create xmlhttprequest

function createXHR() {
        if (typeof XMLHttpRequest != "undefined") {
            return new XMLHttpRequest();
        } else if (typeof ActiveXObject != "undefined") {
            if (typeof arguments.callee.activeXString != "string") {
                var versions = ["MSXML2.XMLHttp.6.0", "MSXML2.XMLHttp.3.0", "MSXML2.XMLHttp"];

                for (var i = 0, len = versions.length; i < len; i++) {
                    try {
                        var xhr = new ActiveXObject(versions[i]);
                        arguments.callee.activeXString = versions[i];
                        return xhr;
                    } catch (ex) {
                        //omit
                    }
                }
            }
            return new ActiveXObject(arguments.callee.activeXString);
        } else {
        throw new Error("no xhr object available.");
        }
    }