1. Make Small Changes

✅ GOOD: One refactoring per commit
❌ BAD: Multiple unrelated changes in one commit

2. Ensure Tests Pass

# Before refactoring
npm test

# After each change
npm test

3. Keep Behavior Unchanged

Refactoring = Improving structure WITHOUT changing behavior

Common Patterns

Extract Function

// Before
function processOrder(order: Order) {
  // validate
  if (!order.items.length) throw new Error('Empty');
  if (order.total < 0) throw new Error('Invalid total');

  // calculate
  const subtotal = order.items.reduce((s, i) => s + i.price, 0);
  const tax = subtotal * 0.1;
  const total = subtotal + tax;

  // save
  db.save({ ...order, total });
}

// After
function validateOrder(order: Order): void {
  if (!order.items.length) throw new Error('Empty');
  if (order.total < 0) throw new Error('Invalid total');
}

function calculateTotal(items: Item[]): number {
  const subtotal = items.reduce((s, i) => s + i.price, 0);
  const tax = subtotal * 0.1;
  return subtotal + tax;
}

function processOrder(order: Order) {
  validateOrder(order);
  const total = calculateTotal(order.items);
  db.save({ ...order, total });
}

Replace Conditionals with Polymorphism

// Before
function getPrice(type: string, base: number): number {
  switch (type) {
    case 'premium': return base * 0.8;
    case 'vip': return base * 0.7;
    default: return base;
  }
}

// After
interface PricingStrategy {
  calculate(base: number): number;
}

class RegularPricing implements PricingStrategy {
  calculate(base: number) { return base; }
}

class PremiumPricing implements PricingStrategy {
  calculate(base: number) { return base * 0.8; }
}

Simplify Conditionals

// Before
if (user !== null && user !== undefined && user.isActive === true) {
  if (user.role === 'admin' || user.role === 'moderator') {
    // ...
  }
}

// After
const isActiveUser = user?.isActive ?? false;
const hasPrivileges = ['admin', 'moderator'].includes(user?.role ?? '');

if (isActiveUser && hasPrivileges) {
  // ...
}

Remove Dead Code

# Find unused exports
npx ts-prune

# Find unused dependencies
npx depcheck

Workflow

Step 1: Identify Smell

Code Smell	Refactoring
Long function	Extract Function
Duplicate code	Extract and reuse
Complex conditionals	Simplify/Polymorphism
God class	Split responsibilities
Feature envy	Move method

Step 2: Write Tests (if missing)

describe('processOrder', () => {
  it('should calculate total correctly', () => {
    const order = { items: [{ price: 100 }] };
    expect(processOrder(order).total).toBe(110);
  });
});

Step 3: Refactor

Small, incremental changes with tests after each.

Step 4: Verify

npm test
npm run lint
npm run typecheck

1. Make Small Changes

2. Ensure Tests Pass

3. Keep Behavior Unchanged

Common Patterns

Extract Function

Replace Conditionals with Polymorphism

Simplify Conditionals

Remove Dead Code

Workflow

Step 1: Identify Smell

Step 2: Write Tests (if missing)

Step 3: Refactor

Step 4: Verify

Checklist

Trigger	Examples
Refactor	"refactor this", "リファクタリングして", "clean up code"
Improve	"improve this code", "コード改善"
Modernize	"update to modern syntax", "モダン化"

refactor-helper

Refactor Helper

Triggers

Refactoring Principles