ajax onreadystatechange object error

Question

js中的代码：function f(){try {if (ajaxObj) {ajaxObj.open("GET", url, true);ajaxObj.onreadystatechange = setViewed();ajaxObj.send(null);} else {alert("Ajax 实例不存在！");}} catch (ex) {alert("错误：" + ex);}}function setViewed() {var t;if (ajaxObj.readyState == 4 && ajaxObj.status == 200) {t = ajaxObj.responseText;alert(t);}}经测试,ajaxObj的创建时成功的，url是正确的，可是运行发生错误“错误：object Error”，要是把 ajaxObj.onreadystatechange = setViewed();一行注释掉，则不会报错，当然也不会调用 setViewed()。请问这是什么原因，这是我从原来成功运行的文件里拷贝过来的代码。（运行环境：IE7，Vista，iis7）

Answer 1

ajaxObj.onreadystatechange = setViewed();
换成
ajaxObj.onreadystatechange =function(){
setViewed();
}

Answer 2

改成
ajaxObj.onreadystatechange = setViewed;
其他地方不用动