Use a fallback image | Mapbox GL JS

Use a coalesce expression to display another image when a requested image is not available.
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Use a fallback image</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no">
<link href="https://api.mapbox.com/mapbox-gl-js/v2.2.0/mapbox-gl.css" rel="stylesheet">
<script src="https://api.mapbox.com/mapbox-gl-js/v2.2.0/mapbox-gl.js"></script>
<style>
body { margin: 0; padding: 0; }
#map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>
<div id="map"></div>
<script>
	// TO MAKE THE MAP APPEAR YOU MUST
	// ADD YOUR ACCESS TOKEN FROM
	// https://account.mapbox.com
	mapboxgl.accessToken = '<your access token here>';
var map = new mapboxgl.Map({
container: 'map',
style: 'mapbox://styles/mapbox/light-v10',
center: [-77, 38.75],
zoom: 5
});
map.on('load', function () {
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', 'rocket-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>