testing-patterns

# Testing Patterns > Principles for reliable test suites. --- ## 1. Testing Pyramid ``` /\ E2E (Few) / \ Critical flows /----\ / \ Integration (Some) /--------\ API, DB queries / \ /------------\ Unit (Many) Functions, classes ``` --- ## 2. AAA Pattern | Step | Purpose | |------|---------| | **Arrange** | Set up test data | | **Act** | Execute code under test | | **Assert** | Verify outcome | --- ## 3. Test Type Selection ### When to Use Each | Type | Best For | Speed | |------|----------|-------| | **Unit** | Pure functions, logic | Fast (<50ms) | | **Integration** | API, DB, services | Medium | | **E2E** | Critical user flows | Slow | --- ## 4. Unit Test Principles ### Good Unit Tests | Principle | Meaning | |-----------|---------| | Fast | < 100ms each | | Isolated | No external deps | | Repeatable | Same result always | | Self-checking | No manual verification | | Timely | Written with code | ### What to Unit Test | Test | Don't Test | |------|------------| | Business logic | Framework code | | Edge cases | Third-party libs | | Error handling | Simple getters | --- ## 5. Integration Test Principles ### What to Test | Area | Focus | |------|-------| | API endpoints | Request/response | | Database | Queries, transactions | | External services | Contracts | ### Setup/Teardown | Phase | Action | |-------|--------| | Before All | Connect resources | | Before Each | Reset state | | After Each | Clean up | | After All | Disconnect | --- ## 6. Mocking Principles ### When to Mock | Mock | Don't Mock | |------|------------| | External APIs | The code under test | | Database (unit) | Simple dependencies | | Time/random | Pure functions | | Network | In-memory stores | ### Mock Types | Type | Use | |------|-----| | Stub | Return fixed values | | Spy | Track calls | | Mock | Set expectations | | Fake | Simplified implementation | --- ## 7. Test Organization ### Naming | Pattern | Example | |---------|---------| | Should behavior | "should return error when..." | | When condition | "when user not found..." | | Given-when-then | "given X, when Y, then Z" | ### Grouping | Level | Use | |-------|-----| | describe | Group related tests | | it/test | Individual case | | beforeEach | Common setup | --- ## 8. Test Data ### Strategies | Approach | Use | |----------|-----| | Factories | Generate test data | | Fixtures | Predefined datasets | | Builders | Fluent object creation | ### Principles - Use realistic data - Randomize non-essential values (faker) - Share common fixtures - Keep data minimal --- ## 9. Best Practices | Practice | Why | |----------|-----| | One assert per test | Clear failure reason | | Independent tests | No order dependency | | Fast tests | Run frequently | | Descriptive names | Self-documenting | | Clean up | Avoid side effects | --- ## 10. Anti-Patterns | ❌ Don't | ✅ Do | |----------|-------| | Test implementation | Test behavior | | Duplicate test code | Use factories | | Complex test setup | Simplify or split | | Ignore flaky tests | Fix root cause | | Skip cleanup | Reset state | --- > **Remember:** Tests are documentation. If someone can't understand what the code does from the tests, rewrite them.