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Create a hover effect

Use events and feature states to create a per feature hover effect.

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Create a hover effect</title>
    <meta property="og:description" content="Use events and feature states to create a per feature hover effect." />
    <meta charset='utf-8'>
    <meta name="viewport" content="width=device-width, initial-scale=1">
    <link rel='stylesheet' href='https://unpkg.com/maplibre-gl@4.4.0/dist/maplibre-gl.css' />
    <script src='https://unpkg.com/maplibre-gl@4.4.0/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',
        style:
            'https://api.maptiler.com/maps/streets/style.json?key=get_your_own_OpIi9ZULNHzrESv6T2vL',
        center: [-100.486052, 37.830348],
        zoom: 2
    });
    let hoveredStateId = null;

    map.on('load', () => {
        map.addSource('states', {
            'type': 'geojson',
            'data':
                'https://maplibre.org/maplibre-gl-js/docs/assets/us_states.geojson'
        });

        // The feature-state dependent fill-opacity expression will render the hover effect
        // when a feature's hover state is set to true.
        map.addLayer({
            'id': 'state-fills',
            'type': 'fill',
            'source': 'states',
            'layout': {},
            'paint': {
                'fill-color': '#627BC1',
                'fill-opacity': [
                    'case',
                    ['boolean', ['feature-state', 'hover'], false],
                    1,
                    0.5
                ]
            }
        });

        map.addLayer({
            'id': 'state-borders',
            'type': 'line',
            'source': 'states',
            'layout': {},
            'paint': {
                'line-color': '#627BC1',
                'line-width': 2
            }
        });

        // When the user moves their mouse over the state-fill layer, we'll update the
        // feature state for the feature under the mouse.
        map.on('mousemove', 'state-fills', (e) => {
            if (e.features.length > 0) {
                if (hoveredStateId) {
                    map.setFeatureState(
                        {source: 'states', id: hoveredStateId},
                        {hover: false}
                    );
                }
                hoveredStateId = e.features[0].id;
                map.setFeatureState(
                    {source: 'states', id: hoveredStateId},
                    {hover: true}
                );
            }
        });

        // When the mouse leaves the state-fill layer, update the feature state of the
        // previously hovered feature.
        map.on('mouseleave', 'state-fills', () => {
            if (hoveredStateId) {
                map.setFeatureState(
                    {source: 'states', id: hoveredStateId},
                    {hover: false}
                );
            }
            hoveredStateId = null;
        });
    });
</script>
</body>
</html>