Custom Polygon Styles

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index.html<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Custom Polygon Styles</title>
    <!-- The line below is only needed for old environments like Internet Explorer and Android 4.x -->
    <script src="https://cdn.polyfill.io/v2/polyfill.min.js?features=requestAnimationFrame,Element.prototype.classList,URL"></script>
    <style>
      .map {
        width: 100%;
        height:400px;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <script src="index.js"></script>
  </body>
</html>

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index.jsimport 'ol/ol.css';
import Map from 'ol/Map';
import View from 'ol/View';
import GeoJSON from 'ol/format/GeoJSON';
import MultiPoint from 'ol/geom/MultiPoint';
import VectorLayer from 'ol/layer/Vector';
import VectorSource from 'ol/source/Vector';
import {Circle as CircleStyle, Fill, Stroke, Style} from 'ol/style';

var styles = [
  /* We are using two different styles for the polygons:
   *  - The first style is for the polygons themselves.
   *  - The second style is to draw the vertices of the polygons.
   *    In a custom `geometry` function the vertices of a polygon are
   *    returned as `MultiPoint` geometry, which will be used to render
   *    the style.
   */
  new Style({
    stroke: new Stroke({
      color: 'blue',
      width: 3
    }),
    fill: new Fill({
      color: 'rgba(0, 0, 255, 0.1)'
    })
  }),
  new Style({
    image: new CircleStyle({
      radius: 5,
      fill: new Fill({
        color: 'orange'
      })
    }),
    geometry: function(feature) {
      // return the coordinates of the first ring of the polygon
      var coordinates = feature.getGeometry().getCoordinates()[0];
      return new MultiPoint(coordinates);
    }
  })
];

var geojsonObject = {
  'type': 'FeatureCollection',
  'crs': {
    'type': 'name',
    'properties': {
      'name': 'EPSG:3857'
    }
  },
  'features': [{
    'type': 'Feature',
    'geometry': {
      'type': 'Polygon',
      'coordinates': [[[-5e6, 6e6], [-5e6, 8e6], [-3e6, 8e6],
        [-3e6, 6e6], [-5e6, 6e6]]]
    }
  }, {
    'type': 'Feature',
    'geometry': {
      'type': 'Polygon',
      'coordinates': [[[-2e6, 6e6], [-2e6, 8e6], [0, 8e6],
        [0, 6e6], [-2e6, 6e6]]]
    }
  }, {
    'type': 'Feature',
    'geometry': {
      'type': 'Polygon',
      'coordinates': [[[1e6, 6e6], [1e6, 8e6], [3e6, 8e6],
        [3e6, 6e6], [1e6, 6e6]]]
    }
  }, {
    'type': 'Feature',
    'geometry': {
      'type': 'Polygon',
      'coordinates': [[[-2e6, -1e6], [-1e6, 1e6],
        [0, -1e6], [-2e6, -1e6]]]
    }
  }]
};

var source = new VectorSource({
  features: (new GeoJSON()).readFeatures(geojsonObject)
});

var layer = new VectorLayer({
  source: source,
  style: styles
});

var map = new Map({
  layers: [layer],
  target: 'map',
  view: new View({
    center: [0, 3000000],
    zoom: 2
  })
});

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package.json{
  "name": "polygon-styles",
  "dependencies": {
    "ol": "6.1.1"
  },
  "devDependencies": {
    "parcel": "1.11.0"
  },
  "scripts": {
    "start": "parcel index.html",
    "build": "parcel build --experimental-scope-hoisting --public-url . index.html"
  }
}