WP Featured image URLs in JSON REST API

functions.php

// WP FEATURED IMAGE URLS in JSON API
// See: https://wordpress.stackexchange.com/a/249769/105228
//
add_action( 'rest_api_init', 'add_thumbnail_to_JSON' );
function add_thumbnail_to_JSON() {
//Add featured image
register_rest_field( 
    'post', // Where to add the field (Here, blog posts. Could be an array)
    'featured_image_src', // Name of new field (You can call this anything)
    array(
        'get_callback'    => 'get_image_src',
        'update_callback' => null,
        'schema'          => null,
         )
    );
}

function get_image_src( $object, $field_name, $request ) {
  $feat_img_array = wp_get_attachment_image_src(
    $object['featured_media'], // Image attachment ID
    'thumbnail',  // Size.  Ex. "thumbnail", "large", "full", etc..
    true // Whether the image should be treated as an icon.
  );
  return $feat_img_array[0];
}