Mapbox turf

HTML

<script src="https://api.mapbox.com/mapbox-gl-js/v0.40.1/mapbox-gl.js"></script>
<link rel="stylesheet" href="https://api.mapbox.com/mapbox-gl-js/v0.40.1/mapbox-gl.css">
<script src="https://npmcdn.com/@turf/turf/turf.min.js"></script>
<div id='map'></div>

CSS

body {
  margin: 0;
  padding: 0;
}

#map {
  position: absolute;
  top: 0;
  bottom: 0;
  width: 100%;
}

.mapboxgl-marker {
    width: 15px;
    height: 15px;
    border-radius: 50%;
    border:1px solid gray;
    background-color:orange;
}

JavaScript

mapboxgl.accessToken = 'pk.eyJ1IjoiYnVsdXRrYXJ0YWwiLCJhIjoiY2o4MTIzODk1Nm1nZDJ4cGtucWlmMGphdiJ9.IeHsRNvJMISAoPccBSxkvQ';

var map = new mapboxgl.Map({
    container: 'map',
    style: 'mapbox://styles/mapbox/streets-v8',
    center: [-.12, 51.51],
    zoom: 9
});

  
  map.on('load', function () {

		var coords = [-.12, 51.51];
    var point = turf.point(coords);
		var buffered = turf.buffer(point, 9, {units: 'miles'});
    
    
  	var marker = new mapboxgl.Marker().setLngLat(coords).addTo(map);

    map.addSource("buffer", {
      type: "geojson",
      data: buffered
    })

    map.addLayer({
        'id': 'poly-buffer',
        'type': 'fill',
        'source': "buffer",
        'layout': {},
        'paint': {
            'fill-color': '#088',
            'fill-opacity': 0.2
        }
    });

    
  });