Under the hood, 'for..in' just calls #each. Therefore, it is easy to make 'for..in' work with a custom collection: just define #each. And that is what you would

Under the hood, 'for..in' just calls #each. Therefore, it is easy to make 'for..in' work with a custom collection: just define #each. And that is what you would do anyway if you were defining an Enumerable custom collection.