This example uses OpenLayers v10.9.0. The latest is v.
Edit

#### Custom Polygon Styles

polygon 3 vector 86 style 27 GeometryFunction 1

#####

Showing the vertices of a polygon with a custom style geometry.

In this example two different styles are created for the polygons:

- The first style is for the polygons themselves.
- The second style is to draw the vertices of the polygons.

##### main.js

    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import GeoJSON from 'ol/format/GeoJSON.js';
    import MultiPoint from 'ol/geom/MultiPoint.js';
    import VectorLayer from 'ol/layer/Vector.js';
    import VectorSource from 'ol/source/Vector.js';
    import CircleStyle from 'ol/style/Circle.js';
    import Fill from 'ol/style/Fill.js';
    import Stroke from 'ol/style/Stroke.js';
    import Style from 'ol/style/Style.js';

    const 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
          const coordinates = feature.getGeometry().getCoordinates()[0];
          return new MultiPoint(coordinates);
        },
      }),
    ];

    const 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],
              ],
            ],
          },
        },
      ],
    };

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

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

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

##### index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="UTF-8">
        <title>Custom Polygon Styles</title>
        <link rel="stylesheet" href="node_modules/ol/ol.css">
        <style>
          .map {
            width: 100%;
            height: 400px;
          }
        </style>
      </head>
      <body>
        <div id="map" class="map"></div>

        <script type="module" src="main.js"></script>
      </body>
    </html>

##### package.json

    {
      "name": "polygon-styles",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }