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' });

async function test() {
  console.log('1');
  
  setTimeout(() => {
    console.log('2');
  }, 0);
  
  await Promise.resolve();
  console.log('3');
  
  new Promise(resolve => {
    console.log('4');
    resolve();
  }).then(() => {
    console.log('5');
  });
  
  console.log('6');
}

test();
console.log('7');

function getCity(person) {
  return person?.address?.city ?? 'Unknown';
}

const data = [
  null,
  { name: 'Alice' },
  { name: 'Bob', address: null },
  { name: 'Charlie', address: { street: '123 Main' } },
  { name: 'David', address: { city: 'Boston' } }
];

const cities = data.map(getCity);
console.log(cities);