Nest js get response body in middleware

Question

I am developing logging middleware, and want to log response body in 500 responses. In middleware im setting a callback for "finish" event on request, that emits after the response was sent, but no response body in Response object is present.

I assume there is no body because nest js with express under the hood sends response by chanks, and does not saves it in Request or Response objects.

But I hope there is a workaround to get response body from Response object in middleware that someone found. The example of middleware class im working on:

@Injectable()
export class LoggerMiddleware implements NestMiddleware {
  use(request: Request, response: Response, next: NextFunction): void {
    const { ip, method, originalUrl } = request;
    const userAgent = request.get("user-agent") || "";

    response.on("finish", () => {
      const { statusCode } = response;
      const contentLength = response.get("content-length");

      const basicRequestMetaInfo = {
        method,
        originalUrl,
        statusCode,
        ... // other data
      };

      if (this.isErroneousStatusCode(statusCode)) {
        const additionalRequestMetaInfo = {
          body: response.body, // The place i want to reach Response body
        };

        this.winstonLoggerErrorLevel.error({
          ...basicRequestMetaInfo,
          ...additionalRequestMetaInfo,
        });
        return;
      }
      this.winstonLoggerInfoLevel.info(basicRequestMetaInfo);
    });
    next();
  }
}

Answer 1

This article explains how to transform the buffer chunks in a readable response.

This is the code from the article that you would need, with some typescript typing you can change as needed:

const getResponseLog = (res: Response) => {
  const rawResponse = res.write;
  const rawResponseEnd = res.end;
  const chunkBuffers = [];
  res.write = (...chunks) => {
    const resArgs = [];
    for (let i = 0; i < chunks.length; i++) {
      resArgs[i] = chunks[i];
      if (!resArgs[i]) {
        res.once('drain', res.write);
        i--;
      }
    }
    if (resArgs[0]) {
      chunkBuffers.push(Buffer.from(resArgs[0]));
    }
    return rawResponse.apply(res, resArgs);
  };
  console.log(`Done writing, beginning res.end`);
  res.end = (...chunk) => {
    const resArgs = [];
    for (let i = 0; i < chunk.length; i++) {
      resArgs[i] = chunk[i];
    }
    if (resArgs[0]) {
      chunkBuffers.push(Buffer.from(resArgs[0]));
    }
    const body = Buffer.concat(chunkBuffers).toString('utf8');
    res.setHeader('origin', 'restjs-req-res-logging-repo');
     const responseLog = {
      response: {
        statusCode: res.statusCode,
        body: JSON.parse(body) || body || {},
        // Returns a shallow copy of the current outgoing headers
        headers: res.getHeaders(),
      },
    };
    console.log('res: ', responseLog);
    rawResponseEnd.apply(res, resArgs);
    return responseLog as unknown as Response;
  };
};