use custom field suite with wp-api

functions.php

<?php

add_action( 'rest_api_init', 'example_add_cfs' );

/**
 * Add cfs fields to api call for post type example
 * Fields will be put in cfs object in json
 * @param none
 *
 * @return null
 */
function example_add_cfs() {
  register_rest_field( 'example',
    'cfs',
    array(
      'get_callback'    => 'add_cfs_to_json',
      'update_callback' => null,
      'schema'          => null,
    )
  );
}

/**
 * @param array $object Details of current post.
 * @param string $field_name Name of field.
 * @param WP_REST_Request $request Current request
 *
 * @return mixed
 */
function add_cfs_to_json( $object, $field_name, $request ) {
  return CFS()->get( false, $object[ 'id' ], array( 'format' => 'raw' ) );
}