refactoring-patterns

Refactoring Patterns Skill

Safe transformations — same behavior, better structure.

Golden Rule

Tests pass before AND after. Never refactor and add features in the same commit.

When to Refactor

Trigger	Action
Feature is hard to add	Refactor first, then add feature
Same bug twice	Refactor to prevent recurrence
"I don't understand"	Refactor for clarity
Duplicate code	Extract and reuse
Long function (>30 lines)	Extract logical units

When NOT to Refactor

• No tests + time pressure
• Code won't change again
• Right before release (deadline pressure)
• Should rewrite instead (>70% changes needed)
• Exploratory/prototype code

Core Refactoring Moves

Extract Function

When a block does one logical thing, give it a name.

// Before
function processOrder(order: Order) {
  // Validate order
  if (!order.items.length) throw new Error('Empty order');
  if (!order.customer) throw new Error('No customer');
  if (order.total < 0) throw new Error('Invalid total');
  
  // Calculate tax
  const taxRate = order.region === 'EU' ? 0.20 : 0.10;
  const tax = order.total * taxRate;
  
  // Apply discount
  const discount = order.customer.isPremium ? 0.15 : 0;
  const finalTotal = order.total + tax - (order.total * discount);
  
  return finalTotal;
}

// After
function processOrder(order: Order) {
  validateOrder(order);
  const tax = calculateTax(order);
  const discount = calculateDiscount(order);
  return order.total + tax - discount;
}

function validateOrder(order: Order): void {
  if (!order.items.length) throw new Error('Empty order');
  if (!order.customer) throw new Error('No customer');
  if (order.total < 0) throw new Error('Invalid total');
}

function calculateTax(order: Order): number {
  const taxRate = order.region === 'EU' ? 0.20 : 0.10;
  return order.total * taxRate;
}

function calculateDiscount(order: Order): number {
  return order.customer.isPremium ? order.total * 0.15 : 0;
}