code: function(i, isSelected, isToday, isDisabled, isEmpty)
      {
          if (isEmpty) {
              return '<td class="is-empty"></td>';
          }
          var arr = [];
          if (isDisabled) {
              arr.push('is-disabled');
          }
          if (isToday) {
              arr.push('is-today');
          }
          if (isSelected) {
              arr.push('is-selected');
          }
          return '<td data-day="' + i + '" class="' + arr.join(' ') + '"><button class="pika-button" type="button">' + i + '</button>' + '</td>';
      }
docs: feature detection and helper functions