Code smell & refactoring

Trước refactor - hàm processOrder làm 5 việc khác nhau

function processOrder(order: Order): void {
  // Kiem tra ton kho
  for (const item of order.items) {
    if (item.qty > getStock(item.id)) throw new Error('Het hang: ' + item.id);
  }
  // Giam ton kho
  for (const item of order.items) {
    decreaseStock(item.id, item.qty);
  }
  // Tinh tong gia
  let total = 0;
  for (const item of order.items) {
    total += item.price * item.qty;
  }
  // Ap dung coupon
  if (order.coupon === 'VIP10') total *= 0.9;
  if (order.coupon === 'SALE20') total *= 0.8;
  // Thanh toan va thong bao
  charge(order.customerId, total);
  sendConfirmation(order.customerId, total);
}

Sau refactor - mỗi hàm con một việc, processOrder chỉ điều phối

// Buoc 1: extract function - chay test sau moi buoc
function validateStock(items: OrderItem[]): void {
  for (const item of items) {
    if (item.qty > getStock(item.id)) throw new Error('Het hang: ' + item.id);
  }
}

function deductStock(items: OrderItem[]): void {
  for (const item of items) decreaseStock(item.id, item.qty);
}

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

// Buoc 2: replace magic string - dat ten ro y dinh
const COUPON_DISCOUNT: Record<string, number> = {
  VIP10: 0.9,
  SALE20: 0.8
};

function applyDiscount(total: number, coupon?: string): number {
  return coupon ? total * (COUPON_DISCOUNT[coupon] ?? 1) : total;
}

// Ket qua: ham dieu phoi doc nhu ke hoach hanh dong
function processOrder(order: Order): void {
  validateStock(order.items);
  deductStock(order.items);
  const total = applyDiscount(calcTotal(order.items), order.coupon);
  charge(order.customerId, total);
  sendConfirmation(order.customerId, total);
}

1. Extract Function - tách logic thành hàm con có tên

// Truoc: doan tinh gia bi chon trong ham dai
function processOrder(order: Order): void {
  // ... nhieu logic khac ...
  let total = 0;
  for (const item of order.items) {
    total += item.price * item.qty;
  }
  // ...
}

// Sau: logic tinh gia duoc dat ten ro rang, co the test doc lap
function calcTotal(items: OrderItem[]): number {
  return items.reduce((sum, item) => sum + item.price * item.qty, 0);
}
// processOrder goi calcTotal - khong doi hanh vi

2. Rename - đặt lại tên nói rõ ý định

// Truoc: ten viet tat, phai doc than ham moi hieu
function d(u: User[]): User[] {
  return u.filter((x) => x.a && x.t <= 30);
}

// Sau: ten noi ro y dinh, doc chu ky la hieu
function activeUsersSeenRecently(users: User[]): User[] {
  return users.filter((u) => u.isActive && u.daysSinceLogin <= 30);
}
// Chi doi ten - hanh vi y het; chay test ngay sau khi doi

3. Replace Magic Number with Constant - đặt tên cho số bí ẩn

// Truoc: 2592000000 la gi? Nguoi doc phai tu tinh
function isSessionExpired(createdAt: Date): boolean {
  return Date.now() - createdAt.getTime() > 2592000000;
}

// Sau: hang so noi ro don vi va y nghia
const THIRTY_DAYS_MS = 30 * 24 * 60 * 60 * 1000;

function isSessionExpired(createdAt: Date): boolean {
  return Date.now() - createdAt.getTime() > THIRTY_DAYS_MS;
}

4. Introduce Parameter Object - gom tham số liên quan thành object

// Truoc: 8 tham so roi, de nham thu tu
function createUser(
  firstName: string, lastName: string,
  email: string, phone: string,
  street: string, city: string, postal: string,
  role: string
): User { /* ... */ }

// Sau: gom nhom theo y nghia, kieu bao dam thu tu dung
interface UserProfile {
  firstName: string;
  lastName: string;
  email: string;
  phone: string;
}

interface Address {
  street: string;
  city: string;
  postalCode: string;
}

function createUser(profile: UserProfile, address: Address, role: string): User {
  /* ... */
}