refactoring

# Refactoring

Refactoring is the third step of TDD. After GREEN, assess if refactoring adds value.

## When to Refactor

- Always assess after green
- Only refactor if it improves the code
- **Commit working code BEFORE refactoring** (critical safety net)

### Commit Before Refactoring - WHY

Having a working baseline before refactoring:
- Allows reverting if refactoring breaks things
- Provides safety net for experimentation
- Makes refactoring less risky
- Shows clear separation in git history

**Workflow:**
1. GREEN: Tests pass
2. COMMIT: Save working code
3. REFACTOR: Improve structure
4. COMMIT: Save refactored code

## Priority Classification

| Priority | Action | Examples |
|----------|--------|----------|
| Critical | Fix now | Mutations, knowledge duplication, >3 levels nesting |
| High | This session | Magic numbers, unclear names, >30 line functions |
| Nice | Later | Minor naming, single-use helpers |
| Skip | Don't change | Already clean code |

## DRY = Knowledge, Not Code

**Abstract when**:
- Same business concept (semantic meaning)
- Would change together if requirements change
- Obvious why grouped together

**Keep separate when**:
- Different concepts that look similar (structural)
- Would evolve independently
- Coupling would be confusing

## Example Assessment

```typescript
// After GREEN:
const processOrder = (order: Order): ProcessedOrder => {
  const itemsTotal = order.items.reduce((sum, item) => sum + item.price, 0);
  const shipping = itemsTotal > 50 ? 0 : 5.99;
  return { ...order, total: itemsTotal + shipping, shippingCost: shipping };
};

// ASSESSMENT:
// ⚠️ High: Magic numbers 50, 5.99 → extract constants
// ✅ Skip: Structure is clear enough
// DECISION: Extract constants only
```

## Speculative Code is a TDD Violation

If code isn't driven by a failing test, don't write it.

**Key lesson**: Every line must have a test that demanded its existence.

❌ **Speculative code examples:**
- "Just in case" logic
- Features not yet needed
- Code written "for future flexibility"
- Untested error handling paths

**What to do**: Delete speculative code. Add behavior tests instead.

---

## When NOT to Refactor

Don't refactor when:

- ❌ Code works correctly (no bug to fix)
- ❌ No test demands the change (speculative refactoring)
- ❌ Would change behavior (that's a feature, not refactoring)
- ❌ Premature optimization
- ❌ Code is "good enough" for current phase

**Remember**: Refactoring should improve code structure without changing behavior.

---

## Commit Messages for Refactoring

```
refactor: extract scenario validation logic
refactor: simplify error handling flow
refactor: rename ambiguous parameter names
```

**Format**: `refactor: <what was changed>`

**Note**: Refactoring commits should NOT be mixed with feature commits.

---

## Refactoring Checklist

- [ ] All tests pass without modification
- [ ] No new public APIs added
- [ ] Code more readable than before
- [ ] Committed separately from features
- [ ] Committed BEFORE refactoring (safety net)
- [ ] No speculative code added
- [ ] Behavior unchanged (tests prove this)