type User = {
  id: number;
  name: string;
  role?: 'admin' | 'user';
};

function processUser(user: Partial<User>): string {
  const defaultUser: User = {
    id: 0,
    name: 'Guest',
    role: 'user'
  };
  
  const mergedUser = { ...defaultUser, ...user };
  
  if (mergedUser.role === 'admin') {
    return `Admin: ${mergedUser.name}`;
  }
  
  return `User: ${mergedUser.name} (ID: ${mergedUser.id})`;
}

console.log(processUser({ name: 'John', role: 'admin' }));

function processConfig(config) {
  const cache = config.cache ?? true;
  const timeout = config.timeout ?? 1000;
  const retries = config.retries ?? 3;
  
  return {
    useCache: cache,
    timeoutMs: timeout,
    maxRetries: retries
  };
}

const result = processConfig({ timeout: 0, retries: false });
console.log(result);

function processInput(userInput) {
  const defaultValue = 'default';
  const value1 = userInput?.value ?? defaultValue;
  const value2 = userInput?.value || defaultValue;
  
  const result = {
    a: 0 ?? 'zero',
    b: '' ?? 'empty',
    c: null ?? 'null',
    d: undefined ?? 'undefined',
    comparison: value1 === value2
  };
  
  console.log(result);
}

processInput({ value: '' });

class ShoppingCart {
  constructor() {
    if (ShoppingCart.instance) {
      return ShoppingCart.instance;
    }
    
    this.items = [];
    ShoppingCart.instance = this;
  }
  
  addItem(item) {
    this.items.push(item);
  }
  
  getItems() {
    return [...this.items];
  }
}

const cart1 = new ShoppingCart();
const cart2 = new ShoppingCart();

cart1.addItem('Book');
cart2.addItem('Laptop');

console.log(cart1.getItems());

function Device(name) {
  this.name = name;
  this.isOn = false;
}

Device.prototype.turnOn = function() {
  this.isOn = true;
  return `${this.name} is now on`;
};

function Smartphone(name, model) {
  Device.call(this, name);
  this.model = model;
}

Smartphone.prototype = Object.create(Device.prototype);
Smartphone.prototype.constructor = Smartphone;

Smartphone.prototype.turnOn = function() {
  const result = Device.prototype.turnOn.call(this);
  return `${result} (model: ${this.model})`;
};

const myPhone = new Smartphone('iPhone', '13 Pro');
console.log(myPhone.turnOn());

let obj1 = { id: 1 };
let obj2 = { id: 2 };
let obj3 = { id: 3 };

const weakSet = new WeakSet([obj1, obj2]);

weakSet.add(obj3);
weakSet.delete(obj1);

obj2 = null;

const remainingObjects = [...weakSet];

console.log(remainingObjects);

const secretData = { password: 'abc123' };
const mySet = new WeakSet();
mySet.add(secretData);

// Later in the code
delete secretData.password;

const checkAccess = (obj) => {
  console.log(mySet.has(obj));
};

checkAccess(secretData);
checkAccess({ password: 'abc123' });