Recursive observable sequence, depending on previous response and concatening results

ObservableSequence.ts

repeatRequests(req, endpoint, callback): any{
    // Array of all results, returned to the subscribe
    let tempArray = [];

    // Recursive lambda function to chain all observables sequentially
    const recursive = (req, endpoint, callback) => {
      return this.httpService.get(endpoint).pipe(
        flatMap((res) => {
          callback(res.data.items);
          // Concat all results in the result array
          tempArray = tempArray.concat(res.data.items);
          if(res.data.next) {
            // Run the next call if needed
            return recursive(req, res.data.next, callback);
          } else {
            // It's the last call, so we return the full array with all calls results
            return of(tempArray);
          }
        })
      );
    }

    return recursive(req, endpoint, callback);
  }