GeoJSONポリゴンの中心(緯度経度)を求める

シンプルにポリゴンの中心を求めたいときに。

const getCentroid =  (geometry) => {
    var polygon = [];

    switch (geometry.type) {
        case "MultiPolygon":
            geometry.coordinates.forEach( (d, i) => {
                if (i == 0 || polygon.length < d[0].length) polygon = d[0];
            });
            break;
        case "Polygon":
            polygon = geometry.coordinates[0];
            break;
    }

    return polygon.reduce(
         (a, b) => {
            return [a[0] + b[0] / polygon.length, a[1] + b[1] / polygon.length];
        },
        [0, 0]
    );
}

geojsonのgeometryプロパティを渡す。

const  geojson =   {
  "type": "Feature",
  "properties": {},
  "geometry": {
    "type": "Polygon",
      "coordinates": [
        [
          [138.7309,35.3628],[138.8079,35.1983],[139.0248,35.2248],[138.7309,35.3628]
        ]
      ]
  }
}

//中心地を求める
const center = getCentroid(geo.geometry);
// ->  [138.823625, 35.287175000000005]