error

5. You want to test a POST route that creates a user. Which approach correctly integrates NestJS and supertest for this integration test?

hard

A. Use Jest mocks to simulate the database and skip app initialization

B. Directly call the controller method without app initialization and check returned value

C. Use Test.createTestingModule with AppModule, compile and init app, then use supertest to POST JSON to '/users' endpoint

D. Write a unit test for the service method that creates users

Solution

1. Step 1: Setup full app for integration test

   Integration tests require the full app context, so createTestingModule with AppModule, compile, and init the app.

2. Step 2: Use supertest to send POST request

   Use supertest to send a POST request with JSON data to the '/users' route to test real HTTP behavior.

3. Final Answer:

   Use Test.createTestingModule with AppModule, compile and init app, then use supertest to POST JSON to '/users' endpoint -> Option C

4. Quick Check:

   Integration test = full app + supertest POST [OK]

Hint: Integration tests use full app and supertest for HTTP calls [OK]

Common Mistakes:

• Testing controller methods directly without app context
• Using mocks instead of real modules for integration tests
• Confusing unit tests with integration tests