不同浏览器创建XMLHttpRequest对象

 function getXHR() {

     if (XMLHttpRequest) {

         return new XMLHttpRequest();

     }

     else {

         return new ActiveXObject("Microsoft.XMLHTTP");

     }

 }

 function getXHR() {

     if (typeof XMLHttpRequest == "undefined") {

         XMLHttpRequest = function() {

             try { return new ActiveXObject("Msxml2.XMLHTTP.6.0"); } catch (e) { }

             try { return new ActiveXObject("Msxml2.XMLHTTP.3.0"); } catch (e) { }

             try { return new ActiveXObject("Msxml2.XMLHTTP"); } catch (e) { }

             return false;

         }

     }

     return new XMLHttpRequest();

 }