如何创建ajax对象？

<script>
    function createAjax(){
        var request=false;
        
        //window对象中有XMLHttpRequest存在就是非IE，包括（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
        }else if(window.ActiveXObject){
            
            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>