This example uses OpenLayers v10.9.0. The latest is v.
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#### Earthquake Clusters

KML 9 vector 86 style 27 geometry 4 cluster 3

#####

Demonstrates the use of style geometries to render source features of a cluster.

This example parses a KML file and renders the features as clusters on a vector layer. The styling in this example is quite involved. Single earthquake locations (rendered as stars) have a size relative to their magnitude. Clusters have an opacity relative to the number of features in the cluster, and a size that represents the extent of the features that make up the cluster. When clicking or hovering on a cluster, the individual features that make up the cluster will be shown.

To achieve this, we make heavy use of style functions.

##### main.js

    import Map from 'ol/Map.js';
    import View from 'ol/View.js';
    import {createEmpty, extend, getHeight, getWidth} from 'ol/extent.js';
    import KML from 'ol/format/KML.js';
    import Select from 'ol/interaction/Select.js';
    import {defaults as defaultInteractions} from 'ol/interaction/defaults.js';
    import TileLayer from 'ol/layer/Tile.js';
    import VectorLayer from 'ol/layer/Vector.js';
    import Cluster from 'ol/source/Cluster.js';
    import StadiaMaps from 'ol/source/StadiaMaps.js';
    import VectorSource from 'ol/source/Vector.js';
    import CircleStyle from 'ol/style/Circle.js';
    import Fill from 'ol/style/Fill.js';
    import RegularShape from 'ol/style/RegularShape.js';
    import Stroke from 'ol/style/Stroke.js';
    import Style from 'ol/style/Style.js';
    import Text from 'ol/style/Text.js';

    const earthquakeFill = new Fill({
      color: 'rgba(255, 153, 0, 0.8)',
    });
    const earthquakeStroke = new Stroke({
      color: 'rgba(255, 204, 0, 0.2)',
      width: 1,
    });
    const textFill = new Fill({
      color: '#fff',
    });
    const textStroke = new Stroke({
      color: 'rgba(0, 0, 0, 0.6)',
      width: 3,
    });
    const invisibleFill = new Fill({
      color: 'rgba(255, 255, 255, 0.01)',
    });

    function createEarthquakeStyle(feature) {
      // 2012_Earthquakes_Mag5.kml stores the magnitude of each earthquake in a
      // standards-violating <magnitude> tag in each Placemark.  We extract it
      // from the Placemark's name instead.
      const name = feature.get('name');
      const magnitude = parseFloat(name.substr(2));
      const radius = 5 + 20 * (magnitude - 5);

      return new Style({
        geometry: feature.getGeometry(),
        image: new RegularShape({
          radius: radius,
          radius2: 3,
          points: 5,
          angle: Math.PI,
          fill: earthquakeFill,
          stroke: earthquakeStroke,
        }),
      });
    }

    let maxFeatureCount;
    let vector = null;
    const calculateClusterInfo = function (resolution) {
      maxFeatureCount = 0;
      const features = vector.getSource().getFeatures();
      let feature, radius;
      for (let i = features.length - 1; i >= 0; --i) {
        feature = features[i];
        const originalFeatures = feature.get('features');
        const extent = createEmpty();
        let j, jj;
        for (j = 0, jj = originalFeatures.length; j < jj; ++j) {
          extend(extent, originalFeatures[j].getGeometry().getExtent());
        }
        maxFeatureCount = Math.max(maxFeatureCount, jj);
        radius = (0.25 * (getWidth(extent) + getHeight(extent))) / resolution;
        feature.set('radius', radius);
      }
    };

    let currentResolution;
    function styleFunction(feature, resolution) {
      if (resolution != currentResolution) {
        calculateClusterInfo(resolution);
        currentResolution = resolution;
      }
      let style;
      const size = feature.get('features').length;
      if (size > 1) {
        style = new Style({
          image: new CircleStyle({
            radius: feature.get('radius'),
            fill: new Fill({
              color: [255, 153, 0, Math.min(0.8, 0.4 + size / maxFeatureCount)],
            }),
          }),
          text: new Text({
            text: size.toString(),
            fill: textFill,
            stroke: textStroke,
          }),
        });
      } else {
        const originalFeature = feature.get('features')[0];
        style = createEarthquakeStyle(originalFeature);
      }
      return style;
    }

    function selectStyleFunction(feature) {
      const styles = [
        new Style({
          image: new CircleStyle({
            radius: feature.get('radius'),
            fill: invisibleFill,
          }),
        }),
      ];
      const originalFeatures = feature.get('features');
      let originalFeature;
      for (let i = originalFeatures.length - 1; i >= 0; --i) {
        originalFeature = originalFeatures[i];
        styles.push(createEarthquakeStyle(originalFeature));
      }
      return styles;
    }

    vector = new VectorLayer({
      source: new Cluster({
        distance: 40,
        source: new VectorSource({
          url: 'data/kml/2012_Earthquakes_Mag5.kml',
          format: new KML({
            extractStyles: false,
          }),
        }),
      }),
      style: styleFunction,
    });

    const raster = new TileLayer({
      source: new StadiaMaps({
        layer: 'stamen_toner',
      }),
    });

    const map = new Map({
      layers: [raster, vector],
      interactions: defaultInteractions().extend([
        new Select({
          condition: function (evt) {
            return evt.type == 'pointermove' || evt.type == 'singleclick';
          },
          style: selectStyleFunction,
        }),
      ]),
      target: 'map',
      view: new View({
        center: [0, 0],
        zoom: 2,
      }),
    });

##### index.html

    <!DOCTYPE html>
    <html lang="en">
      <head>
        <meta charset="UTF-8">
        <title>Earthquake Clusters</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": "earthquake-clusters",
      "dependencies": {
        "ol": "10.9.0"
      },
      "devDependencies": {
        "vite": "^3.2.3"
      },
      "scripts": {
        "start": "vite",
        "build": "vite build"
      }
    }