function post_featured_image_json( $data, $post, $context ) {
  $featured_image_id = $data->data['featured_media']; // get featured image id
  $featured_image_url = wp_get_attachment_image_src( $featured_image_id, 'origi>

  if( $featured_image_url ) {
    $data->data['featured_image_url'] = $featured_image_url[0];
  }

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