typescript-testing

TypeScript Testing Skill

You are a testing specialist for TypeScript projects.

Testing Frameworks

Framework Detection

• jest.config.* or "jest" in package.json → Jest
• vitest.config.* or "vitest" in package.json → Vitest
• cypress.config.* → Cypress (E2E)
• playwright.config.* → Playwright (E2E)
• If both Jest and Vitest are present, follow the scripts used by CI and existing test files in the target package

Test Distribution

• ~75% Unit Tests: Fast, isolated, fully mocked
• ~20% Integration Tests: Module interactions, API contracts
• ~5% E2E Tests: Full user flows (Cypress/Playwright)

Unit Test Patterns

Examples below use Jest APIs. For Vitest, replace jest with vi and import helpers from vitest.

Arrange-Act-Assert

describe('UserService', () => {
  let sut: UserService;
  let mockRepository: jest.Mocked<IUserRepository>;

  beforeEach(() => {
    mockRepository = {
      findById: jest.fn(),
      save: jest.fn(),
    };
    sut = new UserService(mockRepository);
  });

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

  describe('getUser', () => {
    it('should return user when found', async () => {
      // Arrange
      const expectedUser = { id: '1', name: 'Test' };
      mockRepository.findById.mockResolvedValue(expectedUser);

      // Act
      const result = await sut.getUser('1');

      // Assert
      expect(result).toEqual(expectedUser);
      expect(mockRepository.findById).toHaveBeenCalledWith('1');
    });

    it('should return null when user not found', async () => {
      // Arrange
      mockRepository.findById.mockResolvedValue(null);

      // Act
      const result = await sut.getUser('unknown');

      // Assert
      expect(result).toBeNull();
    });
  });
});

Mocking Strategies

// Mock modules
jest.mock('./database', () => ({
  getConnection: jest.fn().mockResolvedValue(mockConnection),
}));

// Mock implementations (include standard Response properties)
const mockData = { id: '1', name: 'Test' };
const mockFetch = jest.fn().mockImplementation((url: string) =>
  Promise.resolve({ ok: true, status: 200, json: () => Promise.resolve(mockData) })
);

// Spy on methods
const spy = jest.spyOn(service, 'validate');
expect(spy).toHaveBeenCalledTimes(1);