Modifying responses in WP Rest Api. Add featured image thumbnail to the response for a custom post type (by default return ID).

modify-response-wp-rest-api.php

// in function.php

// Modify Responses WP Rest Api
add_action( 'rest_api_init', 'featured_image_thumbnail_url' );

// Modifying Responses
function featured_image_thumbnail_url() {

	// More info http://v2.wp-api.org/extending/modifying/
	register_api_field(
		'portfolio',
		'featured_image_url',
		array(
			'get_callback'    => 'get_featured_image_url',
			'update_callback' => null,
			'schema'          => null,
		)
	);
}

/**
 * Get the value of the "featured_image_url" field
 *
 * @param array $object Details of current post.
 * @param string $field_name Name of field.
 * @param WP_REST_Request $request Current request
 *
 * @return mixed
 */
function get_featured_image_url( $object, $field_name, $request ) {
	$thumbnail_id = get_post_thumbnail_id( $object->ID );
	return wp_get_attachment_image_src($thumbnail_id);
}

Now return a new key with and array for the featured image.