const users = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Charlie' }
];

const userScores = new WeakMap();

// Set scores for users
userScores.set(users[0], 95);
userScores.set(users[1], 80);

// Remove reference to Bob
users[1] = null;

let sum = 0;
for (const user of users) {
  if (user && userScores.has(user)) {
    sum += userScores.get(user);
  }
}

console.log(sum);

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());

const team = {
  lead: { name: 'Alice', projects: ['Alpha', 'Beta'] },
  dev: { name: 'Bob', projects: ['Gamma'] },
  tester: { name: 'Charlie', projects: [] }
};

const { 
  lead: { projects: [leadProject] },
  dev: { projects: [devProject = 'Delta'] },
  tester: { projects: [testerProject = 'Epsilon'] }
} = team;

console.log(`${leadProject}-${devProject}-${testerProject}`);

const target = { a: 1, b: 2 };
const handler = {
  get(obj, prop) {
    return prop in obj ? obj[prop] * 2 : 'not found';
  },
  set(obj, prop, value) {
    if (typeof value !== 'number') {
      return false;
    }
    obj[prop] = value + 10;
    return true;
  }
};

const proxy = new Proxy(target, handler);
proxy.c = '5';
proxy.d = 5;

console.log(JSON.stringify({
  a: proxy.a,
  b: proxy.b,
  c: proxy.c,
  d: target.d,
  hasC: Reflect.has(target, 'c')
}));

function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) {
      return fn.apply(this, args);
    }
    return function(...moreArgs) {
      return curried.apply(this, [...args, ...moreArgs]);
    };
  };
}

const multiply = curry((a, b, c) => a * b * c);
const double = multiply(2);
const result = double(3)(4);

console.log(result);

class EventEmitter {
  constructor() {
    this.events = {};
  }
  
  on(event, listener) {
    if (!this.events[event]) {
      this.events[event] = [];
    }
    this.events[event].push(listener);
    return () => this.off(event, listener);
  }
  
  off(event, listener) {
    if (!this.events[event]) return;
    this.events[event] = this.events[event].filter(l => l !== listener);
  }
  
  emit(event, ...args) {
    if (!this.events[event]) return false;
    this.events[event].forEach(listener => listener(...args));
    return true;
  }
}

const emitter = new EventEmitter();
const unsubscribe = emitter.on('message', data => console.log(data));
emitter.emit('message', 'Hello');
emitter.emit('message', 'World');
unsubscribe();
emitter.emit('message', 'Ignored');
console.log(emitter.emit('message', 'Still ignored'));

const numbers = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

const result = numbers
  .filter(num => num % 2 === 0)
  .map(num => num * 2)
  .reduce((acc, num, index, array) => {
    if (index === array.length - 1) {
      return (acc + num) / array.length;
    }
    return acc + num;
  }, 0);

console.log(result);