get localWeekNumber() {
    return this.isValid ? possiblyCachedLocalWeekData(this).weekNumber : NaN;
  }
Get the week number of the week year according to the locale. Different locales assign week numbers differently,
because the week can start on different days of the week (see localWeekday) and because a different number of days
is required for a week to count as the first week of a year.
@returns {number}