req.on('response') not working in plugins

I’m creating a custom policy plugin to transform response. Since EG middleware is essentially express middleware, I tried to add a listener for response event:

policy: () => (req, res, next) => {
  req.on('response', () => {
  }
}

But the response event is never triggered, and neither is any of the other events for HttpClientRequest (https://nodejs.org/api/http.html#http_class_http_clientrequest).

For context, I’m creating a plugin to transform the response body. But the response I’m getting from the server is in chunks (header set to Transfer-Encoding: chunked) Overriding res.write, as described here doesn’t work because I have to join all the chunks before doing any transformation.

[jay-finn]

@XVincentX thanks for looking into this.

I have a policy plugin which is based on the response-transformer. I’m overriding the res.write as well.

The difference is that the service sending the response is sending it in chunks, so the data in

return (req, res, next) => {
  res.write = (data) => { 
    JSON.parse(data)
  }
}

is an invalid JSON. In this specific case the response from the service is sent in 3 chunks, and all 3 chunks combined make a valid JSON. But the policy is trying to parse the JSON data for each chunk which obviously fails.

I thought the req in this case is the request that’s being proxied to my service and I could just add a req.on('response') event listener and wait for the full response to be received before doing JSON.parse(). But like you pointed out, req is an instance of IncomingMessage so there’s no response event. What would you recommend in this case?

[xinjiguaike]

I guess you could be using a streaming serialiser (you can pick up any on NPM) to accumulate the buffer and then send it once calling the original _write.

@XVincentX can you give an example, I really have no idea how to process with this !