AJAX filter on custom types and custom fields in wordpress?

Question

WordPress

Hello. When creating an AJAX filter by custom fields, there was a problem, the request is sent but there is no response, I follow the instructions https://misha.blog/wordpress/filtryi-zapisey-i-tov...
Tell me what I'm doing wrong?
I suspect that the $args array is not being passed back, I don’t know how to pass it and add it correctly to query_posts on the page?
Thanks in advance.
Here is the code of the page with filters
form

<form action="<?php echo site_url() ?>/wp-admin/admin-ajax.php" method="POST" id="filter">
                <div class="form-group">
                    <label for="dateStart">С даты</label>
                    <input type="date" class="form-control" id="dateStart" name="dateStart">
                </div>
                <div class="form-group">
                    <label for="dateEnd">По дату</label>
                    <input type="date" class="form-control" id="dateEnd" name='dateEnd'>
                </div>
                <div class="form-group">
                    <label for="seminarTypeSelect">Город</label>
                    <select id="seminarTypeSelect" class="form-control" name='city'>
                        <option>Москва</option>
                        <option>Киев</option>
                    </select>
                </div>
                <div class="form-group">
                    <label for="seminarTypeSelect2">Тип семнара</label>
                    <select id="seminarTypeSelect2" class="form-control" name='stype'>
                        <option value="seminar">Семинар</option>
                        <option value="webinar">Вебинар</option>
                    </select>
                </div>
                <button type="submit" class="btn btn-default">Применить фильтр</button>
                <input type="hidden" name="action" value="myfilter">
            </form>
            <div id="response"></div>
        </div>

Arguments for query_posts

<?php
                                $paged = (get_query_var('paged')) ? get_query_var('paged') : 1;
                                $args = array(
                                'post_type' => 'kursy',
                                'order' => 'DESC',
                                'posts_per_page' => 15,
                                'orderby' => 'meta_value',
                                'paged' => $paged,
                                'meta_query' => array(
                                array('key' => 'datestart'),
                                ),
                                );
  ?>

Script for sending AJAX :

<script type="text/javascript">
    jQuery(function ($) {
        $('#filter').submit(function () {
            var filter = $(this);
            $.ajax({
                url: ajaxurl, // обработчик
                data: filter.serialize(), // данные
                type: filter.attr('method'), // тип запроса
                beforeSend: function (xhr) {
                    filter.find('button').text('Загружаю...'); // изменяем текст кнопки
                },
                success: function (data) {
                    filter.find('button').text(
                    'Применить фильтр'); // возвращаеи текст кнопки
                    $('#response').html(data);
                }
            });
            return false;
        });
    });
</script>

Function handler in function.php file

function true_filter_function(){
  
    $args['meta_query'] = array( 'relation'=>'AND' ); // AND значит все условия meta_query должны выполняться
 
  // условие 1: Фильтр по датам если установленны.
  if( isset( $_POST['dateStart'] ))
    $args['meta_query'][] = array(
      'key' => 'datestart',
      'value' => $_POST['dateStart'],
      'compare' => '=',
            'type' => 'DATE'
    );
 
  
  if( isset( $_POST['stype'] ) )
    $args['meta_query'][] = array(
      'key' => 'stype',
      'value' => $_POST['stype'],
      'type' => 'CHAR',
      'compare' => '='
    );
 
  
 
  die();
}
 
 
add_action('wp_ajax_myfilter', 'true_filter_function'); 
add_action('wp_ajax_nopriv_myfilter', 'true_filter_function');

Answer 1

Misha has a lot of bugs in the Russian-language version, at least there are cuts in the code, he doesn’t even know about it himself, yes, yes!!...)))
If you don’t believe me, I can poke a link - the difference is a few lines, the script is fully functional destroyed.
Misha scored in the Russian Federation, scored ...
Google his English site - everything is OK there !!! Don't forget the dictionary.