Listen ajax responses in phantomJS

//example
listenAjaxResponses(page, 'getUserDetails', function(responseData)
{
    // this will be executed within the sandboxed page, not within the phantomjs
    alert('User details has requested!');
    var username = eval('(' + responseData + ')').user.username;
    alert(username);
});

//listener for all ajax responses of the page
function listenAjaxResponses(page, urlPattern, callback)
{
    page.evaluate(function(urlPattern, callback)
    {
        try
        {
            (function(open)
            {
                XMLHttpRequest.prototype.open = function(method, url, async, user, pass)
                {
                    this.addEventListener("readystatechange", function()
                    {
                        if (url.indexOf(urlPattern) === 0)
                        {
                            if (this.readyState == XMLHttpRequest.DONE)
                            {
                                callback(this.responseText);
                            }
                        }
                    }, false);

                    open.call(this, method, url, async, user, pass);
                };

            })(XMLHttpRequest.prototype.open);
        }
        catch (ex)
        {
            alert(ex);
        }
    }, urlPattern, callback);
}

//create a blank page
function createPage(viewportSize)
{
    var page = webpage.create();

    page.settings.webSecurityEnabled = false;
    page.settings.localToRemoteUrlAccessEnabled = true;
    page.settings.userAgent = 'Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/39.0.2171.65 Safari/537.36';
    if (viewportSize) page.viewportSize = viewportSize;

    page.onAlert = function(error)
    {
        console.log('Alert: {0}'.format(error));
    };

    page.onError = function(error, trace)
    {
        var msgStack = ['ERROR: {0}'.format(error)];

        if (trace && trace.length)
        {
            msgStack.push('TRACE:');

            trace.forEach(function(t)
            {
                msgStack.push(' -> {file}: {line} (in function "{function}")'.format(t));
            });
        }
        console.log('Error: {0}'.format(JSON.stringify(trace)));
    };

    return page;
}

//formatting function
//example 1: 'loren {0} dolor {1} amet'.format(['ipsum', 'sit']) = 'loren ipsum dolor sit amet'
//example 2: '{city} is a {{0}} of {country}'.format({city: 'London', country: 'Great Britain'}) = 'London is a {0} of Great Britain'
String.prototype.format = function format()
{
    if (arguments.length == 0) return this;

    var data;
    if (arguments.length > 1) data = arguments;
    else if (typeof arguments[0] == 'object') data = arguments[0];
    else data = arguments;

    return this.split('{{').map(function(q)
    {
        return q.replace(/\{([^\{\}]+)\}/g, function ($0, $1)
            {
                return data[$1];
            });
    }).join('{').replace(/\}\}/g, '}');
};