Wordpress AJAX resquest example

functions.php

<?php 

add_action( 'wp_enqueue_scripts', function() {
  // Register the script
  wp_register_script('ajax-script', get_template_directory_uri() . '/js/my-ajax-script.js', ['jquery']);
  // Localize the script with new data
  wp_localize_script('ajax-script', 'my_ajax_object', ['ajax_url' => admin_url( 'admin-ajax.php' )]);
  // Enqueued script with localized data.
  wp_enqueue_script('ajax-script');
});

add_action( 'wp_ajax_my_custom_action', function() {
    if ($_GET['data'] == 'val') {
        wp_send_json_success(['message' => 'Success']);
    }
    wp_send_json_error();
});

my-ajax-script.js

jQuery(function($){
   $.get(my_ajax_object.ajax_url, { 'action': 'my_custom_action', 'data': 'val' }, function (data) {
      console.log(data);
   });
});