format(items) {
    if (LocaleFeature.USE_ECMASCRIPT_I18N_LISTFORMAT) {
      return this.intlFormatter_.format(items);
    } else {
      // Implementation using JavaScript, requiring data and code.
      return this.formatJavaScript(items);
    }
  }
Formats a list of items in either conjunctive or disjunctive form
with locale-specific punctuation and joining words.
@param {!Array<string|number>} items list of strings to be formatted
@return {string} formatted string for these items