Function useDefaultProps

packages/react-stately/src/datepicker/utils.ts:285–314

(
  v: DateValue | null,
  granularity: Granularity | undefined
)

}

export function useDefaultProps(
  v: DateValue | null,
  granularity: Granularity | undefined
): [Granularity, string | undefined] {
  // Compute default granularity and time zone from the value. If the value becomes null, keep the last values.
  let defaultTimeZone = v && 'timeZone' in v ? v.timeZone : undefined;
  let defaultGranularity: Granularity = v && 'minute' in v ? 'minute' : 'day';

  // props.granularity must actually exist in the value if one is provided.
  if (v && granularity && !(granularity in v)) {
    throw new Error('Invalid granularity ' + granularity + ' for value ' + v.toString());
  }

  let [lastValue, setLastValue] = useState<[Granularity, string | undefined]>([
    defaultGranularity,
    defaultTimeZone
  ]);

  // If the granularity or time zone changed, update the last value.
  if (v && (lastValue[0] !== defaultGranularity || lastValue[1] !== defaultTimeZone)) {
    setLastValue([defaultGranularity, defaultTimeZone]);
  }

  if (!granularity) {
    granularity = v ? defaultGranularity : lastValue[0];
  }

  let timeZone = v ? defaultTimeZone : lastValue[1];
  return [granularity, timeZone];
}