Ajax的完整兼容各种浏览器版本代码

<script type="text/javascript">

function createAjax(){
        var request=false;

        //window对象中有XMLHttpRequest存在就是非IE，有可能是（IE7，IE8）
        if(window.XMLHttpRequest){
            request=new XMLHttpRequest();

            if(request.overrideMimeType){
                request.overrideMimeType("text/xml");
            }

            //window对象中有ActiveXObject属性存在就是IE中的Microsoft.XMLHTTP代表不同IE版本的参数

        }else if(window.ActiveXObject){
            //ActiveXOject('Microsoft.XMLHTTP')  
            var versions=['Microsoft.XMLHTTP', 'MSXML.XMLHTTP', 'Msxml2.XMLHTTP.7.0','Msxml2.XMLHTTP.6.0','Msxml2.XMLHTTP.5.0', 'Msxml2.XMLHTTP.4.0', 'MSXML2.XMLHTTP.3.0', 'MSXML2.XMLHTTP'];

            for(var i=0; i<versions.length; i++){
                try{
                    request=new ActiveXObject(versions[i]);

                    if(request){
                        return request;
                    }
                }catch(e){
                    request=false;
                }
            }
        }
        return request;
}

    var ajax=createAjax();

    alert(ajax);
        </script>