javascript跨浏览器创建XMLHttpRequest

//定义xmlhttprequest对象
    var xmlrequest;
    //完成xmlhttprequest对象的初始化
    function createXMLHttpRequest() {
        if (window.XMLHttpRequest) {
            //DOM2浏览器
            xmlrequest = new XMLHttpRequest();
        }

        else if (window.ActiveXObject) {
            //将Internet Explorer的不同xmlhttp实现声明成数组
            var MSXML = ['MSXML2.XMLHTTP.5.0','MSXML2.XMLHTTP.4.0',
                'MSXML2.XMLHTTP.3.0', 'MSXML2.XMLHTTP', 'Microsoft.XMLHTTP'];
            //依次对每个XMLHTTP实现创建XMLHttpRequest对象
            for(var n= 0;n<MSXML.length;n++){
                try{
                    xmlrequest=new ActiveXObject(MSXML[n]);
                    break;
                }
                catch (e){}
            }
        }
    }