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Use a fallback image

Use a coalesce expression to display another image when a requested image is not available.

<!DOCTYPE html>
<html lang="en">
<head>
    <title>Use a fallback image</title>
    <meta property="og:description" content="Use a coalesce expression to display another image when a requested image is not available." />
    <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',
        style: 'https://api.maptiler.com/maps/streets/style.json?key=get_your_own_OpIi9ZULNHzrESv6T2vL',
        center: [-77, 38.75],
        zoom: 5
    });
    map.on('load', () => {
        map.addSource('points', {
            'type': 'geojson',
            'data': {
                'type': 'FeatureCollection',
                'features': [
                    {
                        'type': 'Feature',
                        'geometry': {
                            'type': 'Point',
                            'coordinates': [
                                -77.03238901390978, 38.913188059745586
                            ]
                        },
                        'properties': {
                            'title': 'Washington DC',
                            'icon': 'monument'
                        }
                    },
                    {
                        'type': 'Feature',
                        'geometry': {
                            'type': 'Point',
                            'coordinates': [-79.9959, 40.4406]
                        },
                        'properties': {
                            'title': 'Pittsburgh',
                            'icon': 'bridges'
                        }
                    },
                    {
                        'type': 'Feature',
                        'geometry': {
                            'type': 'Point',
                            'coordinates': [-76.2859, 36.8508]
                        },
                        'properties': {
                            'title': 'Norfolk',
                            'icon': 'harbor'
                        }
                    }
                ]
            }
        });
        map.addLayer({
            'id': 'points',
            'type': 'symbol',
            'source': 'points',
            'layout': {
                'icon-image': [
                    'coalesce',
                    ['image', ['concat', ['get', 'icon'], '_15']],
                    ['image', 'marker_15']
                ],
                'text-field': ['get', 'title'],
                'text-font': ['Open Sans Semibold', 'Arial Unicode MS Bold'],
                'text-offset': [0, 0.6],
                'text-anchor': 'top'
            }
        });
    });
</script>
</body>
</html>