apppp.module.ts

providers: [
  { provide: HTTP_INTERCEPTORS, useClass: OfflineInterceptor, multi: true }
]

OfflineInterceptor.ts

@Injectable()
export class OfflineInterceptor {
  private onlineChanges$ = fromEvent(window, 'online').pipe(mapTo(true));
  
  get isOnline() {
    return navigator.onLine;
  }

  intercept(req, next) {
    return next.handle(req).pipe(
      retryWhen(errors => {
        if (this.isOnline) {
          return errors.pipe(switchMap(err => throwError(err)));
        }

        return this.onlineChanges$;
      })
    );
  }
}