Finnish postal codes by intersection

wfs.ts

import axios from 'axios';
import * as proj4 from 'proj4';

interface Result {
  readonly features: ReadonlyArray<{
    readonly properties: {
      readonly posti_alue: string;
    };
  }>;
}

proj4.defs(
  'TM35FIN',
  '+proj=utm +zone=35 +ellps=GRS80 +towgs84=0,0,0,-0,-0,-0,0 +units=m +no_defs'
);

const getFilter = (coordinates: [number, number][]) => `
<ogc:Filter xmlns:ogc="http://www.opengis.net/ogc">
  <ogc:Intersects>
    <ogc:PropertyName>geom</ogc:PropertyName>
    <gml:Polygon xmlns:gml="http://www.opengis.net/gml" srsName="EPSG:3067">
      <gml:exterior>
        <gml:LinearRing>
          <gml:posList>
          ${coordinates.map((coordinate) => coordinate.join(' ')).join(' ')}
          </gml:posList>
        </gml:LinearRing>
      </gml:exterior>
    </gml:Polygon>
  </ogc:Intersects>
</ogc:Filter>
`;

const polygon: [number, number][] = [
  [61.511309, 23.726957],
  [61.513618, 23.786633],
  [61.491548, 23.797155],
  [61.490941, 23.737657],
  [61.511309, 23.726957],
];

const filter = getFilter(
  polygon.map(([lat, lon]) => proj4('WGS84', 'TM35FIN', [lon, lat]))
);

const endpoint = 'http://geo.stat.fi/geoserver/postialue/wfs';
const params = new URLSearchParams({
  service: 'wfs',
  version: '1.1.0',
  request: 'GetFeature',
  srsName: 'EPSG:3067',
  outputFormat: 'json',
  typeName: 'postialue:pno',
  filter,
  propertyName: 'posti_alue',
});

if (!module.parent) {
  axios
    .get(endpoint, { params })
    .then((response) => response.data)
    .then((geojson: Result) =>
      geojson.features
        .map((feature) => feature.properties.posti_alue)
        .join('\n')
    )
    .then(console.log)
    .catch(console.error);
}

The hard-coded polygon covers central Tampere area, and script prints out

33100
33180
33200
33210
33230
33250
33500
33540