check_forwarded_request.ts

export function buildRequestHandler(): RequestHandler {

  return (externalRequest: Request, externalResponse: Response, next: NextFunction) => {

    const errorHandler = (error: any) => {
      if (externalResponse.headersSent) {
        return next(error);
      }
      externalResponse.status(INTERNAL_SERVER_ERROR).json(createDefaultErrorBody(error));
    };

    const requestCallback: RequestCallback = (error: any, internalResponse: RequestResponse, internalBody: any) => {
      if (error) {
        errorHandler(error);
      } else {
        if (isResponseOk(internalResponse.statusCode, internalBody)) {
          // cannot pipe anymore, because response was already consumed from stream to read body.
          externalResponse.status(internalResponse.statusCode);
          externalResponse.header(internalResponse.headers);
          externalResponse.send(internalBody);
        } else {
          const invalidResponseError = new Error("Internal service sent invalid response.");
          externalResponse.status(INTERNAL_SERVER_ERROR).json(createDefaultErrorBody(invalidResponseError));
        }
      }
    };

    const internalRequest = request(targetUrl, requestCallback); 

    externalRequest.pipe(internalRequest);
  };
}