If you are looking for a javascript command line interpreter you should really consider using Firebug.

http://www.getfirebug.com/

Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

Firebug includes a powerful JavaScript debugger that lets you pause execution at any time and have look at the state of the world. If your code is a little sluggish, use the JavaScript profiler to measure performance and find bottlenecks fast.

Having a fancy JavaScript debugger is great, but sometimes the fastest way to find bugs is just to dump as much information to the console as you can. Firebug gives you a set of powerful logging functions that help you get answers fast.

Read this article to find out

Lets assume we have a simple Form with a 2 state radio button

<form name="example">
One:
<input name="test" type="radio" />
Two:
<input name="test" type="radio" />
</form>

We can set the first to be checked like this…

<script type="text/javascript">
<!--
document.example.test[0].checked = true;
//-->
</script>

and the second to be checked like this…

<script type="text/javascript">
<!--
document.example.test[1].checked = true;
//-->
</script>

Some other sites that might help you :

http://www.webmasterworld.com/forum91/889.htm
http://www.javascriptkit.com/javatutors/radiocheck.shtml

http://www.blueshoes.org/en/javascript/tree/