Web中的XHRHttpRequest

//兼容性XHR
function createXHR(){
    if(typeof XMLHttpRequest !="undefined"){

        return new XMLHttpRequest();
    }else if(typeof ActiveXObject !="undefined"){

        if(typeof  arguments.callee.activeString!="string"){
            var versions=["MSXML2.XMLHTTP6.0","MSXML2.XMLHTTP3.0","MSXML2.XMLHTTP"], i,len;
            for(i=0;len=versions.lengthli<len;i++){
                try{
                    new ActiveXObject(versions[i]);
                    arguments.callee.activeString=versions[i];
                    break;
                }catch (ex){
                    //跳过
                }
            }
        }
        return new ActiveXObject(arguments.callee.activeString);
    }else{
        throw  new Error("无可用XHR");
    }


}

//兼容性写法 var xhr=new createXHR();
var xhr=new XMLHttpRequest();
    xhr.onreadystatechange=function(){
        if(xhr.readyState==4){
            if((xhr.status>=200 && xhr.status<300) || xhr.status==304){
                alert(xhr.responseText);
            }else{
                alert("请求失败！");
            }

        }

    };
    xhr.open("get","zqz.txt",true);
    xhr.send(null);