vue-wp/plugins/cia-endpoints/includes/formats.php

<?php

function default_post_format( $item ) {
    $filtered = array();
    $filtered[id] = $item->ID;
    $filtered[slug] = $item->post_name;
    $filtered[type] = $item->post_type;
    $filtered[title] = $item->post_title;
    $filtered[excerpt] = $item->post_excerpt;
    $filtered[date] = $item->post_date;
    $filtered[content] = $item->post_content;

    $filtered[hero] = get_post_meta( $item->ID, 'hero_header', true );

    if($item->post_type === 'artist') {
        $filtered[sortname] = get_post_meta( $item->ID, 'artist-sort-name', true );
    
        // Parse category IDs and store just the slugs
        $category_slugs = [];
        foreach (wp_get_post_categories( $item->ID ) as $id):
            $category = get_category($id);
            array_push($category_slugs, $category->slug);
        endforeach;
        $filtered[categories] = $category_slugs;
    }

    return $filtered;
}

?>