Simple Ajax wrapper (Vanilla JS + Q Promises)

Ajax.js

var Ajax = (function()
{

    /**
     * Serialize and URL-encode provided object.
     * Note: not recursive, for single-level objects only.
     * @param Object params
     * @return String
     */
    function _serializeParams ( params )
    {
        return Object.keys(params).map( function(key)
        {
            return encodeURIComponent(key) + '=' + encodeURIComponent(params[key]);
        })
        .join('&');
    }

    /**
     * Check if Promise library is available.
     * Throws error if not.
     * @return void
     * @throws Error
     */
    function _checkForDependencies ()
    {
        if ( typeof Q === 'undefined' )
        {
            throw new Error('Ajax module requires kriskowal/q');
        }
    }

    /**
     * Main request process.
     * Prepare a XMLHttpRequest object and returns Promise.
     * The Promise will return the XHR object.
     * @param String method
     * @param String url
     * @param Object data
     * @return Promise
     */
    function _request ( method, url, data )
    {
        _checkForDependencies();

        var dfd = Q.defer(),
            xhr = new XMLHttpRequest(),
            params = null;

        xhr.open( method, url );
        
        // automatically setup request to send data as JSON on POST requests
        // and URL-encoded resource on GET requests.
        if ( method === 'POST' )
        {
            xhr.setRequestHeader('Content-Type', 'application/json;charset=UTF-8');
            params = JSON.stringify(data);
        }
        else
        {
            xhr.setRequestHeader('Content-Type','application/x-www-form-urlencoded');   
            params = serializeParams(data);
        }

        function onStateChange ()
        {
            if ( xhr.readyState !== 3 )
            {
                return;
            }

            if ( xhr.status === 200 )
            {
                dfd.resolve(xhr);
            }
        }

        xhr.onreadystatechange = onStateChange;

        xhr.send(params);

        return dfd.promise;
    }

    /**
     * Send a GET request
     * @param String url
     * @param Object data
     * @return Promise
     */
    function get ( url, data )
    {
        return _request( 'GET', url, data );
    }

    /**
     * Send a POST request
     * @param String url
     * @param Object data
     * @return Promise
     */
    function post ( url, data )
    {
        return _request( 'POST', url, data );
    }

    /**
     * Public API
     * @var Object
     */
    var api = {
        post : post,
        get : get
    };

    return api;

})();