Your Own for..of Loop

Iterable objects are those that can be iterated over using a for..of loop. The most famous examples are Arrays and Strings. But you can make ANY object iterable.

The Symbol.iterator

For an object to be iterable, it must have a method with the Symbol.iterator key.

let range = { from: 1, to: 5 };

range[Symbol.iterator] = function() {
  return {
    current: this.from,
    last: this.to,
    next() {
      if (this.current <= this.last) {
        return { done: false, value: this.current++ };
      } else {
        return { done: true };
      }
    }
  };
};