Why Javascript can't get value in Struts?

Question

I have a button in Struts,when click this button,it will show a value in Struts,like follows:
<%@ taglib uri="/WEB-INF/struts-html.tld" prefix="form" %>
<script language="JavaScript">
function a(){
alert(username.value);
</script>
<form:form action="/register">
UserName:<form:text property="username"/><br>
enter password:<form:password property="password1"/><br>
re-enter password:<form:password property="password2"/><br>
<form:hidden property="abc" value="3"/><br>
<form:button property="button" value="Test" onclick="a()"/>     
<form:submit value="Register"/>
</form:form>
But when I click Test button,it can't show the value of parameter "username",why? How to show the value of it?
I have a another question,how many attribute of the taglib <form:>? Is there a document for it?
Thanks!

Answer 1

But when I click Test button,it can't show the value of parameter "username",why? How to show the value of it?
Paz>>
<script language="JavaScript">
function a(){
alert(document.forms[0].username.value);
</script>I have a another question,how many attribute of the taglib <form:>? Is there a document for it?
Paz>>
http://struts.apache.org//struts-doc-1.1/userGuide/struts-html.html