exists in public/javascripts folder

application.js

jQuery(document).ready(function() {
    jQuery('#regform_state_id').change(function() {
        var data=$('#regform_state_id').val();                        
                    $.ajax({
                            type: "POST",
                            url: "http://"+location.host+"dynamic_districts/"+data,
                            data: data,
                            });
                   });
});