Refactoring

Golden Rule

Change behavior OR change structure, never both at once.

Workflow

1. Ensure tests exist - Coverage before refactoring
2. Make small changes - One refactoring at a time
3. Run tests frequently - After each change
4. Commit often - Easy to revert if needed

Common Refactorings

Extract Function

Before:

function processOrder(order: Order) {
  // 20 lines of validation
  // 30 lines of calculation
  // 15 lines of formatting
}

After:

function processOrder(order: Order) {
  validateOrder(order);
  const total = calculateTotal(order);
  return formatReceipt(order, total);
}

Replace Conditionals with Polymorphism

Before:

function getPrice(type: string) {
  if (type === 'regular') return basePrice;
  if (type === 'premium') return basePrice * 1.5;
  if (type === 'vip') return basePrice * 2;
}

After:

interface PricingStrategy {
  getPrice(basePrice: number): number;
}

const strategies: Record<string, PricingStrategy> = {
  regular: { getPrice: (p) => p },
  premium: { getPrice: (p) => p * 1.5 },
  vip: { getPrice: (p) => p * 2 },
};

Simplify Conditionals

Before:

if (user && user.isActive && user.subscription && user.subscription.isValid) {

After:

function hasValidSubscription(user: User): boolean {
  return user?.isActive && user?.subscription?.isValid;
}

if (hasValidSubscription(user)) {

Code Smells to Address

Smell	Refactoring
Long function	Extract function
Duplicate code	Extract and reuse
Long parameter list	Introduce parameter object
Feature envy	Move method to appropriate class
Primitive obsession	Replace with value object
Switch statements	Replace with polymorphism

When NOT to Refactor

• No test coverage (add tests first)
• Under time pressure (technical debt is ok short-term)
• Code is about to be replaced
• Refactoring scope is unclear