code: function assertString(attr, data) {
    if (data === null || data === undefined) {
      return null;
    }
  
    if (typeof data !== 'string') {
      throw new ParseError(`Attribute ${attr} must be of type "string"`);
    }
  
    return data;
  }
docs: Throws if the provided data is not a string
  Returns the unmodified data if validated
  
  @throws
  @param {string} attr - tested attribute name
  @param {*} data
  @return {null|string}