switchMap

initTime = new Date().getTime();
lowerOrders: Observable<string>[] = [];
higherOrder = timer(0, 2000).pipe(
  take(4),
  mapNumberToChar(),
);
firstOrder = this.higherOrder.pipe(
  switchMap(val => {
    const lowerOrder = timer(0, 1000).pipe(
      take(4),
      map(innerVal => val + innerVal),
    );
    this.lowerOrders.push(lowerOrder);
    return lowerOrder;
  }),
);