创建兼容的XHR对象

     function createXHR () {//创建XMLHttpRequest对象
         var xhr=null;
         if(window.XMLHttpRequest){
             createXHR=createStandardXHR; // Firefox, Opera 8.0+,Safari，IE7+
         }
         else{
             createXHR=createCompatibleXHR; // IE5、IE6
         }

         try{
             xhr=createXHR();
         }
         catch(e){
             createXHR = createErrorXHR;
             xhr=createXHR();
         }

         return xhr;
     }

     function createStandardXHR(){
         return new XMLHttpRequest();
     }

     function createCompatibleXHR(){
         return new ActiveXObject("Microsoft.XMLHTTP");
     }

     function createErrorXHR(){
         alert("Your browser does not support XMLHTTP.");
         return null;
     }