code: function(evt, deltaZ) {
          if (!this.map.fractionalZoom) {
              deltaZ =  Math.round(deltaZ);
          }
          var currentZoom = this.map.getZoom(),
              newZoom = currentZoom + deltaZ;
          newZoom = Math.max(newZoom, 0);
          newZoom = Math.min(newZoom, this.map.getNumZoomLevels());
          if (newZoom === currentZoom) {
              return;
          }
          this.map.zoomTo(newZoom, evt.xy);
      }
docs: Method: wheelChange
  
  Parameters:
  evt - {Event}
  deltaZ - {Integer}