/** add featured img url to wp rest api **
 * Created 11.15.18
 * to test, add this code to functions.php under the current wp theme "../wp-content/themes/themeName/functions.php"
 * then check out the api: "../wp-json/wp/v2/posts" and inspect the json,
 * under post data object, you should see a new key: 'ftd_img_sm', with value: 'URL of image'
 **/

function prepare_rest($data, $post, $request){
    $_data = $data ->data;

    $thumbnail_id = get_post_thumbnail_id( $post->ID );
    $thumbnail_sm = wp_get_attachment_image_src( $thumbnail_id, 'small' ); #get img url

    $_data['ftd_img_sm'] = $thumbnail_sm[0];  #name of new data attribute
    $data->data = $_data;  #add url to api data

    return $data;

}
add_filter('rest_prepare_post', 'prepare_rest', 10, 3);