Cách viết unit tests cho Node.js API với Jest?

• Test pyramid cho Node.js API: unit tests (service/utils logic) → integration tests (controller + real DB) → e2e tests (full HTTP).
• Controller testing với supertest: const res = await request(app).post('/users').send({ email: 'test@test.com' }); expect(res.status).toBe(201) — không cần start server, supertest inject request trực tiếp.
• Service testing: mock DB layer với jest.mock('../db') → (db.findUser as jest.Mock).mockResolvedValue({ id: 1 }) — test business logic độc lập với DB.
• Middleware testing: gọi middleware với mock req, res, next objects — verify next() được gọi hay res.status() được set.
• Integration tests: dùng test DB riêng (TEST_DATABASE_URL), chạy migrations trước test suite, truncate tables giữa tests (afterEach).
• Mock strategies: jest.mock() cho modules, jest.spyOn() cho methods, jest.useFakeTimers() cho time-dependent code.
• Test database setup: globalSetup chạy migration một lần, beforeEach truncate data, afterAll disconnect.
• Coverage: jest --coverage + coverageThreshold: { global: { lines: 80 } } enforce minimum.
• Lưu ý: không isolate tests → test order dependent failures; mock tất cả external calls (HTTP, email) để tests deterministic.

• Test pyramid for a Node.js API: unit tests (service/utils logic) → integration tests (controller + real DB) → E2E tests (full HTTP).
• Controller testing with supertest: const res = await request(app).post('/users').send({ email: 'test@test.com' }); expect(res.status).toBe(201) — no need to start a real server; supertest injects requests directly.
• Service testing: mock the DB layer with jest.mock('../db') → (db.findUser as jest.Mock).mockResolvedValue({ id: 1 }) — test business logic independently of the DB.
• Middleware testing: call middleware with mock req, res, and next objects — verify next() was called or res.status() was set correctly.
• Integration tests: use a separate test DB (TEST_DATABASE_URL), run migrations before the test suite, and truncate tables between tests (afterEach).
• Mock strategies: jest.mock() for modules, jest.spyOn() for methods, jest.useFakeTimers() for time-dependent code.
• Test database setup: globalSetup runs migrations once, beforeEach truncates data, afterAll disconnects.
• Coverage: jest --coverage + coverageThreshold: { global: { lines: 80 } } enforces a minimum.

Pitfall: not isolating tests → test-order-dependent failures; mock all external calls (HTTP, email) to keep tests deterministic.