Pass a variable value from javascript to xsl

I am trying to pass a variable value from javascript to xsl. For
example, I have a xsl:variable called instance:
 
 
 <xsl:variable name="instance"/>
 
 
  And I want to assign a javascript variable value to 'instance'
 The javascript code is:
  
 
 <script language="JavaScript">
 function example(){
 		var art
 		art = document.exampleForm.artistOnChange[document.noadsterForm.artistOnChange.selectedIndex].text
 }
 </script>
 
I call the javascript function in my xsl code when someone select
something in a form:
  
  
 <select name="artistOnChange" onchange="example">
 
 
  I want to save the value of the javascript variable 'art' into the
xsl:variable 'instance'. How can I do that?
 I'm using xalan. 
  
  Thanks
  .ohtar