code: function( page ) {
  
              if ( this.nextButton ) {
                  if(!this.tileSources || this.tileSources.length - 1 === page) {
                      //Disable next button
                      if ( !this.navPrevNextWrap ) {
                          this.nextButton.disable();
                      }
                  } else {
                      this.nextButton.enable();
                  }
              }
              if ( this.previousButton ) {
                  if ( page > 0 ) {
                      //Enable previous button
                      this.previousButton.enable();
                  } else {
                      if ( !this.navPrevNextWrap ) {
                          this.previousButton.disable();
                      }
                  }
              }
        }
docs: Updates the sequence buttons.
  @function OpenSeadragon.Viewer.prototype._updateSequenceButtons
  @private
  @param {Number} Sequence Value