testing-nestjs.md

---

title: NestJS Testing Patterns impact: HIGH impactDescription: Ensures reliable backend through proper test structure tags: testing, nestjs, jest, mocking

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NestJS Testing Patterns

Impact: HIGH

Unit Tests (Services)

Mock all dependencies. Test business logic in isolation.

describe('CatalogService', () => {
  let service: CatalogService;
  let repo: jest.Mocked<CatalogRepository>;
  let cache: jest.Mocked<CacheService>;

  beforeEach(async () => {
    const module = await Test.createTestingModule({
      providers: [
        CatalogService,
        { provide: CatalogRepository, useValue: createMock<CatalogRepository>() },
        { provide: CacheService, useValue: createMock<CacheService>() },
      ],
    }).compile();

    service = module.get(CatalogService);
    repo = module.get(CatalogRepository);
    cache = module.get(CacheService);
  });

  it('uses cache for catalog queries', async () => {
    const items = [{ id: '1', title: 'Course' }];
    cache.getOrSet.mockImplementation((_key, _ttl, factory) => factory());
    repo.findPublished.mockResolvedValue(items);

    const result = await service.getPublishedItems({});
    expect(cache.getOrSet).toHaveBeenCalled();
    expect(result).toEqual(items);
  });
});

Integration Tests (Controllers)

Use Supertest to test HTTP endpoints with real NestJS app.

describe('CatalogController (e2e)', () => {
  let app: INestApplication;

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

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

  it('GET /catalog returns published items', () => {
    return request(app.getHttpServer())
      .get('/catalog')
      .expect(200)
      .expect(res => {
        expect(Array.isArray(res.body.items)).toBe(true);
      });
  });

  afterAll(() => app.close());
});

Rules

• Unit test services with mocked dependencies
• Integration test controllers with real HTTP requests
• Never test private methods — test through the public API
• Use factories for test data (createMockItem(), createMockSeller())