Trial of my own JS AJAX module

ajax.js

(function(window, document, undefined) {

    /**
     * @returns {XMLHttpRequest|ActiveXObject}
     */
    function initialise () {
        if (window.XMLHttpRequest) {
            // [ IE7+, FireFox, Chrome, Opera, Safari ]
            return new XMLHttpRequest();
        } else {
            // [ IE5, IE6 ]
            return new ActiveXObject("Microsoft.XMLHTTP");
        };
    }

    var Ajax = {

        /**
         * Get a resource via a URL
         *
         * @param string   url
         * @param callable callback
         */
        get: function(url, callback) {

            var xmlHttp = initialise();

            xmlHttp.onreadystatechange = function() {
                if (xmlHttp.readyState == 4 ) {
                    if (typeof callback != 'undefined') {
                        return callback(xmlHttp);
                    };
                }
            }

            xmlHttp.open('GET', url, true);
            xmlHttp.setRequestHeader('X-Requested-With','XMLHttpRequest');
            xmlHttp.send();
        },

        /**
         * Put content onto the server
         *
         * Used to *create* or *replace* the entity content.
         *
         * @param string   url
         * @param mixed    data
         * @param callable callback
         */
        put: function(url, data, callback) {
            var xmlHttp = initialise();

            xmlHttp.onreadystatechange = function() {
                if (xmlHttp.readyState == 4) {
                    if (typeof callback === 'function') {
                        return callback(xmlHttp);
                    };
                };
            }

            xmlHttp.open('PUT', url, false);
            xmlHttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
            xmlHttp.setRequestHeader('X-Requested-With','XMLHttpRequest');
            xmlHttp.send(data);
        },

        /**
         * Patch content on the server
         *
         * Used to *update* part of an entity
         *
         * @param string   url
         * @param mixed    data
         * @param callable callback
         */
        patch: function(url, data, callback) {
            var xmlHttp = initialise();

            xmlHttp.onreadystatechange = function() {
                if (xmlHttp.readyState == 4) {
                    if (typeof callback == 'function') {
                        return callback(xmlHttp);
                    }
                }
            }

            xmlHttp.open('PATCH', url, false);
            xmlHttp.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded');
            xmlHttp.setRequestHeader('X-Requested-With', 'XMLHttpRequest');
            xmlHttp.send(data);
        },

        /**
         * Delete a resource from the server
         *
         * Used to *remove* an entity
         *
         * @param url
         * @param callback
         */
        delete: function(url, callback) {
            var xmlHttp = initialise();

            xmlHttp.onreadystatechange = function() {
                if (xmlHttp.readyState == 4) {
                    if (typeof callback === 'function') {
                        return callback(xmlHttp);
                    };
                };
            }

            xmlHttp.open('DELETE', url, false);
            xmlHttp.setRequestHeader('X-Requested-With','XMLHttpRequest');
            xmlHttp.send();
        }

    }

    window.Ajax = Ajax;

    if (typeof define === 'function' && define.amd) {
        define(Ajax);
    };

}) (window, document);