refactoring-patterns

Extract Method

When: A code block does one logical thing within a function that does more.

BEFORE:

function processOrder(order: Order) {
  // Validate order
  if (!order.items || order.items.length === 0) {
    throw new Error('Order must have items');
  }
  if (order.total <= 0) {
    throw new Error('Order total must be positive');
  }

  // Calculate tax
  let tax = 0;
  for (const item of order.items) {
    tax += item.price * item.quantity * 0.08;
  }

  // Save to database
  db.orders.insert({ ...order, tax });
}

AFTER:

function processOrder(order: Order) {
  validateOrder(order);
  const tax = calculateTax(order.items);
  saveOrder(order, tax);
}

function validateOrder(order: Order) {
  if (!order.items || order.items.length === 0) {
    throw new Error('Order must have items');
  }
  if (order.total <= 0) {
    throw new Error('Order total must be positive');
  }
}

function calculateTax(items: OrderItem[]): number {
  return items.reduce((sum, item) => sum + item.price * item.quantity * 0.08, 0);
}

function saveOrder(order: Order, tax: number) {
  db.orders.insert({ ...order, tax });
}

Why: Each function has a single responsibility. Method names replace comments.

Extract Class

When: A class has multiple responsibilities or a group of fields are always used together.

BEFORE:

class User {
  id: number;
  name: string;
  email: string;
  street: string;
  city: string;
  zipCode: string;
  country: string;

  getFullAddress(): string {
    return `${this.street}, ${this.city}, ${this.zipCode}, ${this.country}`;
  }
}

AFTER:

class Address {
  constructor(
    public street: string,
    public city: string,
    public zipCode: string,
    public country: string
  ) {}

  getFullAddress(): string {
    return `${this.street}, ${this.city}, ${this.zipCode}, ${this.country}`;
  }
}

class User {
  id: number;
  name: string;
  email: string;
  address: Address;
}