react-day-picker/src/formatters/formatWeekNumber.ts

/**
 * Format the week number.
 *
 * @defaultValue `weekNumber.toLocaleString()` with a leading zero for single-digit numbers
 * @group Formatters
 * @see https://daypicker.dev/docs/translation#custom-formatters
 */
export function formatWeekNumber(weekNumber: number) {
  if (weekNumber < 10) {
    return `0${weekNumber.toLocaleString()}`;
  }
  return `${weekNumber.toLocaleString()}`;
}