refactoring-patterns

# 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. ```javascript // 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. ```javascript // 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 | 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 |