code: function() {
          var lowestLevel = Math.max(
              this.source.minLevel,
              Math.floor(Math.log(this.minZoomImageRatio) / Math.log(2))
          );
          var currentZeroRatio = this.viewport.deltaPixelsFromPointsNoRotate(
              this.source.getPixelRatio(0), true).x *
              this._scaleSpring.current.value;
          var highestLevel = Math.min(
              Math.abs(this.source.maxLevel),
              Math.abs(Math.floor(
                  Math.log(currentZeroRatio / this.minPixelRatio) / Math.log(2)
              ))
          );
  
          // Calculations for the interval of levels to draw
          // can return invalid intervals; fix that here if necessary
          highestLevel = Math.max(highestLevel, this.source.minLevel || 0);
          lowestLevel = Math.min(lowestLevel, highestLevel);
          return {
              lowestLevel: lowestLevel,
              highestLevel: highestLevel
          };
      }
docs: private