获取XMLHttpRequest

function getXMLHttpRequest()
    {
        if(window.XMLHttpRequest)
        {
            return new XMLHttpRequest();
        }
        else
        {
            var progIDs=['Msxml12.XMLHTTP','Microsoft.XMLHTTP'];
            for(var i=0;i<progIDs.length;i++)
            {
            try{
                var xmlhttp=new ActiveXObject(progIDs[i]);
                return xmlhttp;
                }
                catch(ex){}
            }
           
            return null;
        }
    }