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();
 }