Imported from shipshitdev/skills (skills/nestjs-testing-expert/SKILL.md) via skills.sh. Install upstream with npx skills add shipshitdev/skills --skill nestjs-testing-expert. Copyright stays with the author.

NestJS Testing Expert

Build reliable Jest suites for NestJS modules, services, controllers, and HTTP endpoints.

Scope

This skill owns NestJS mechanics: testing modules, provider wiring, and request-level end-to-end flows. For framework-agnostic questions — which level a behavior belongs at, what a coverage number means, how to design a test that survives refactoring, how to kill a flake — use testing-expert.

When to Use

• Writing unit, integration, or end-to-end tests for a NestJS application
• Setting up a testing module, overriding providers, or faking a database
• Testing controllers, guards, interceptors, or pipes
• Exercising HTTP endpoints against a booted Nest application

Levels in a Nest Application

• Unit — a service or provider in isolation, collaborators replaced with test doubles. No module graph beyond the providers under test.
• Integration — a module compiled with its real providers, external systems faked. Proves the wiring and the contracts between layers.
• End-to-end — a booted application driven over HTTP. Proves the full request path: pipes, guards, controller, service, persistence.

Service Specs

Compile a testing module with the subject real and every collaborator supplied explicitly. Injection tokens come from whatever integration provides them — an ORM's token helper, a class reference, or a custom token constant.

describe('UsersService', () => {
  let service: UsersService;
  let repository: jest.Mocked<UsersRepository>;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      providers: [
        UsersService,
        { provide: UsersRepository, useValue: createUsersRepositoryMock() },
      ],
    }).compile();

    service = module.get(UsersService);
    repository = module.get(UsersRepository);
  });

  afterEach(() => {
    jest.resetAllMocks();
  });

  it('returns only the active users of the requested organization', async () => {
    const expected = [{ id: '1', organization: 'org1' }];
    repository.find.mockResolvedValue(expected);

    const result = await service.findAll('org1');

    expect(result).toEqual(expected);
    expect(repository.find).toHaveBeenCalledWith({
      organization: 'org1',
      isDeleted: false,
    });
  });
});

Assert on the arguments the collaborator received when they encode a business rule — the isDeleted: false filter above is the behavior, not an implementation detail.

Controller Specs

Register the controller, stub its service, and verify only the controller's own job: argument extraction, delegation, and response shaping. Business rules belong to the service spec.

describe('UsersController', () => {
  let controller: UsersController;
  let service: jest.Mocked<UsersService>;

  beforeEach(async () => {
    const module: TestingModule = await Test.createTestingModule({
      controllers: [UsersController],
      providers: [{ provide: UsersService, useValue: createUsersServiceMock() }],
    }).compile();

    controller = module.get(UsersController);
    service = module.get(UsersService);
  });

  it('passes the organization from the request through to the service', async () => {
    const expected = [{ id: '1', email: 'test@example.com' }];
    service.findAll.mockResolvedValue(expected);

    const result = await controller.findAll('org1');

    expect(result).toEqual(expected);
    expect(service.findAll).toHaveBeenCalledWith('org1');
  });
});

Overriding Providers

Compile the real module and swap only what must not run for real. overrideProvider and overrideGuard keep the rest of the graph authentic, so the test still proves the wiring.

const module: TestingModule = await Test.createTestingModule({
  imports: [UsersModule],
})
  .overrideProvider(MailerService)
  .useValue(mailerMock)
  .overrideGuard(AuthGuard)
  .useValue({ canActivate: () => true })
  .compile();

Override the guard only in tests about something else. Authorization itself deserves end-to-end tests that run the real guard.

Integration Tests

Boot the application, keep persistence real against a disposable database, and reset state between tests.

describe('Users (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();
  });

  beforeEach(async () => {
    await resetDatabase();
  });

  it('persists a created user and returns it on read', async () => {
    const created = await request(app.getHttpServer())
      .post('/api/users')
      .send({ email: 'test@example.com', name: 'Test' })
      .expect(201);

    const read = await request(app.getHttpServer())
      .get(`/api/users/${created.body.id}`)
      .expect(200);

    expect(read.body.email).toBe('test@example.com');
  });
});

Close the application in afterAll. A leaked Nest application holds its connection pool open and hangs the Jest run.

End-to-End Tests

Same boot, driven as a real client — authentication included.

describe('Users API (e2e)', () => {
  let app: INestApplication;
  let authToken: string;

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

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

    authToken = await signInTestUser(app);
  });

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

  it('rejects an unauthenticated list request', () =>
    request(app.getHttpServer()).get('/api/users').expect(401));

  it('returns the caller organization users when authenticated', () =>
    request(app.getHttpServer())
      .get('/api/users')
      .set('Authorization', `Bearer ${authToken}`)
      .expect(200)
      .expect((res) => {
        expect(Array.isArray(res.body)).toBe(true);
      }));
});

A shared signInTestUser helper keeps the token flow in one place and out of every spec.

Database Strategy

Reset between tests by truncating or rolling back a transaction rather than recreating the schema — schema rebuilds dominate suite runtime.

Tips

• Give each spec its own module compilation; a shared one leaks state between tests.
• Reset mocks in afterEach so a stub set in one test cannot satisfy the next.
• Boot the application once per describe block and reset data per test — booting per test is the usual cause of a slow Nest suite.
• Keep tests deterministic: freeze the clock and pin the timezone rather than asserting on the real one.

Checklist

• Clear arrange/act/assert structure
• Subject real, collaborators explicit — never the reverse
• Error and unauthorized paths covered, not just the happy path
• Application closed and mocks reset in teardown
• Fast to run