Ajax--readyState一直为0

readyState 一直为0表示对象已建立，连接未建立的状态

我写的代码：

    function LoadXMLDoc(url,scope,orderID){
        if(window.XMLHttpRequest){
            xmlhttp = new XMLHttpRequest();
        }else if(window.ActiveXObject){
            xmlhttp = new ActiveXObject("Microsoft.XMLHTTP");
        }
        alert(xmlhttp);
        if(xmlhttp!=null){
            xmlhttp.onreadystatechange = changeState();
            xmlhttp.open("post",url,true);
            xmlhttp.setRequestHeader("Content-type","application/x-www-form-urlencoded");
            xmlhttp.send("scope="+scope+"&orderID="+orderID);
        }
    }

    function changeState(){
        alert("q");
        alert('state:'+xmlhttp.readyState);
        alert('status:'+xmlhttp.status);
        if(xmlhttp.readyState==4 && xmlhttp.status==200){
            alert("Q");
            window.location.reload();
        }
    }

后来终于解决了，是因为这句：xmlhttp.onreadystatechange = changeState();这样写的话xmlhttp.onreadystatechange为将changeState的返回值赋给xmlhttp.onreadystatechange；而xmlhttp.onreadystatechange为当状态改变时调用函数changeState，所以应该这样写：xmlhttp.onreadystatechange=changgeState，如果有参数的话这样写：xmlhttp.onreadystatechange = functiong (){changeState(a,b);};