code: function getModifierClassNames(props) {
    return modifierKeys.reduce((result, key) => {
      if (props[key]) {
        return result.concat(styles[`${key}-${props[key]}`]);
      }
      return result;
    }, []);
  }
docs: Converts xs="middle" to class "middle-xs"
  @param {Object} props incoming props
  @returns {Array} result modifier classes