import { catchError, map, tap } from 'rxjs/operators';
getHeroes(): Observable<Hero[]> {
      return this.http.get<Hero[]>(this.heroesUrl)
        .pipe(
          catchError(this.handleError<Hero[]>('getHeroes', []))
        );
    }

/**
 * Handle Http operation that failed.
 * Let the app continue.
 * @param operation - name of the operation that failed
 * @param result - optional value to return as the observable result
 */
private handleError<T>(operation = 'operation', result?: T) {
  return (error: any): Observable<T> => {

    // TODO: send the error to remote logging infrastructure
    console.error(error); // log to console instead

    // TODO: better job of transforming error for user consumption
    this.log(`${operation} failed: ${error.message}`);

    // Let the app keep running by returning an empty result.
    return of(result as T);
  };
}