Optional та Readonly-властивості

const firstUser: User = {
  name: "Олена",
};

const secondUser: User = {
  name: "Андрій",
  age: 28,
};

const invalidUser: User = {
  age: 25,
};

type User = {
  name: string;
  age?: number;
};

function printUserAge(user: User): void {
  if (user.age !== undefined) {
    console.log(`${user.name}: ${user.age} років`);
  } else {
    console.log(`${user.name}: вік не вказано`);
  }
}

printUserAge({ name: "Олена" });
printUserAge({ name: "Андрій", age: 28 });

type Product = {
  name: string;
  discount?: number;
};

function getDiscount(product: Product): number {
  return product.discount ?? 0;
}

console.log(getDiscount({ name: "Книга" })); // 0
console.log(getDiscount({ name: "Навушники", discount: 15 })); // 15

type User = {
  readonly id: number;
  name: string;
};

const user: User = {
  id: 1,
  name: "Олена",
};

user.name = "Андрій"; // Коректно
user.id = 2; // Помилка TypeScript

console.log(user.id);

interface Article {
  readonly id: number;
  title: string;
  description?: string;
}

const article: Article = {
  id: 101,
  title: "Основи TypeScript",
};

article.title = "Оновлені основи TypeScript"; // Коректно
article.description = "Короткий опис"; // Коректно
article.id = 102; // Помилка TypeScript

type Settings = {
  readonly id: number;
  theme?: string;
};

const settings: Settings = {
  id: 1,
};

settings.theme = "dark"; // Коректно
settings.id = 2; // Помилка TypeScript

type Book = {
  title: string;
  readonly isbn: string;
  readonly publishedAt?: string;
};

const book: Book = {
  title: "TypeScript для початківців",
  isbn: "978-1-234-56789-0",
};

book.title = "TypeScript: практичний курс"; // Коректно
book.publishedAt = "2026-08-08"; // Помилка TypeScript
book.isbn = "978-9-876-54321-0"; // Помилка TypeScript

type Profile = {
  readonly userId: number;
  username: string;
  email: string;
  bio?: string;
};

function printProfile(profile: Profile): void {
  console.log(`Користувач: ${profile.username}`);
  console.log(`Email: ${profile.email}`);
  console.log(`ID: ${profile.userId}`);
  console.log(`Опис: ${profile.bio ?? "відсутній"}`);
}

const profile: Profile = {
  userId: 42,
  username: "olena",
  email: "olena@example.com",
};

printProfile(profile);

profile.username = "olena_dev"; // Коректно
profile.bio = "Розробниця TypeScript"; // Коректно

// profile.userId = 100; // Помилка: userId доступна лише для читання

type Product = {
  name: string;
  description: string;
};

const product: Product = {
  name: "Мишка",
};

type Product = {
  name: string;
  description?: string;
};

type User = {
  name: string;
  age?: number;
};

function printAge(user: User): void {
  console.log(user.age.toFixed(0));
}

function printAge(user: User): void {
  if (user.age !== undefined) {
    console.log(user.age.toFixed(0));
  }
}

type Order = {
  readonly id: number;
  status: string;
};

const order: Order = {
  id: 10,
  status: "new",
};

order.id = 11;

order.status = "paid";

type Example = {
  optionalValue?: string;
  readonly fixedValue: number;
};