Оператор keyof та ключі об’єктних типів

let key: UserKey;

key = "id";
key = "name";
key = "isActive";

// Помилка: такого ключа немає в типі User
// key = "email";

type UserKey = "id" | "name" | "isActive";

interface User {
  id: number;
  name: string;
  isActive: boolean;
}

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

const key: keyof User = "name";

const value = user[key];

console.log(value); // Олена

// typeof value:
// number | string | boolean

interface User {
  id: number;
  name: string;
  isActive: boolean;
}

function getProperty<T, K extends keyof T>(object: T, key: K): T[K] {
  return object[key];
}

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

const userName = getProperty(user, "name");
const userId = getProperty(user, "id");
const userStatus = getProperty(user, "isActive");

console.log(userName);   // Олена
console.log(userId);     // 1
console.log(userStatus); // true

const name = getProperty(user, "name");
// string

const id = getProperty(user, "id");
// number

const isActive = getProperty(user, "isActive");
// boolean

// Помилка: "email" не є ключем типу User
// const email = getProperty(user, "email");

function getPropertyUnsafe(object: User, key: string) {
  // TypeScript не може гарантувати, що key є ключем User
  // return object[key];
}

const settings = {
  theme: "dark",
  language: "uk",
  notifications: true,
};

type SettingKey = keyof typeof settings;
// "theme" | "language" | "notifications"

function readSetting(key: SettingKey) {
  return settings[key];
}

console.log(readSetting("theme")); // dark
console.log(readSetting("language")); // uk

// Помилка: "fontSize" не є ключем об’єкта settings
// readSetting("fontSize");

interface Product {
  id: number;
  title: string;
  price: number;
}

type ProductKey = keyof Product;
// "id" | "title" | "price"

interface Article {
  title: string;
  author: string;
  views: number;
  published: boolean;
}

type TextKey = "title" | "author";

function printText(article: Article, key: TextKey): void {
  console.log(article[key]);
}

const article: Article = {
  title: "TypeScript для початківців",
  author: "Ірина",
  views: 120,
  published: true,
};

printText(article, "title");
printText(article, "author");

// Помилка: "views" не належить до TextKey
// printText(article, "views");

function printValue(article: Article, key: keyof Article): void {
  console.log(article[key]);
}

printValue(article, "title");
printValue(article, "views");
printValue(article, "published");

interface User {
  id: number;
  name: string;
}

type UserKey = keyof User;

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

type UserKey = keyof typeof user;

function readUser(user: User, key: string) {
  // key може містити будь-який рядок
}

function readUser(user: User, key: keyof User) {
  return user[key];
}

type UserKey = keyof User;
// "id" | "name"

interface User {
  id: number;
  name: string;
}

type UserValue = User[keyof User];
// number | string