let obj1 = { key: 'value1' };
let obj2 = { key: 'value2' };

const weakMap = new WeakMap();
weakMap.set(obj1, 'data1');
weakMap.set(obj2, 'data2');

obj1 = null;
setTimeout(() => {
  console.log(weakMap.has(obj1));
  console.log(weakMap.has(obj2));
}, 100);

const target = {
  secret: "hidden",
  reveal: "nothing"
};

const handler = {
  get: function(obj, prop, receiver) {
    if (prop === "secret") {
      return "revealed";
    }
    return Reflect.get(...arguments);
  }
};

const proxy = new Proxy(target, handler);

with (proxy) {
  console.log(secret);
  console.log(reveal);
}

function* numberDoubler(arr) {
  for (const num of arr) {
    yield num * 2;
  }
}

const result = [...numberDoubler([1, 2, 3])].some(n => n % 4 === 0);

console.log(result);

function* idGenerator() {
  let id = 1;
  while (true) {
    yield id++;
  }
}

const gen = idGenerator();
const ids = Array.from({ length: 3 }, () => gen.next().value).map(id => `ID${id}`);

console.log(ids);

function* fibGenerator() {
  let a = 0, b = 1;
  while (true) {
    let next = a + b;
    a = b;
    b = next;
    yield next;
  }
}

const gen = fibGenerator();
const fibArray = Array.from({ length: 5 }, () => gen.next().value);

console.log(fibArray);