Method _assertFunction

lib/test.js:339–347

(fn, res)

   * @api private
   */// eslint-disable-next-line class-methods-use-this
  _assertFunction(fn, res) {
    let err;
    try {
      err = fn(res);
    } catch (e) {
      err = e;
    }
    if (err instanceof Error) return err;
  }
}

/**