cellxgene/client/src/util/maybeScientific.js

import significantDigits from "./significantDigits";

/**
 * The function will conditionally convert a d3 axis label to scientific notation
 * @param x - a d3 axis, ie:
 *  d3
 *    .axisBottom(x)
 *    .ticks(4)
 *    .tickFormat(d3.format(maybeScientific(x)))
 * @returns - the number formatted as scientific, if it's big enough
 */

export default function maybeScientific(x) {
  let format = ",";
  const _ticks = x.ticks(4);

  if (x.domain().some((n) => Math.abs(n) >= 10000)) {
    /* 
          heuristic: if the last tick d3 wants to render has one significant
          digit ie., 2000, render 2e+3, but if it's anything else ie., 42000000 render
          4.20e+n
        */
    format = significantDigits(_ticks[_ticks.length - 1]) === 1 ? ".0e" : ".2e";
  }

  return format;
}