Question

I am using node-http-proxy. However, in addition to relaying HTTP requests, I also need to listen to the incoming and outgoing data.

Intercepting the response data is where I’m struggling. Node’s ServerResponse object (and more generically the WritableStream interface) doesn’t broadcast a 'data' event. http-proxy seems to create it’s own internal request, which produces a ClientResponse object (which does broadcast the 'data' event) however this object is not exposed publically outside the proxy.

Any ideas how to solve this without monkey-patching node-http-proxy or creating a wrapper around the response object?

Answer 1

Related issue in issues of node-http-proxy on Github seems to imply this is not possible. For future attempts by others, here is how I hacked the issue:

• you’ll quickly find out that the proxy is only calling writeHead(), write() and end() methods of the res object
• since res is already an EventEmitter, you can start emitting new custom events
• listen for these new events to assemble the response data and then use it

var eventifyResponse = function(res) {
  var methods = ['writeHead', 'write', 'end'];
  methods.forEach(function(method){
    var oldMethod = res[method]; // remember original method
    res[method] = function() {   // replace with a wrapper
      oldMethod.apply(this, arguments); // call original method
      arguments = Array.prototype.slice.call(arguments, 0);
      arguments.unshift("method_" + method);
      this.emit.apply(this, arguments); // broadcast the event
    };
  });
};

res = eventifyResponse(res), outputData = '';

res.on('method_writeHead', function(statusCode, headers) { saveHeaders(); });
res.on('method_write',     function(data) { outputData += data; });
res.on('method_end',       function(data) { use_data(outputData + data); });
proxy.proxyRequest(req, res, options)