Can anybody show me how to grab a parameter in ruby?

I have:

<%
require "dbi"
require 'cgi'
cgi = CGI.new

year     = cgi.params['year']
make     = cgi.params['make']
category = cgi.params['category']

%>

for passing in from another form.

What I cannot get to work is:

<script>

function someFunc(str_string)
{

var js_string = str_string;
alert(js_string);               <-----everything looks good here

My web page won't even come up because of the following line(sudo).
It doesn't like the 'js_string'

<% some ruby code equals= js_string%>


}

</script>

How do I dereference a Javascript var?

Any input would be great!

Thanks