Function getHourCycle

core/src/components/datetime/utils/helpers.ts:19–71

(locale: string, hourCycle?: DatetimeHourCycle)

 * locale extension tags or from Intl.DateTimeFormat directly.
 */
export const getHourCycle = (locale: string, hourCycle?: DatetimeHourCycle) => {
  /**
   * If developer has explicitly enabled 24-hour time
   * then return early and do not look at the system default.
   */
  if (hourCycle !== undefined) {
    return hourCycle;
  }

  /**
   * If hourCycle was not specified, check the locale
   * that is set on the user's device. We first check the
   * Intl.DateTimeFormat hourCycle option as developers can encode this
   * option into the locale string. Example: `en-US-u-hc-h23`
   */
  const formatted = new Intl.DateTimeFormat(locale, { hour: 'numeric' });
  const options = formatted.resolvedOptions();
  if (options.hourCycle !== undefined) {
    return options.hourCycle;
  }

  /**
   * If hourCycle is not specified (either through lack
   * of browser support or locale information) then fall
   * back to this slower hourCycle check.
   */
  const date = new Date('5/18/2021 00:00');
  const parts = formatted.formatToParts(date);
  const hour = parts.find((p) => p.type === 'hour');

  if (!hour) {
    throw new Error('Hour value not found from DateTimeFormat');
  }

  /**
   * Midnight for h11 starts at 0:00am
   * Midnight for h12 starts at 12:00am
   * Midnight for h23 starts at 00:00
   * Midnight for h24 starts at 24:00
   */
  switch (hour.value) {
    case '0':
      return 'h11';
    case '12':
      return 'h12';
    case '00':
      return 'h23';
    case '24':
      return 'h24';
    default:
      throw new Error(`Invalid hour cycle "${hourCycle}"`);
  }
};

/**
 * Determine if the hour cycle uses a 24-hour format.