404 how make a rote

Firstly define base_url in header.

<script type="text/javascript">
      var site_url = '<?php echo site_url(); ?>';
      var base_url = site_url;
      var siteurl = site_url;
</script>

after that you can use url something like that where you want in whole project files.
$.ajax({
          type: 'POST',
          dataType:'json',
          url: siteurl+'functions_ajax/check_all_Ukraine',