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/*
 * Отправляем AJAX-запрос типа ncAction
 */
$.ajax({
  type: 'POST',
  url: ncVar.ajaxurl,
  data: {
    'test'   : 'Hello, world!',
    'action' : 'ncAction',
  },
  dataType: 'json',
  success: function(result){
    console.log('success: ' + result.success);
    console.log(result.data);
  }
});

/*
 * Обрабатываем AJAX-запрос типа ncAction
 */
add_action( 'wp_ajax_ncAction', 'ncAction_callback' );
add_action( 'wp_ajax_nopriv_ncAction', 'ncAction_callback' );
function ncAction_callback() {
  $args = wp_parse_args( $_POST, array(
    'test' => false,
  ) );

  $result = array(
    'test' => $args['test'],
  );

  if ( $result['test'] ) {
    wp_send_json_success( $result );
  } else {
    wp_send_json_error( $result );
  }
}