Filter tokyo.geojson to remove all islands

filter-tokyo-geojson.js

const fs = require("fs");

// https://github.com/dataofjapan/land/blob/master/tokyo.geojson
const tokyo = JSON.parse(fs.readFileSync("tokyo.geojson"));

const tokyoMainland = {
  type: "FeatureCollection",
  features: [],
};

for (let feature of tokyo.features) {
  if (
    feature &&
    feature.properties &&
    feature.properties.area_ja &&
    feature.properties.area_ja != "島嶼部" &&
    feature.properties.area_ja != "null"
  ) {
    // not an island
    tokyoMainland.features.push(feature);
  }
}

fs.writeFileSync(
  "tokyo-mainland.geojson",
  JSON.stringify(tokyoMainland, null, 0)
);