const arr = [1, 2, 3];
arr[10] = 11;
console.log(arr.length);

const obj = {
  a: 1,
  b: 2,
  a: 3
};
console.log(obj.a);

console.log([..."hello"]);

Number.prototype[Symbol.iterator] = function*() { yield *['a','b','c'] };
console.log([...1]);

console.log([…({ [Symbol.iterator]: function*(){ yield *[1, yield *[2,3] ] } })])