Make any JavaScript value iterable

JavaScript's Symbol.iterator is a very powerful tool that every web developer should learn how to use. It allows you to define and customize the way a value is iterated, effectively allowing you to make any value iterable. You can easily apply this knowledge to plain JavaScript objects and even classes.

@Quick refresher

All you need to correctly define an iterator is a generator function yielding each of the iteration values. This could be used to retrieve key-value pairs in an object, call specific getter functions from a class or split a number into an array of digits:

const obj = { a: 1, b: 2, c: 3 };

obj[Symbol.iterator] = function* () {
  for (let key of Object.keys(obj)) yield { [key]: obj[key] };
};

[...obj]; // [ { a: 1 }, { b: 2 }, { c: 3 }]

class IterableNumber extends Number {
  *[Symbol.iterator]() {
    for (let digit of [...`${this}`].map(d => Number.parseInt(d))) yield digit;
  }
}

const num = new IterableNumber(1337);
[...num]; // [ 1, 3, 3, 7]