add_filter('rest_prepare_post', 'my_rest_modifier', 10, 3);

function add_featured_image_url($data, $post, $context) {
  $id = $data->data['featured_media'];

  if (!$id) {
    return $data;
  }

  $full = wp_get_attachment_image_src($id, 'full');
  $thumb = image_get_intermediate_size($id, 'thumbnail');
  $medium = image_get_intermediate_size($id, 'medium');

  if ($thumb) {
    $data->data['featured_image_url']['thumbnail'] = $thumb['url'];
  }
  if ($medium) {
    $data->data['featured_image_url']['medium'] = $medium['url'];
  }
  if ($full) {
    $data->data['featured_image_url']['full'] = $full[0];
  }

  return $data;
}
add_filter('rest_prepare_post', 'add_featured_image_url', 10, 3);

function add_comment_count($data, $post, $context) {
  $count = get_comments([
    'post_id' => $post->ID,
    'count' => true,
    'status' => 'approve',
  ]);

  $data->data['comment_count'] = $count;
  return $data;
}
add_filter('rest_prepare_post', 'add_comment_count', 10, 3);

function custom_excerpt($data, $post, $context) {
  if (!has_excerpt($post->ID)) {
    $data->data['excerpt']['raw'] = wp_trim_words($post->post_content, 55, ' ... ');
  }
  return $data;
}
add_filter('rest_prepare_post', 'custom_excerpt', 10, 3);