Refactoring Patterns#

Systematically improve code without changing its behavior.

The Refactoring Workflow#

1. Identify a code smell or pain point
2. Ensure you have tests (write them first if not)
3. Apply one refactoring at a time
4. Test after each change (green = continue, red = revert)
5. Commit — small, focused, descriptive
6. Repeat

Extract Method#

When: A method does multiple things or is too long. How: Find a cohesive block → extract to new method → call it.

// Before
function processOrder(order) {
  const total = order.items.reduce((sum, i) => sum + i.price, 0);
  const tax = total * 0.08;
  const discount = order.coupon ? total * 0.1 : 0;
  return total + tax - discount;
}

// After
function processOrder(order) {
  const subtotal = calculateSubtotal(order);
  const tax = calculateTax(subtotal);
  const discount = calculateDiscount(order, subtotal);
  return subtotal + tax - discount;
}

Replace Conditional with Polymorphism#

When: Switch/if-else chains based on type grow too long.

// Before
function calculateShipping(order) {
  if (order.type === 'standard') return order.weight * 0.5;
  if (order.type === 'express') return order.weight * 1.5 + 5;
  if (order.type === 'overnight') return order.weight * 3 + 15;
}

// After
class StandardShipping {
  calculate(weight) { return weight * 0.5; }
}
class ExpressShipping {
  calculate(weight) { return weight * 1.5 + 5; }
}

Legacy Code Strategy#

1. Characterize with tests: Write tests that capture current behavior
2. Sprout method: Add new code in new methods, don't modify old
3. Wrap method: Wrap entire methods with new behavior (logging, caching)
4. Step-by-step: Extract small pieces over time
5. Strangler pattern: New code replaces old incrementally, old gets retired

Common Refactorings#