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Display HTML clusters with custom properties

Extend clustering with HTML markers and custom property expressions.

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Display HTML clusters with custom properties</title>
    <meta property="og:description" content="Extend clustering with HTML markers and custom property expressions. " />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/[email protected]/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/[email protected]/dist/maplibre-gl.js'></script>
    <style>
        body { margin: 0; padding: 0; }
        html, body, #map { height: 100%; }
    </style>
</head>
<body>
<div id="map"></div>

<script>
    const map = new maplibregl.Map({
        container: 'map',
        zoom: 0.3,
        center: [0, 20],
        style:
            'https://api.maptiler.com/maps/streets/style.json?key=get_your_own_OpIi9ZULNHzrESv6T2vL'
    });

    map.addControl(new maplibregl.NavigationControl());

    // filters for classifying earthquakes into five categories based on magnitude
    const mag1 = ['<', ['get', 'mag'], 2];
    const mag2 = ['all', ['>=', ['get', 'mag'], 2], ['<', ['get', 'mag'], 3]];
    const mag3 = ['all', ['>=', ['get', 'mag'], 3], ['<', ['get', 'mag'], 4]];
    const mag4 = ['all', ['>=', ['get', 'mag'], 4], ['<', ['get', 'mag'], 5]];
    const mag5 = ['>=', ['get', 'mag'], 5];

    // colors to use for the categories
    const colors = ['#fed976', '#feb24c', '#fd8d3c', '#fc4e2a', '#e31a1c'];

    map.on('load', () => {
        // add a clustered GeoJSON source for a sample set of earthquakes
        map.addSource('earthquakes', {
            'type': 'geojson',
            'data':
                'https://maplibre.org/maplibre-gl-js/docs/assets/earthquakes.geojson',
            'cluster': true,
            'clusterRadius': 80,
            'clusterProperties': {
                // keep separate counts for each magnitude category in a cluster
                'mag1': ['+', ['case', mag1, 1, 0]],
                'mag2': ['+', ['case', mag2, 1, 0]],
                'mag3': ['+', ['case', mag3, 1, 0]],
                'mag4': ['+', ['case', mag4, 1, 0]],
                'mag5': ['+', ['case', mag5, 1, 0]]
            }
        });
        // circle and symbol layers for rendering individual earthquakes (unclustered points)
        map.addLayer({
            'id': 'earthquake_circle',
            'type': 'circle',
            'source': 'earthquakes',
            'filter': ['!=', 'cluster', true],
            'paint': {
                'circle-color': [
                    'case',
                    mag1,
                    colors[0],
                    mag2,
                    colors[1],
                    mag3,
                    colors[2],
                    mag4,
                    colors[3],
                    colors[4]
                ],
                'circle-opacity': 0.6,
                'circle-radius': 12
            }
        });
        map.addLayer({
            'id': 'earthquake_label',
            'type': 'symbol',
            'source': 'earthquakes',
            'filter': ['!=', 'cluster', true],
            'layout': {
                'text-field': [
                    'number-format',
                    ['get', 'mag'],
                    {'min-fraction-digits': 1, 'max-fraction-digits': 1}
                ],
                'text-font': ['Open Sans Semibold', 'Arial Unicode MS Bold'],
                'text-size': 10
            },
            'paint': {
                'text-color': [
                    'case',
                    ['<', ['get', 'mag'], 3],
                    'black',
                    'white'
                ]
            }
        });

        // objects for caching and keeping track of HTML marker objects (for performance)
        const markers = {};
        let markersOnScreen = {};

        function updateMarkers() {
            const newMarkers = {};
            const features = map.querySourceFeatures('earthquakes');

            // for every cluster on the screen, create an HTML marker for it (if we didn't yet),
            // and add it to the map if it's not there already
            for (let i = 0; i < features.length; i++) {
                const coords = features[i].geometry.coordinates;
                const props = features[i].properties;
                if (!props.cluster) continue;
                const id = props.cluster_id;

                let marker = markers[id];
                if (!marker) {
                    const el = createDonutChart(props);
                    marker = markers[id] = new maplibregl.Marker({
                        element: el
                    }).setLngLat(coords);
                }
                newMarkers[id] = marker;

                if (!markersOnScreen[id]) marker.addTo(map);
            }
            // for every marker we've added previously, remove those that are no longer visible
            for (id in markersOnScreen) {
                if (!newMarkers[id]) markersOnScreen[id].remove();
            }
            markersOnScreen = newMarkers;
        }

        // after the GeoJSON data is loaded, update markers on the screen and do so on every map move/moveend
        map.on('data', (e) => {
            if (e.sourceId !== 'earthquakes' || !e.isSourceLoaded) return;

            map.on('move', updateMarkers);
            map.on('moveend', updateMarkers);
            updateMarkers();
        });
    });

    // code for creating an SVG donut chart from feature properties
    function createDonutChart(props) {
        const offsets = [];
        const counts = [
            props.mag1,
            props.mag2,
            props.mag3,
            props.mag4,
            props.mag5
        ];
        let total = 0;
        for (let i = 0; i < counts.length; i++) {
            offsets.push(total);
            total += counts[i];
        }
        const fontSize =
        total >= 1000 ? 22 : total >= 100 ? 20 : total >= 10 ? 18 : 16;
        const r = total >= 1000 ? 50 : total >= 100 ? 32 : total >= 10 ? 24 : 18;
        const r0 = Math.round(r * 0.6);
        const w = r * 2;

        let html =
            `<div><svg width="${
                w
            }" height="${
                w
            }" viewbox="0 0 ${
                w
            } ${
                w
            }" text-anchor="middle" style="font: ${
                fontSize
            }px sans-serif; display: block">`;

        for (i = 0; i < counts.length; i++) {
            html += donutSegment(
                offsets[i] / total,
                (offsets[i] + counts[i]) / total,
                r,
                r0,
                colors[i]
            );
        }
        html +=
            `<circle cx="${
                r
            }" cy="${
                r
            }" r="${
                r0
            }" fill="white" /><text dominant-baseline="central" transform="translate(${
                r
            }, ${
                r
            })">${
                total.toLocaleString()
            }</text></svg></div>`;

        const el = document.createElement('div');
        el.innerHTML = html;
        return el.firstChild;
    }

    function donutSegment(start, end, r, r0, color) {
        if (end - start === 1) end -= 0.00001;
        const a0 = 2 * Math.PI * (start - 0.25);
        const a1 = 2 * Math.PI * (end - 0.25);
        const x0 = Math.cos(a0),
            y0 = Math.sin(a0);
        const x1 = Math.cos(a1),
            y1 = Math.sin(a1);
        const largeArc = end - start > 0.5 ? 1 : 0;

        return [
            '<path d="M',
            r + r0 * x0,
            r + r0 * y0,
            'L',
            r + r * x0,
            r + r * y0,
            'A',
            r,
            r,
            0,
            largeArc,
            1,
            r + r * x1,
            r + r * y1,
            'L',
            r + r0 * x1,
            r + r0 * y1,
            'A',
            r0,
            r0,
            0,
            largeArc,
            0,
            r + r0 * x0,
            r + r0 * y0,
            `" fill="${color}" />`
        ].join(' ');
    }
</script>
</body>
</html>