function* generator() {
  yield* [1, 2];
  yield* (function* () { yield 3; yield 4; })();
}

const gen = generator();

console.log([...gen]);

const obj = {
  a: 1,
  b() {
    return this.a + 1;
  }
};

const { b } = obj;
console.log(b());

const obj = { a: 1, b: 2 };
Object.defineProperty(obj, 'b', { value: 3, writable: false });

obj.b = 4;
console.log(obj.b);

const a = { value: 1 };
const b = Object.create(a);
b.value = 2;

console.log(b.value);
console.log(a.value);

let count = 0;

const counter = (function() {
  count = 0;
  return function() {
    count += 1;
    return count;
  };
})();

count = 10;
counter();
console.log(count);

const target = {
  age: 30
};

const handler = {
  get: function(obj, prop) {
    return obj[prop]++;  
  }
};

const proxy = new Proxy(target, handler);

console.log(proxy.age);
console.log(target.age);
console.log(proxy.age);

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);