Refactoring Patterns

Refactoring Patterns

Systematically improve code without changing its behavior.

The Refactoring Workflow

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

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

Common Refactorings

| Refactoring | When | Risk |

|-------------|------|------|

| Rename Variable | Unclear name | Low |

| Extract Method | Long function | Low |

| Replace Magic Literal | Hard-coded values | Low |

| Introduce Parameter Object | Many parameters | Medium |

| Replace Temp with Query | Reused expressions | Low |

| Decompose Conditional | Complex condition | Medium |

| Extract Class | Class doing too much | Medium-High |