Creating a simple Node.js web service for use with AJAX

I have a basic Node.js http server set up as per the docs, I'm trying to create a very simple web service I can talk to with Javascript (AJAX) which I'll then hook up to Arduino. It's a test piece, I may go down the road of something else but this is proving to be fun. Anyway, here is the server-side javascript:

var http = require('http');
var _return = 'Bing Bong';

  http.createServer(function (req, res) {
        res.writeHead(200, {'Content-Type': 'application/jsonp'});

    //do stuff
    res.end('_return(\'Hello :-)\' + _return)');
}).listen(process.env.VMC_APP_PORT || 1337, null);

And this is the client side:

function experimentFive(node) {
    console.log('X5 Func started');

    console.log('Calling Node.js service');

    var nodeURL = node;

    $.ajax({
        url: nodeURL,
        dataType: "jsonp",
        jsonpCallback: "_return",
        cache: false,
        timeout: 50000,
        success: function(data) {
            console.log('Data is: ' + data);
            $("#nodeString").text(" ");
            $("#nodeString").append(data);
        },
        error: function(jqXHR, textStatus, errorThrown) {
            console.log('error : ' + textStatus + " " + errorThrown);
        }
    });
}

experimentFive('http://fingerpuk.eu01.aws.af.cm/');

This works, I get the data back but not as I'd expect. What I'd like to be able to do is change some data server side and receive that back. I get back the string in the res.end plus:

function () {
        responseContainer = arguments;
    } 

Instead of the variable's data. No matter how I try to get that data back, it is either undefined or this response.

From research I think the variable is not having it's data set before the callback fires, so it's empty. But if I set an initial value the data is still undefined.

Am I missing something very simple here? Should I just go back to C#?

Cheers.