Source: centroid.js

import { check } from "./helpers/check.js";
import { geoArea, geoCentroid, geoIdentity, geoPath } from "d3-geo";
const d3 = Object.assign({}, { geoArea, geoCentroid, geoIdentity, geoPath });
/**
 * @function centroid
 * @summary Calculates the centroids of geometries.
 * @description Based on `d3.geoArea()` and `d3.geoCentroid()`
 * @param {object|array} data - A GeoJSON FeatureCollection, an array of features, an array of geometries, a single feature or a single geometry.
 * @param {object} options - Optional parameters.
 * @param {boolean} [options.larget = true] - If true, set the point at the centre of the largest polygon.
 * @param {boolean} [options.geo = true] - Use true to consider the centroid from world coordinates on the globe. If you use false, then you are considering the coordinates within the svg document.
 * @returns {object|array} - A GeoJSON FeatureCollection, an array of features, an array of geometries, a single feature or a single geometry (it depends on what you've set as `data`)
 * @example
 * geotoolbox.centroid(*a geojson*, {largest: true})
 */
export function centroid(data, { largest = true, geo = true } = {}) {
  const handle = check(data);
  let x = handle.import(data);
  let path = d3.geoPath(d3.geoIdentity());
  function largestPolygon(d) {
    var best = {};
    var bestArea = 0;
    d.geometry.coordinates.forEach(function (coords) {
      var poly = { type: "Polygon", coordinates: coords };
      var area = geo ? d3.geoArea(poly) : path.area(poly);
      if (area > bestArea) {
        bestArea = area;
        best = poly;
      }
    });
    return best;
  }
  let centers = x.features.map((d) => {
    if (geo) {
      console.log(d);
      d.geometry.coordinates = d3.geoCentroid(
        largest == true
          ? d.geometry.type == "Polygon"
            ? d
            : largestPolygon(d, true)
          : d
      );
    } else {
      d.geometry.coordinates = path.centroid(
        largest == true
          ? d.geometry.type == "Polygon"
            ? d
            : largestPolygon(d, false)
          : d
      );
    }
    d.geometry.type = "Point";
    return d;
  });
  x.features = centers;
  x.name = "centroid";
  return handle.export(x);
}