toFormat(fmt, opts = {}) {
    return this.isValid
      ? Formatter.create(this.loc.redefaultToEN(opts)).formatDateTimeFromString(this, fmt)
      : INVALID;
  }
Returns a string representation of this DateTime formatted according to the specified format string.
**You may not want this.** See {@link DateTime#toLocaleString} for a more flexible formatting tool. For a table of tokens and their interpretations, see [here](https://moment.github.io/luxon/#/formatting?id=table-of-tokens).
Defaults to en-US if no locale has been specified, regardless of the system's locale.
@param {string} fmt - the format string
@param {Object} opts - opts to override the configuration options on this DateTime
@example DateTime.now().toFormat('yyyy LLL dd') //=> '2017 Apr 22'
@example DateTime.now().setLocale('fr').toFormat('yyyy LLL dd') //=> '2017 avr. 22'
@example DateTime.now().toFormat('yyyy LLL dd', { locale: "fr" }) //=> '2017 avr. 22'
@example DateTime.now().toFormat("HH 'hours and' mm 'minutes'") //=> '20 hours and 55 minutes'
@return {string}