Integration testing in NestJS

Introduction

Integration testing checks if different parts of your NestJS app work well together. It helps find problems between modules or services.

When you want to test how controllers and services work together.

To verify database operations with real or test databases.

When you need to check if middleware and pipes interact correctly.

To ensure external APIs or modules integrate properly.

Before deploying to catch issues between components.

Syntax

NestJS

describe('YourModule Integration Test', () => {
  let app: INestApplication;

  beforeAll(async () => {
    const moduleRef = await Test.createTestingModule({
      imports: [YourModule],
    }).compile();

    app = moduleRef.createNestApplication();
    await app.init();
  });

  afterAll(async () => {
    await app.close();
  });

  it('should test something', () => {
    // your test code here
  });
});

Use Test.createTestingModule to build a test module with real imports.

Call app.init() to start the NestJS app before tests.

Examples

This tests the /cats route to ensure it returns status 200.

NestJS

describe('CatsController Integration', () => {
  let app: INestApplication;

  beforeAll(async () => {
    const moduleRef = await Test.createTestingModule({
      imports: [CatsModule],
    }).compile();

    app = moduleRef.createNestApplication();
    await app.init();
  });

  afterAll(async () => {
    await app.close();
  });

  it('GET /cats returns 200', () => {
    return request(app.getHttpServer())
      .get('/cats')
      .expect(200);
  });
});

This tests the UserService to check if it creates a user correctly.

NestJS

describe('UserService Integration', () => {
  let service: UserService;

  beforeAll(async () => {
    const moduleRef = await Test.createTestingModule({
      imports: [UserModule],
    }).compile();

    service = moduleRef.get<UserService>(UserService);
  });

  it('should create a user', async () => {
    const user = await service.create({ name: 'Anna' });
    expect(user.name).toBe('Anna');
  });
});

Sample Program

This integration test starts the full NestJS app from AppModule. It sends a GET request to the root URL and checks if the response is 'Hello World!'. This shows how to test the app's real HTTP behavior.

NestJS

import { Test, TestingModule } from '@nestjs/testing';
import { INestApplication } from '@nestjs/common';
import * as request from 'supertest';
import { AppModule } from '../src/app.module';

describe('AppController (Integration)', () => {
  let app: INestApplication;

  beforeAll(async () => {
    const moduleFixture: TestingModule = await Test.createTestingModule({
      imports: [AppModule],
    }).compile();

    app = moduleFixture.createNestApplication();
    await app.init();
  });

  afterAll(async () => {
    await app.close();
  });

  it('/ (GET) should return Hello World', () => {
    return request(app.getHttpServer())
      .get('/')
      .expect(200)
      .expect('Hello World!');
  });
});

Important Notes

Use supertest to simulate HTTP requests in integration tests.

Integration tests are slower than unit tests but catch more real issues.

Close the app with app.close() to free resources after tests.

Summary

Integration testing checks how NestJS parts work together.

Use Test.createTestingModule with real modules and app.init().

Test HTTP routes with supertest for realistic behavior.

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