JavaScript custom iterable

custom-iterable.js

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

countToThree[Symbol.iterator] = function* () {
  const keys = Object.keys(this);
  const length = keys.length;

  for (const key in this) {
    yield this[key];
  }
};

let [...three] = countToThree;
console.log(three); // [ 1, 2, 3 ]

The MDN says that you can make your own iterators with just the Symbol, but you also need to implement the Iterable interface and the next() method as far as i know.

Link to spec: http://www.ecma-international.org/ecma-262/6.0/#sec-iterable-interface

I'm wrong?

What you're missing is that the [Symbol.iterator] we implemented was a generator function, which creates the iterable methods for you automatically.

As you can see if you try to run that code in a recent version of Node, it works just fine.

let [...three] = countToThree;
console.log(three); // [ 1, 2, 3 ]

for...of also works:

for (i of countToThree) console.log(i);
// 1
// 2
// 3