NestJS Unit Testing

Testing a Service in Isolation

Create a testing module that provides the service under test and value stubs for its dependencies. Compile the module, get the service, and assert method results and mock call arguments.

describe('CatsService', () => {
  let service: CatsService;
  const repo = { findAll: jest.fn(), findOne: jest.fn() };

  beforeEach(async () => {
    const moduleRef = await Test.createTestingModule({
      providers: [CatsService, { provide: CatsRepository, useValue: repo }],
    }).compile();
    service = moduleRef.get(CatsService);
    jest.clearAllMocks();
  });

  it('throws when missing', async () => {
    repo.findOne.mockResolvedValue(null);
    await expect(service.findOne(1)).rejects.toBeInstanceOf(NotFoundException);
  });
});

Clear mocks between tests so call counts and return values do not leak across cases.

Useful TestingModule APIs

moduleRef.get(CatsService)
moduleRef.resolve(RequestScopedService) // for REQUEST scope
overrideProvider(X).useValue(mock)

• get() retrieves singleton providers; resolve() for transient/request scoped.
• Override providers after importing a real module with .overrideProvider().
• Prefer asserting thrown Nest exceptions for expected failure paths.
• Keep tests readable with Arrange-Act-Assert structure.

Unit Testing Cheatsheet

Jest + Nest patterns.

Partial Mocks

Only mock methods the test touches. Over-specified mocks become brittle when signatures evolve.

Controlling Time

Use Jest fake timers when testing expiration, retries, or scheduled logic.

Common Mistakes

• Not awaiting async assertions.
• Reusing the same mock object mutations across tests.
• Testing private methods directly.
• Importing the entire AppModule for a single service unit test.

Key Takeaways

• TestingModule isolates providers with mocked dependencies.
• Assert return values, thrown exceptions, and mock interactions.
• Clear mocks between tests.
• Override providers when building from real feature modules.