3.1.2. Handle permissions callback

permissions-callback.php

<?php
function check_initiative_permissions( WP_REST_Request $request ) : bool {
    $post_id = $request->get_param( 'id' ) ?? 0;

    if ( ! is_user_logged_in() ) {
        return false;
    }

    if ( current_user_can( 'edit_published_posts' ) ) {
        return true;
    }

    $post = get_post( $post_id );

    if ( null === $post ) {
        return false;
    }

    return get_current_user_id() === $post->post_author;
}