D3 Examples - Create geojson; add a polygon and circle layer from source

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<!DOCTYPE html>
<html>
<head>
    <meta charset='utf-8' />
    <title></title>
    <meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
    <script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.32.1/mapbox-gl.js'></script>
    <link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.32.1/mapbox-gl.css' rel='stylesheet' />
    <style>
        body { margin:0; padding:0; }
        #map { position:absolute; top:0; bottom:0; width:100%; }
    </style>
</head>
<body>
​
<div id="map"></div>
<script>
mapboxgl.accessToken = 'pk.eyJ1IjoidGtheW5lMjMiLCJhIjoiN2ZuVll5MCJ9.INSHe8gm7HMfO8HZPUuAhg';
var map = new mapboxgl.Map({
    container: "map",
    style: "mapbox://styles/mapbox/outdoors-v9",
    center: [-121.403732, 40.492392],
    zoom: 10
});
​
map.on("load", function() {
    map.addSource("national-park", {
        "type": "geojson",
        "data": {
            "type": "FeatureCollection",
            "features": [{
                "type": "Feature",
                "geometry": {
                    "type": "Polygon",
                    "coordinates": [
                        [
                          [-121.353637, 40.584978],
                          [-121.284551, 40.584758],
                          [-121.275349, 40.541646],
                          [-121.246768, 40.541017],
                          [-121.251343, 40.423383],
                          [-121.326870, 40.423768],
                          [-121.360619, 40.434790],
                          [-121.363694, 40.409124],
                          [-121.439713, 40.409197],
                          [-121.439711, 40.423791],
                          [-121.572133, 40.423548],
                          [-121.577415, 40.550766],
                          [-121.539486, 40.558107],
                          [-121.520284, 40.572459],
                          [-121.487219, 40.550822],
                          [-121.446951, 40.563190],
                          [-121.370644, 40.563267],
                          [-121.353637, 40.584978]
                        ]
                    ]
                }
            }, {
                "type": "Feature",
                "geometry": {
                    "type": "Point",
                    "coordinates": [-121.415061, 40.506229]
                }
            }, {
                "type": "Feature",
                "geometry": {
                    "type": "Point",
                    "coordinates": [-121.505184, 40.488084]
                }
            }, {
                "type": "Feature",
                "geometry": {
                    "type": "Point",
                    "coordinates": [-121.354465, 40.488737]
                }
            }]
        }
    });
​
    map.addLayer({
        "id": "park-boundary",
        "type": "fill",
        "source": "national-park",
        "paint": {
            "fill-color": "#888888",
            "fill-opacity": 0.4
        },
        "filter": ["==", "$type", "Polygon"]
    });
​
    map.addLayer({
        "id": "park-volcanoes",
        "type": "circle",
        "source": "national-park",
        "paint": {
            "circle-radius": 6,
            "circle-color": "#B42222"
        },
        "filter": ["==", "$type", "Point"],
    });
});
</script>
​
</body>
</html>
​
​
    