Object vs Data Structure

Hình học kiểu OOP - object giấu dữ liệu, lộ hành vi

// Moi hinh biet tinh dien tich cua chinh no
interface Shape {
  area(): number;
}

class Circle implements Shape {
  constructor(private radius: number) {}
  area() { return Math.PI * this.radius * this.radius; }
}

class Rect implements Shape {
  constructor(private width: number, private height: number) {}
  area() { return this.width * this.height; }
}

function printArea(s: Shape) {
  console.log(s.area().toFixed(2));
}
printArea(new Circle(5));  // 78.54
printArea(new Rect(4, 6)); // 24.00

Hình học kiểu data + function - data lộ ra, hàm switch theo loại

// Data structure: field cong khai, khong co method
type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'rect';   width: number; height: number };

// Ham xu ly dieu phoi tu ben ngoai
function area(s: Shape): number {
  switch (s.kind) {
    case 'circle': return Math.PI * s.radius * s.radius;
    case 'rect':   return s.width * s.height;
  }
}

console.log(area({ kind: 'circle', radius: 5 }).toFixed(2));  // 78.54
console.log(area({ kind: 'rect', width: 4, height: 6 }).toFixed(2)); // 24.00

❌ Hybrid - tệ cả hai hướng

// DTO hay object? Khong ro - do la hybrid
class Order {
  public customerId: string = '';   // public field - kieu data structure
  public items: string[] = [];      // public field
  public status: string = 'new';    // public field

  // Nhung lai co method nghiep vu - kieu object
  canCancel(): boolean {
    return this.status === 'new' || this.status === 'pending';
  }
  totalItems(): number {
    return this.items.length;
  }
}

// Van de: them kieu moi -> phai dong cham ca field lan method
// Them thao tac moi -> cung phai mo class nay ra
// Khong ro rang nen doi xu nhu du lieu hay object

✅ Tách rõ: data structure cho DTO, object cho nghiệp vụ

// Data structure: chi du lieu, kieu ro rang
interface OrderDto {
  customerId: string;
  items: string[];
  status: 'new' | 'pending' | 'shipped' | 'cancelled';
}

// Object: data an, chi lo method
class Order {
  constructor(private data: OrderDto) {}

  canCancel(): boolean {
    return this.data.status === 'new' || this.data.status === 'pending';
  }
  itemCount(): number {
    return this.data.items.length;
  }
}

❌ Train wreck - leo qua nhiều lớp

// Caller biet: Department co manager, manager co address, address co city
// Doi ten truong? Doi kieu? Vo day chuyen ngay
const city = department.getManager().getAddress().getCity();

// Te hon nua: lai leo het chuoi mot lan nua de quyet dinh
if (city === 'Ha Noi') {
  applyHanoiPolicy(department.getManager().getAddress().getCity());
}

✅ Tell, Don't Ask - bảo object tự xử lý

// Them method vao Department: no tu biet lay city o dau
class Department {
  private manager: Employee;
  constructor(manager: Employee) { this.manager = manager; }

  // "Tell don't ask": dong goi logic, caller khong can biet cau truc ben trong
  managerCity(): string {
    return this.manager.address.city;
  }
}

// Caller chi goi mot method, khong leo qua cac lop
const city = department.managerCity();

DTO trong TypeScript - interface cho dữ liệu, class cho object nghiệp vụ

// interface / type: compile-time only, khong sinh JS thua - chon cho DTO
interface UserDto {
  id: string;
  name: string;
  email: string;
  createdAt: string; // ISO 8601 string tu API
}

interface OrderDto {
  orderId: string;
  totalAmount: number;
  customerEmail: string;
}

// Dung o bien: API response, tham so truyen qua layer
function processOrder(order: OrderDto): void {
  console.log(order.orderId, order.totalAmount);
  // ORD-001 350000
}

// class: khi can instanceof, constructor validation, hoac default phuc tap
class Money {
  constructor(
    private amount: number,
    private currency: string
  ) {
    if (amount < 0) throw new Error('So tien khong the am');
  }
  // Object co hanh vi: format, cong, tru...
  format(): string { return `${this.amount} ${this.currency}`; }
}