Unable to chain iterators into a new iterator that isn't iterable

Question

I've got two iterators that may or may not be iterable. If they were both guaranteed to be iterable, I could probably use an array and call .values() to get another iterable iterator:

[...iter1, ...iter2].values(); // simple :)

However, I'm struggling to find a way to chain them together if they aren't iterable. I considered using a generator function, but that would return a generator and not an iterator. Ideally, the returned iterator should also not be iterable.

This returns an iterable iterator and only works for iterable iterators:

const iter1 = [1, 2, 3].values();
const iter2 = [4, 5, 6].values();

console.log([...[...iter1, ...iter2].values()]);

This works for iterators that aren't iterable but instead returns a generator:

const iter1 = [1, 2, 3].values();
const iter2 = [4, 5, 6].values();

console.log([...(function* () {
  let item;
  while (!(item = iter1.next()).done) yield item.value;
  while (!(item = iter2.next()).done) yield item.value;
})()]);

So how would I take two iterators and chain them together to create a new iterator?

---

Maybe an XY problem; I'm concerned about iterability since TypeScript has separate interfaces: Iterator and IterableIterator. Is it implying that you can create an iterator that isn't iterable, or are all iterators in JavaScript iterable, and TypeScript is just being weird?

Answer 1

I'll post the obvious answer that despite being old-fashioned and baroque, seems to fit the requirements:

class ConsIterable{
    constructor(...iterators){
        this.iterators = iterators;
        this.current = 0;
    }
    next(){
        let result;
        do{
            result = this.iterators[this.current].next();
            if(result.done){
                this.current++;
            }
        }while(result.done && this.current < this.iterators.length);
        return result;
    }
}

const iter1 = [1, 2, 3].values();
const iter2 = [4, 5, 6].values();

const chained_iterable = new ConsIterable(iter1, iter2);
let nxt;
do{
    nxt = chained_iterable.next();
    console.log(nxt);
}while(!nxt.done)