Check out example codes for "where to put toaster on http service calls typescript". It will help you in understanding the concepts better.

Code Example 1

testApi(viewData: any): Observable<any> {

    return this.http
      .delete<any[]>(URL, viewData)
      .pipe(
        map((res: any) => {
		  // add snackbar or toast function here;
          return res;
        })
      )
      .pipe(
        catchError((error: any) => {
          // add snackbar or toast function here;
          return throwError(error);
        })
      );
  }

Code Example 2

@Injectable()
export class HttpInterceptor implements HttpInterceptor {
constructor(public toasterService: ToastrService) {}

intercept(
    req: HttpRequest<any>,
    next: HttpHandler
  ): Observable<HttpEvent<any>> {

    return next.handle(req).pipe(
        tap(evt => {
            if (evt instanceof HttpResponse) {
                if(evt.body && evt.body.success)
                    this.toasterService.success(evt.body.success.message, evt.body.success.title, { positionClass: 'toast-bottom-center' });
            }
        }),
        catchError((err: any) => {
            if(err instanceof HttpErrorResponse) {
                try {
                    this.toasterService.error(err.error.message, err.error.title, { positionClass: 'toast-bottom-center' });
                } catch(e) {
                    this.toasterService.error('An error occurred', '', { positionClass: 'toast-bottom-center' });
                }
                //log error 
            }
            return of(err);
        }));
  }
}

Learn ReactJs, React Native from akashmittal.com