Get featured image for WordPress RESTful API

functions.php

// Add to theme functions.php (Tested on WordPress 5.0.3)

<?php

    function post_fetured_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, 'original' ); // get url of the original size

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

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

?>