🔷 TypeScript Lesson 02 — Interfaces & Type Aliases

1. Interface — Object Shape

// interface = object ရဲ့ shape define
interface User {
  id:        number;
  name:      string;
  email:     string;
  role:      "admin" | "user";
  phone?:    string;          // ? = optional field
  readonly createdAt: Date;   // readonly = can't change after creation
}

// Use interface
const user: User = {
  id: 1,
  name: "Ko Ko",
  email: "ko@test.com",
  role: "admin",
  createdAt: new Date()
};

user.name = "Ma Ma";     // ✅ OK
user.createdAt = new Date(); // ❌ Error: readonly!
user.phone = "09123";    // ✅ optional OK
user.age = 25;           // ❌ Error: not in interface

2. Interface vs type

interface Animal {
  name: string;
  sound(): string;
}

// Extend
interface Dog extends Animal {
  breed: string;
}

// Declaration merging
// (can add to existing interface)
interface User { id: number; }
interface User { name: string; }
// merged: { id, name }

type Animal = {
  name: string;
  sound(): string;
};

// Extend (intersection)
type Dog = Animal & {
  breed: string;
};

// Can't merge
// type User = { id: number }
// type User = { name: string }
// ❌ Duplicate identifier error!

// Can do unions
type ID = string | number;

3. Extending & Combining

interface BaseEntity {
  id: number;
  createdAt: Date;
  updatedAt: Date;
}

interface User extends BaseEntity {
  name:  string;
  email: string;
  role:  "admin" | "user";
}

interface Post extends BaseEntity {
  title:   string;
  body:    string;
  userId:  number;
  tags?:   string[];
}

// Intersection type (type alias)
type AdminUser = User & {
  permissions: string[];
  lastLogin: Date;
};

// Partial — all fields optional (DTO for update)
type UpdateUser = Partial<User>;
// { id?: number; name?: string; email?: string; ... }

// Pick — select some fields
type UserPreview = Pick<User, 'id' | 'name'>;

// Omit — exclude some fields
type CreateUser = Omit<User, 'id' | 'createdAt' | 'updatedAt'>;

// Required — all optional → required
type RequiredUser = Required<Partial<User>>;

4. Nested & Complex Interfaces

interface Address {
  street: string;
  city:   string;
  zip?:   string;
  country: string;
}

interface Profile {
  avatar?: string;
  bio?:    string;
  social?: {
    twitter?: string;
    github?:  string;
    linkedin?: string;
  };
}

interface User {
  id:      number;
  name:    string;
  email:   string;
  address: Address;
  profile: Profile;
  tags:    string[];
  scores:  Record<string, number>;   // { math: 95, english: 88 }
}

// Record type — key-value map
type LangScore = Record<string, number>;
const scores: LangScore = { javascript: 90, python: 85 };

// Index signature — any string key
interface StringMap {
  [key: string]: string;
}
const labels: StringMap = { hello: "ဟဲလို", bye: "နှုတ်ဆက်" };