创建httpRequest对象

    var xmlHttp;    try    {        //Firefox Opera 8.0+ Safari        xmlHttp = new XMLHttpRequest();    }    catch (e)     {        //Internet Explorer 6.0+        try         {            xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");        }        catch (e)         {            //Internet Explorer 5.5+            try             {                xmlHttp = new ActiveXObject("Microsoft.XMLHTTP");            }            catch (e)             {                alert("Your browser does not support AJAX");                return false;            }                    }    }