获取xmlhttp对象

/创建XHR
    function CreateXHR()
    {
        if(typeof XMLHttpRequest!="undefined")
        {
            return new XMLHttpRequest(); 
        }
        else if(window.ActiveXObject)
        {
            var aVersions=["MSXML2.XMLHttp.5.0","MSXML2.XMLHttp.4.0","MSXML2.XMLHttp.3.0","MSXML2.XMLHttp","Microsoft.XMLHttp"]
            for(var i=0;i<aVersions.length;i++)
            {
                try
                {
                    var oXHR=new ActiveXObject(aVersions[i]);
                    return oXHR;
                }
                catch(oError)
                {
                    //do nothing
                }
            }
        }
        throw new Error("XMLHttp Object could be created.")
    }