javascript不执行

    <script type="text/javascript">
        function submit()
        {
            //alert("test");
            //1.创建XMLHttpRequest对象;
            if(window.XMLHttpRequest)
            {
                alert("firefox");
                var xmlhttp = new XMLHttpRequest;
                if(xmlhttp.overrideMimeType)
                {
                    xmlhttp.overrideMimeType("text/html");
                }
            }
            else if(window.ActiveXObject)
            {
                activexName = ["MSXML2.XMLHTTP.6.0","MSXML2.XMLHTTP.5.0","MSXML2.XMLHTTP.4.0"
                ,"MSXML2.XMLHTTP.3.0","MSXML2.XMLHTTP","Miscrosoft.XMLHTTP"];
                for(var i=0;i<activexName.length;i++)
                    {
                        try{
                            var xmlhttp = new ActiveXObject(activexName[i]);
                            break;
                        }
                        catch(e){
                        }
                    }
            }
            
            if(xmlhttp==undefinded||xmlhttp==null)
            {
                alert("当前浏览器不支持");
                return;
            }
            alert(xmlhttp);
        }
    </script>