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public class servlet extends HttpServlet {

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        request.getRequestDispatcher("/test.jsp").forward(request,response);
    }
}

<%@ page language="java" contentType="text/html; charset=windows-1255" pageEncoding="windows-1255"%>
<!DOCTYPE html>
<html>
<head>
        <script src="test.js"></script>
    </head>
    <body>
        <% String param = "foo";%>
    </body>
</html>

var param_js_context = '<%=param%>';
alert(param_js_context);