how to pass javascript return variable to code behind

Question

hi  folks
i am write one javascript function
as below

i want to pass the a value to the my code behind .how i pass it

  <script type="text/javascript">

       function myKeyPress(e) {

           var keynum;

           if (window.event) { // IE

               keynum = e.keyCode;

           } else

               if (e.keyCode==13) { // Netscape/Firefox/Opera

                   keynum = e.which;

                   alert(String.fromCharCode(keynum));

               }

               else if (e.keyCode == 65) {

                  

                   return false;

               }

           var a = String.fromCharCode(keynum);

            

       }

</script>