XHR的对象及用法

function  createXHR(){         //检测原生XHR对象是否存在，如果存在刚返回它的新实例；     //如果不存在，则检测ActiveX对象;     //如果两个都不存在，就抛出一个错误。         if(typeof xmlHttPRequest != "undefined"){         return new xmlhttpRequest();     }else if(typeof ActiveXObject != "undefined"){         //适合IE7之前的版本         if(typeof arguments.callee.activeXString != "string"){             var versions = ["MSXML2.XMLHttp.6.0","MSXML2.XMLHttp.3.0","MSXML.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){                     //跳过                 }             }         }                 return new ActiveXObject(arguments.callee.activeXString);     }else{         throw new Error("No XHR object available.");     }; }

//创建XHR对象 var xhr = createXHR(); xhr.onreadystatechange =  function(){     if(xhr.readyState == 4){         if((xhr.status >=200 && xhr.status < 300 ) || xhr.status == 304 ){             alert(xhr.responseText);         }else{             alert("Request was unsuccessful : " + xhr.status);         }     } }   

//读取example文本 xhr.open("get","example.txt",true); xhr.send(null);