问通过AJAX (内嵌Wordpress)接收php中的数据

EN

<script type='text/javascript'>

    jQuery(document).ready(function() {

        jQuery("#fetch_data_button").click(function() {

            searchstring =  "My search text";
                                    
            jQuery.ajax({
                type: "POST",
                url: "admin-ajax.php",
                data: {         
                    action: 'fetch_data', 
                    _ajax_nonce: '<?php echo $nonce; ?>',
                    data: searchstring
                }
            });
            return false;                   
        });                                         
    });
    
</script>

add_action( 'wp_ajax_fetch_data', 'fetch_data_fn' );

function fetch_data_fn() {

    $searchstring = $_POST['searchstring'];
    // why is $searchstring empty ... ?

    die(); 
}