recharts/src/state/selectors/combiners/combineActiveTooltipIndex.ts

import { TooltipIndex, TooltipInteractionState } from '../../tooltipSlice';
import { ChartData } from '../../chartDataSlice';
import { isWellBehavedNumber } from '../../../util/isWellBehavedNumber';

export const combineActiveTooltipIndex = (
  tooltipInteraction: TooltipInteractionState,
  chartData: ChartData,
): TooltipIndex | null => {
  const desiredIndex: TooltipIndex | undefined = tooltipInteraction?.index;
  if (desiredIndex == null) {
    return null;
  }

  const indexAsNumber = Number(desiredIndex);
  if (!isWellBehavedNumber(indexAsNumber)) {
    // this is for charts like Sankey and Treemap that do not support numerical indexes. We need a proper solution for this before we can start supporting keyboard events on these charts.
    return desiredIndex;
  }

  /*
   * Zero is a trivial limit for single-dimensional charts like Line and Area,
   * but this also needs a support for multidimensional charts like Sankey and Treemap! TODO
   */
  const lowerLimit = 0;
  let upperLimit: number = +Infinity;

  if (chartData.length > 0) {
    upperLimit = chartData.length - 1;
  }

  // now let's clamp the desiredIndex between the limits
  return String(Math.max(lowerLimit, Math.min(indexAsNumber, upperLimit)));
};