ジオコーダの結果をリージョン名に制限

Mapbox Geocoding APIのmapbox-gl-geocoderコントロール機能を使って、結果領域を制限しながら、リージョンを検索します。
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8" />
<title>ジオコーダの結果をリージョン名に制限</title>
<meta name="viewport" content="initial-scale=1,maximum-scale=1,user-scalable=no" />
<script src="https://api.mapbox.com/mapbox-gl-js/v1.6.1/mapbox-gl.js"></script>
<link href="https://api.mapbox.com/mapbox-gl-js/v1.6.1/mapbox-gl.css" rel="stylesheet" />
<style>
	body { margin: 0; padding: 0; }
	#map { position: absolute; top: 0; bottom: 0; width: 100%; }
</style>
</head>
<body>
<script src="https://api.mapbox.com/mapbox-gl-js/plugins/mapbox-gl-geocoder/v4.4.2/mapbox-gl-geocoder.min.js"></script>
<link
rel="stylesheet"
href="https://api.mapbox.com/mapbox-gl-js/plugins/mapbox-gl-geocoder/v4.4.2/mapbox-gl-geocoder.css"
type="text/css"
/>
<!-- Promise polyfill script required to use Mapbox GL Geocoder in IE 11 -->
<script src="https://cdn.jsdelivr.net/npm/es6-promise@4/dist/es6-promise.min.js"></script>
<script src="https://cdn.jsdelivr.net/npm/es6-promise@4/dist/es6-promise.auto.min.js"></script>
<div id="map"></div>
 
<script>
	mapboxgl.accessToken = '<your access token here>';
var map = new mapboxgl.Map({
container: 'map',
style: 'mapbox://styles/mapbox/streets-v11',
center: [146.359, -32.648],
zoom: 4
});
 
map.addControl(
new MapboxGeocoder({
accessToken: mapboxgl.accessToken,
 
// limit results to Australia
countries: 'au',
 
// further limit results to the geographic bounds representing the region of
// New South Wales
bbox: [139.965, -38.03, 155.258, -27.839],
 
// apply a client side filter to further limit results to those strictly within
// the New South Wales region
filter: function(item) {
// returns true if item contains New South Wales region
return item.context
.map(function(i) {
// id is in the form {index}.{id} per https://github.com/mapbox/carmen/blob/master/carmen-geojson.md
// this example attempts to find the `region` named `New South Wales`
return (
i.id.split('.').shift() === 'region' &&
i.text === 'New South Wales'
);
})
.reduce(function(acc, cur) {
return acc || cur;
});
},
mapboxgl: mapboxgl
})
);
</script>
 
</body>
</html>