Test isolation: tại sao quan trọng và cách đảm bảo?

Test isolation đảm bảo tests không phụ thuộc thứ tự — dùng jest.resetAllMocks() trong afterEach, transaction rollback cho DB tests, tránh global state mutation.

• Test isolation đảm bảo mỗi test chạy độc lập — không phụ thuộc vào thứ tự, không share state với tests khác.
• Tại sao quan trọng: test order-dependent là flaky (pass individually, fail khi chạy cùng); shared state làm debug khó (failing test có thể do test khác gây ra).
• Kỹ thuật isolation: Module mocks: jest.resetModules() trong beforeEach để reset module registry — cần thiết khi mock modules có side effects; Environment reset: jest.clearAllMocks() (reset call count), jest.resetAllMocks() (clear implementations cũng), jest.restoreAllMocks() (restore spies); Database isolation: transaction rollback sau mỗi test (Prisma: prisma.$transaction với rollback trong afterEach), hoặc truncate tables; Test-scoped servers: mỗi test suite tạo Express instance riêng thay vì shared global app. --runInBand flag chạy tests sequential (không parallel) để debug isolation issues — nếu test pass với --runInBand nhưng fail parallel, đó là shared state bug.
• Jest worker isolation: mỗi file chạy trong separate Node.js worker — file-level isolation tốt; within file mới cần manual isolation.
• Lưu ý: global variable mutations trong test không cleanup → leak sang test tiếp theo.

Test isolation ensures tests don't depend on order — use jest.resetAllMocks() in afterEach, transaction rollback for DB tests, avoid global state mutations.

• Test isolation ensures each test runs independently — no ordering dependencies, no shared state.
• Why it matters: order-dependent tests are flaky (pass individually, fail together); shared state makes debugging hard (a failing test may be caused by a different test).
• Isolation techniques: Module mocks: jest.resetModules() in beforeEach to reset the module registry — needed when mocked modules have side effects; Environment reset: jest.clearAllMocks() (reset call counts), jest.resetAllMocks() (also clears implementations), jest.restoreAllMocks() (restores spies); Database isolation: transaction rollback after each test (Prisma: prisma.$transaction with rollback in afterEach), or truncate tables; Test-scoped servers: each test suite creates its own Express instance instead of a shared global app.
• The --runInBand flag runs tests sequentially to debug isolation issues — if tests pass with --runInBand but fail in parallel, it's a shared state bug.
• Jest worker isolation: each file runs in a separate Node.js worker — good file-level isolation; within a file, manual isolation is still needed.

Pitfall: global variable mutations in a test that aren't cleaned up leak into subsequent tests.