Getting returned value from a JS / Node Module

I've created a simple module that posts some data to an external service which returns a message and some other results.

Am trying to test this with Mocha but I'm finding it hard to understand how to access the returned value.

I can see it logged in the console but don't know how to set it as a variable. As you'll not doubt notice, I am a novice javacripter. I'm sure it's simple, I just can't see how.

My module:

module.exports = {

  foo: function(id,serial) {
    var querystring = require('querystring');
    var http = require('http');
    var fs = require('fs');
    var post_data = querystring.stringify({
        'serial' : serial,
        'id': id
    });

    var post_options = {
        host: 'localhost',
        port: '8080',
        path: '/api/v1/status',
        method: 'POST',
        headers: {
            'Content-Type': 'application/x-www-form-urlencoded',
            'Content-Length': post_data.length
        }
    };

    var post_req = http.request(post_options, function(res) {
        res.setEncoding('utf8');
        res.on('data', function (chunk) {
            console.log(chunk);
            return chunk;
        });
    });

    post_req.write(post_data);
    post_req.end();
  }
}

And I've called this with:

describe('Functions and modules', function() {
  it('does some shizzle', function(done) {
    var tools = require('../tools');
    chunk = '';
    id = 123;
    serial =456;
    tools.foo(id,serial);
    chunk.should.equal.......
  });
});

I basically need the returned message from tools.foo(id,serial) However chunk is blanker than a blank thing.

In my terminal I can see something like:

{"message":"This device is still in use","live":"nop"}

You cannot access the "returned" value the way you would in other languages. Http requests in node are asynchronous, and don't return their values. Instead, you pass in a call back function, or create a callback function within the scope of the same request. For example you could complete your function like this: (I removed some of the filler)

module.exports = {

    foo: function (options, data, callback) {
        'use strict';

        var completeData = '';

        var post_req = http.request(options, function (res) {
            res.setEncoding('utf8');

            res.on('data', function (chunk) {
                console.log(chunk);
                completeData += chunk;
                return chunk;
            });

            res.on('end', function () {
                callback(completeData);
                //You can't return the data, but you can pass foo a callback,
                //and call that function with the data you collected
                //as the argument.
            });

        });

        post_req.write(data);
        post_req.end();
    }
};

function someCallBackFunction (data) {
    console.log("THE DATA: " + data);
}

var someOptions = "whatever your options are";
var someData = "whatever your data is";

module.exports.foo(someOptions, someData, someCallBackFunction);

If the function your defining is at the same scope, you could also access someCallBackFunction directly within the scope of foo, but passing in the callback is better style.