Dorus

const flatMap = (fn, stream, resultSelector) => 
    stream.flatMap((x, xi) => fn(x).map((y, yi) => resultSelector(x, y, xi, yi)));

const flatMapLatest = (fn, stream) => 
    stream.publish(s => s.flatMap(v => fn(v).takeUntil(s)));

const flatMapLatest = (fn, stream, resultSelector) => stream.publish(s => {
    return s.flatMap(v => fn(v), resultSelector).takeUntil(s));
});