不同浏览器中创建XHR对象

function createXmlHttp()
{
    var MSXML = ["Msxml2.XMLHTTP.6.0","Msxml2.XMLHTTP.5.0","Msxml2.XMLHTTP.4.0","Msxml2.XMLHTTP.3.0","Msxml2.XMLHTTP","Microsoft.XMLHTTP"];  
    if( window.XMLHttpRequest )    //W3C标准(firefox、360、IE8等)
    {
        xmlHttp = new XMLHttpRequest();
    }
    else if( window.ActiveXObject )    //主要IE的低版本
    {       
        for( var n = 0; n < MSXML.length; n++)
        {
            try
            {
                xmlHttp = new ActiveXObject(MSXML[n]);
                if( xmlHttp )
                {
                    break;
                }
            }
            catch(e) {  }
        }
    }
    else
    {
        xmlHttp = null;
    }
}