Node.js Javascript Scope Issue

Question

I have a Node.js HTTP server running which goes like this (simplified):

http = require('http');
help = require('./modules/help').run;
router = require('./modules/router').run;
m = {something: require('./modules/something')};

var server = http.createServer(router).listen(8001);

"help" is a set of functions-helpers, for example:

module.exports.run = {
  joinObjects: function(obj1, obj2) {
    for (var prop in obj2) {
       obj1[prop] = obj2[prop];
    }
    return obj1;
  }
}

"router" handles the request (passes it further down and handles response to the client):

module.exports.run = function(req, res) {

  var urlPath = url.parse(req.url).pathname;

  switch(urlPath) {
    case '/something':
      requestHandler(req, res, 'something');
      break;
    ...
  }

  function requestHandler(req, res, handler) {
    var data = '';
    req.on('data', function(chunk) {
      data += chunk;
    }
    req.on('end', function() {
      m[handler].run(req, data, function(response) {
        response.headers = help.joinObjects(config.responseHeaders, response.headers);
        res.writeHead(response.code, response.headers);
        res.end(response.data);
      });
    }
  }

}

The "handler" module/function runs the callback function and passes the response (code, headers and data) to it. The callback function then merges headers with a set of default headers that are set in the config file.

THE ISSUE: When there are two connections calling help.joinObjects() at the same time (that's my guess), response.headers property collides with the one of another user/connection and returns bad data. If I comment out the line that does the merging, this does not occur.

THE QUESTION: What's wrong with my approach to the "help" module? There is some kind of scope issue that I do not understand here.

Answer 1

As lazyhammer pointed out, the problem was the order in which I passed objects into the helper function: help.joinObjects(config.responseHeaders, response.headers) .

Javascript passes variables by reference, so every time help.joinObjects() is called, it overwrites the config property instead of the user-specific object.