code: function getSetDayOfWeek (input) {
          var day = this._isUTC ? this._d.getUTCDay() : this._d.getDay();
          if (input != null) {
              input = parseWeekday(input, this.localeData());
              return this.add(input - day, 'd');
          } else {
              return day;
          }
      }
docs: MOMENTS