iteratables.js

var obj = { x: 1, y: 2, z: 3 };
obj[Symbol.iterator] = function() {
  // An iterator is an object which has a next method,
  // which also returns an object with atleast one of two properties: value & done.

  // returning an iterator object
  return {
    next: function() {
      if (this._countDown === 3) {
        const lastValue = this._countDown;
        return { value: this._countDown, done: true };
      }
      this._countDown = this._countDown + 1;
      return { value: this._countDown, done: false };
    },
    _countDown: 0
  };
};
[...obj]; // will print [1, 2, 3]