Code Refactoring
Refactoring Principles
When to Refactor
- Before adding new features (make change easy, then make easy change)
- After getting tests passing (red-green-refactor)
- When you see code smells
- During code review feedback
When NOT to Refactor
- Without tests covering the code
- Under tight deadlines with no safety net
- Code that will be replaced soon
- When you don't understand what the code does
Common Code Smells
Long Methods
// BEFORE: Method doing too much
function processOrder(order: Order) {
// 100 lines of validation, calculation, notification, logging...
}
// AFTER: Extract into focused methods
function processOrder(order: Order) {
validateOrder(order);
const total = calculateTotal(order);
saveOrder(order, total);
notifyCustomer(order);
}
Deeply Nested Conditionals
// BEFORE: Arrow code
function getDiscount(user: User, order: Order) {
if (user) {
if (user.isPremium) {
if (order.total > 100) {
if (order.items.length > 5) {
return 0.2;
}
}
}
}
return 0;
}
// AFTER: Early returns (guard clauses)
function getDiscount(user: User, order: Order) {
if (!user) return 0;
if (!user.isPremium) return 0;
if (order.total <= 100) return 0;
if (order.items.length <= 5) return 0;
return 0.2;
}
Primitive Obsession
// BEFORE: Primitives everywhere
function createUser(name: string, email: string, phone: string) {
if (!email.includes('@')) throw new Error('Invalid email');
// more validation...
}
// AFTER: Value objects
class Email {
constructor(private value: string) {
if (!value.includes('@')) throw new Error('Invalid email');
}
toString() { return this.value; }
}
function createUser(name: string, email: Email, phone: Phone) {
// Email is already validated
}
Feature Envy
// BEFORE: Method uses another object's data extensively
function calculateShipping(order: Order) {
const address = order.customer.address;
const weight = order.items.reduce((sum, i) => sum + i.weight, 0);
const distance = calculateDistance(address.zip);
return weight * distance * 0.01;
}
// AFTER: Move method to where the data is
class Order {
calculateShipping() {
return this.totalWeight * this.customer.shippingDistance * 0.01;
}
}
Refactoring Techniques
Extract Method
// Identify a code block that does one thing
// Move it to a new method with a descriptive name
// Replace original code with method call
function printReport(data: ReportData) {
// Extract this block...
const header = `Report: ${data.title}\nDate: ${data.date}\n${'='.repeat(40)}`;
console.log(header);
// ...into a method
printHeader(data);
}
Replace Conditional with Polymorphism
// BEFORE: Switch on type
function getArea(shape: Shape) {
switch (shape.type) {
case 'circle': return Math.PI * shape.radius ** 2;
case 'rectangle': return shape.width * shape.height;
case 'triangle': return shape.base * shape.height / 2;
}
}
// AFTER: Polymorphic classes
interface Shape {
getArea(): number;
}
class Circle implements Shape {
constructor(private radius: number) {}
getArea() { return Math.PI * this.radius ** 2; }
}
class Rectangle implements Shape {
constructor(private width: number, private height: number) {}
getArea() { return this.width * this.height; }
}
Introduce Parameter Object
// BEFORE: Too many parameters
function searchProducts(
query: string,
minPrice: number,
maxPrice: number,
category: string,
inStock: boolean,
sortBy: string,
sortOrder: string
) { ... }
// AFTER: Parameter object
interface SearchParams {
query: string;
priceRange: { min: number; max: number };
category?: string;
inStock?: boolean;
sort?: { by: string; order: 'asc' | 'desc' };
}
function searchProducts(params: SearchParams) { ... }
Replace Magic Numbers with Constants
// BEFORE
if (user.age >= 18 && order.total >= 50) {
applyDiscount(order, 0.1);
}
// AFTER
const MINIMUM_AGE = 18;
const DISCOUNT_THRESHOLD = 50;
const STANDARD_DISCOUNT = 0.1;
if (user.age >= MINIMUM_AGE && order.total >= DISCOUNT_THRESHOLD) {
applyDiscount(order, STANDARD_DISCOUNT);
}
Safe Refactoring Process
- Ensure tests exist - Write tests if they don't
- Make small changes - One refactoring at a time
- Run tests after each change - Catch regressions immediately
- Commit frequently - Easy to revert if something breaks
- Review the diff - Make sure behavior hasn't changed
Refactoring Checklist
