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

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

const obj = {
  valueOf: () => 42,
  toString: () => "Hello"
};
console.log(`${obj}`, obj + 10);

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

const x = (() => {
  try {
    return 10;
  } finally {
    return 20;
  }
})();
console.log(x);