stats-provider.ts

class StatsProvider {
  private getSwitchedCellStrength(): Observable<number> {
      return this.switch$
        .switchMap((deviceId: string) =>
          this.db
            .list(this.paths.sensorStatList(deviceId, CELL_STRENGTH_KEY), {
              query: {
                orderByChild: 'datetime',
                limitToLast: 1
              }
            })
            .takeUntil(this.device.deviceChange$)
        )
        .filter((stats: Stat<any>[]) => stats != null)
        .filter(stats => stats[0])
        .map((stats): number => stats[0].val)
    }
  }