Example of extending REST API for Wordpress to include a field not normally returned in result

extending_wp_rest_result.php

/*
 |--------------------------------------------------------------------------
 | Prepare REST
 |--------------------------------------------------------------------------
 */
 
 function prepare_rest($data, $post, $request){
     $_data = $data->data;
 
     $thumbnail_id = get_post_thumbnail_id( $post->ID );
     $thumbnail300x180 = wp_get_attachment_image_src( $thumbnail_id, '300x180' );
     $thumbnailMedium = wp_get_attachment_image_src( $thumbnail_id, 'medium' );
 
 
     $_data['fi_300x180'] = $thumbnail300x180[0];
     $_data['fi_medium'] = $thumbnailMedium[0];
     $data->data = $_data;
 
     return $data;
 }
 add_filter('rest_prepare_post', 'prepare_rest', 10, 3);