keywords$ = this.keywordsControl.valueChanges;
data$ = keywords$.pipe(
  /* Wait for the user to stop typing for 100ms and emit last value. */
  debounceTime(100),
  /* Ignore identical successive values
   * (e.g. user pastes the same value in the input). */
  distinctUntilChanged(), 
  /* when new keywords are emitted, this unsubscribes from the previous
   * search result (canceling the underlying http request)
   * and subscribes to the new one. */
  switchMap(keywords => this.search(keywords))
)

function interval(period) {
  return new Observable(observer => {
    let i = 0;
    const handle = setInterval(() => observer.next(i++), period);
    /* This is the teardown logic. */
    return () => clearInterval(handle);
  });
}

function getFiles(directoryPath) {
  return new Observable(observer => {
    ...
    return () => walker.pause();
  }
}

function readLines(filePath) {
  return new Observable(observer => {
    ...
    return () => reader.close();
  }
}

function search(): Observable<Line[]> {
  return getFiles(nodeModulesPath)
    .pipe(
      mergeMap(file => readLines(file)),
      ...
    );
}

  public async transformToResult(resultOrDeferred: any) {
    if (resultOrDeferred && isFunction(resultOrDeferred.subscribe)) {
      return resultOrDeferred.toPromise();
    }
    return resultOrDeferred;
  }

@Injectable()
export class NoopInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
    return next.handle();
  }
}

@Injectable()
export class UnsubscribeOnCloseInterceptor implements NestInterceptor {
  intercept(context: ExecutionContext, next: CallHandler): Observable<unknown> {
    if (context.getType() !== 'http') {
      return next.handle();
    }

    const request = context.switchToHttp().getRequest() as Request;

    const close$ = fromEvent(request, 'close');

    return next.handle().pipe(takeUntil(close$));
  }
}