JS实现对ajax的封装

//本来想写个ajax的封装的，但在情封的项目中无意看到这段源码，和我刚开始的构思差不多，果断copy过来参考，做了一点修改。 
var Ajax = { 
    createXHR:function () { 
        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) { 
 
                    } 
                } 
            } 
            return new ActiveXObject(arguments.callee.activeXString); 
        } else { 
            throw new Error('No xhr object available'); 
        } 
    }, 
    responseType:function (request) { 
        var _responseType = request.getResponseHeader('Content-Type'); 
        switch (_responseType) { 
            case 'text/xml': 
                return request.responseXML; 
            case 'text/json': 
            case 'text/javascript': 
            case 'application/javascript': 
            case 'application/x-javascript': 
                return eval('(' + request.responseText + ')'); 
            default: 
                return request.responseText; 
        } 
    }, 
    encodeFormData:function (data) { 
        var _pairs = []; 
        var _regexp = /%20/g; 
        for (var name in data) { 
            var _value = data[name].toString(); 
            var _pair = encodeURIComponent(name).replace(_regexp, '+') + '=' + encodeURIComponent(_value).replace(_regexp, '+'); 
            _pairs.push(_pair); 
        } 
        return _pair.join('&'); 
    }, 
    /** 
     * 执行ajax请求 
     * @param {String} method 'GET' || 'POST' 
     * @param {String} url  
     * @param {Boolean} asyn 是否异步加载 
     * @param {Object} callback {success,failure} 回调对象，存放函数 
     * @param value 
     */ 
    ajaxRequest:function (method, url, asyn, callback, value) { 
        var _xhr = this.createXHR(); 
        _xhr.onreadystatechange = function () { 
            if (_xhr.readyState === 4) { 
                if (_xhr.status >= 200 && _xhr.status < 300 || _xhr.status === 300) { 
                    callback.success(this.responseType(_xhr)); 
                } else { 
                    if (callback.failure) { 
                        callback.failure(_xhr.status, _xhr.statusText); 
                    } 
                } 
            } 
        }; 
 
        _xhr.open(method, url, asyn); 
 
        if (method === 'GET') { 
            _xhr.send(null); 
        } else if (method === 'POST') { 
            _xhr.setRequestHeader('Content-Type', 'application/x-www-form-urlencoded'); 
            _xhr.send(this.encodeFormData(value)); 
        } 
    } 
};