wp ajax boilerplate with fetch

wp-ajax-fetch-boilerplate.php

<?php
add_action('wp_ajax_ACTION_HERE', function(){
    $response = [
        'post' => $_POST,
    ];
    wp_send_json($response);
});
?>

<script>
(function(){
    button.addEventListener('click', ajax_request)
    function ajax_request(){
        let form_data = new FormData();
        form_data.append('action', 'ACTION_HERE');
        form_data.append('custom_key', 'custom_data');
        fetch( ajaxurl, { method: 'POST', body: form_data }).then(res => res.text()).then(process);
    }
    function process(data){
        data = JSON.parse(data);
        console.log('process', data)
    }
})();
</script>