code: function validate({ separator, transformer }) {
    /* separator */
    if (null != separator) {
      if ('string' !== typeof separator) {
        throw new Error(`The separator must be a string: "${separator}".`)
      } else if (INVALID_SEPARATOR.test(separator)) {
        throw new Error(`The separator has invalid characters: "${separator}".`)
      }
    }
    /* transformer */
    if (null != transformer) {
      if (false !== transformer
        && 'function' !== typeof transformer
        && !defaultTransformers[transformer] /* TODO Deprecate */
      ) {
        throw new Error(`The transformer must be a function: "${transformer}".`)
      }
    }
  }
docs: Validate options