code-refactoring-patterns

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• SKILL.md
• refactoring-catalog.md · references/

SKILL.md (first lines)

# Code Refactoring Patterns

A comprehensive guide to refactoring code systematically while maintaining functionality and improving quality.

## When to Refactor

- Code smells detected (duplicated code, long functions, etc.)
- Before adding new features to complex areas
- After understanding improves ("now I see a better way")
- When tests are in place
- Performance optimization needed

## Refactoring Rules

1. **Never refactor without tests**: Write tests first if they don't exist
2. **Small steps**: Make one change at a time
3. **Run tests after each change**: Ensure nothing breaks
4. **Commit often**: Each working refactor is a commit
5. **Don't mix refactoring with feature work**: Separate concerns

## Common Code Smells

### 1. Long Method/Function

**Smell**: Functions over 20-30 lines

**Refactor**: Extract Method

```typescript
// Before
function processOrder(order: Order) {
  // Validate order (10 lines)
  // Calculate totals (15 lines)
  // Apply discounts (12 lines)
  // Send confirmation (8 lines)
}

// After
function processOrder(order: Order) {
  validateOrder(order);
  const totals = calculateTotals(order);
  const finalPrice = applyDiscounts(totals, order);
  sendConfirmation(order, finalPrice);
}
```

### 2. Duplicated Code

**Smell**: Same code in multiple places

**Refactor**: Extract Function/Class

```typescript
// Before
function formatUserName(user: User) {
  return `${user.firstName} ${user.lastName}`;
}

function formatAuthorName(author: Author) {
  return `${author.firstName} ${author.lastName}`;
}

// After
function formatFullName(person: { firstName: string; lastName: string }) {
  return `${person.firstName} ${person.lastName}`;
}
```

### 3. Long Parameter List

**Smell**: Functions with 4+ parameters

**Refactor**: Parameter Object

```typescript
// Before
function createUser(
  firstName: string,
  lastName: string,
  email: string,
  phone: string,
  address: string
) { }

// After
interface UserDetails {
  firstName: string;
  lastName: string;
  email: string;
  phone: string;
  address: string;
}

function createUser(details: UserDetails) { }
```

### 4. Large Class

**Smell**: Classes with many responsibilities

**Refactor**: Extract Class

```typescript
// Before
class UserManager {
  createUser() { }
  deleteUser() { }
  sendEmail() { }
  generateReport() { }
  logActivity() { }
}

// After
class UserService {
  createUser() { }
  deleteUser() { }
}

class EmailService {

From 4444J99/a-i--skills, MIT.

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