Testability

Testability Quality Dimension

Every component must be testable in isolation. If it's hard to test, the design is wrong.

Checklist

Mark each item ✅ Pass / ❌ Fail / ⚠️ N/A:

Dependency Injection

• Dependencies passed in (not imported directly)
• Mocking is straightforward
• No static method abuse
• No global state in business logic

Pure Functions

• Business logic is pure (same input = same output)
• Side effects isolated to boundaries
• No hidden dependencies
• Deterministic test results

Seams

• Integration points clearly defined
• Easy to inject test doubles
• Unit tests don't need full system
• Boundary interfaces are thin

Test Coverage

• All new functions have tests
• Edge cases covered (null, empty, error)
• Happy path AND error path tested
• No test-free zones in business logic

Test Quality

• Tests are readable (describe intent)
• Tests are fast (no sleep, no real I/O)
• Tests are independent (no shared state)
• Tests are maintainable (no magic numbers)

When to Check

• Design-time: verify testability in the design
• Pre-ship: verify all functions have tests

Fix if Failing

Refactor to use dependency injection. Extract side effects. Add tests. Use test doubles at boundaries.