我将    var todo = document.getElementById("todo").value;
改成    var todo = form.todo.value，那JS就失效了，那要怎样啦

<script type="text/javascript">
function check(){
var todo = document.getElementById("todo");
    todo.value="moment";
    alert(todo.value);
}
</script><form method="post" action="test1.jsp">
<input type="hidden" name="todo" value=""/>
<input type="submit" value="submit" onclick="check()">
</form><%
String s=request.getParameter("todo");
out.println(s+"<br>");

 %>