Ajax的方法和使用代码

 //1.创建Ajax对象，已经封装好的函数：
 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"],
                 i, len;

             for (i=0,len=versions.length; i < len; i++){
                 try {
                     var xhr = new ActiveXObject(versions[i]);
                     arguments.callee.activeXString = versions[i];
                     return xhr;
                 } catch (ex){
                     //skip
                 }
             }
         }

         return new ActiveXObject(arguments.callee.activeXString);
     } else {
         throw new Error("No XHR object available.");
     }
 }

 //直接引用

 var xhr = createXHR(); 

 //2.请求状态监控

 xhr.onreadystatechange = function(event){
     if (xhr.readyState == 4){
         if ((xhr.status >= 200 && xhr.status < 300) || xhr.status == 304){
             alert(xhr.responseText);
         } else {
             alert("Request was unsuccessful: " + xhr.status);
         }
     }
 };
 //3.连接服务器（方法，文件名，异步传输）
 xhr.open("get", "example.txt", true);
 //4.发送请求
 xhr.send(null);