testing-strategy

# Testing Strategy

Comprehensive testing guidance for test architecture, coverage strategy, and test design.

## Test Pyramid

```
                    ▲
                   ╱ ╲   E2E Tests
                  ╱───╲  (5-10% of tests)
                 ╱     ╲  Real browser, full stack
                ╱───────╲
               ╱         ╲  Integration Tests
              ╱───────────╲  (15-25% of tests)
             ╱             ╲  Multiple units, real dependencies
            ╱───────────────╲
           ╱                 ╲  Unit Tests
          ╱───────────────────╲  (65-80% of tests)
         ╱                     ╲  Single units, isolated, fast
```

| Level | Speed | Cost | Confidence | Isolation |
|-------|-------|------|------------|-----------|
| **Unit** | ~1ms | Low | Narrow | High |
| **Integration** | ~100ms | Medium | Medium | Medium |
| **E2E** | ~1-10s | High | High | Low |

### When to Deviate

Invert the pyramid when:
- **Legacy code without unit tests** → Start with E2E for safety net, add units as you refactor
- **Highly integrated systems** → More integration tests, fewer isolated units
- **Critical user journeys** → Extra E2E coverage for checkout, auth, payments
- **UI-heavy apps** → More component/visual tests, fewer traditional units

## Decision Framework

**What test type should I write?**

```
Is it a pure function or utility?
  └─ YES → Unit test
  └─ NO ↓

Does it involve UI rendering?
  └─ YES → Component test (RTL/Vue Test Utils)
  └─ NO ↓

Does it cross system boundaries (DB, API, services)?
  └─ YES → Integration test
  └─ NO ↓

Is it a critical user flow spanning multiple pages?
  └─ YES → E2E test (Playwright/Cypress)
  └─ NO → Unit or integration test
```

**What NOT to test:**

- Framework internals (React hooks work, Next.js routing works)
- Third-party libraries (axios sends requests correctly)
- Implementation details (which internal method was called)
- Styling (unless visual regression is set up)
- Trivial code (getters, setters,