Negative buffer with turf.js

<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8 />
<title>turf-buffer</title>
<meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
<script src='https://cdn.jsdelivr.net/gh/turfjs/turf/turf.min.js'></script>
<script src='https://api.tiles.mapbox.com/mapbox.js/v2.0.1/mapbox.js'></script>
<link href='https://api.tiles.mapbox.com/mapbox.js/v2.0.1/mapbox.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>

var polygon = turf.polygon([
    [
        [-121.5847,40.1002],[-121.5080,40.1002],[-121.4368,40.1495],[-121.4040,40.1495],[-121.3656,40.0838],[-121.4204,40.0181],[-121.4094,39.8702],[-121.0753,39.5963],[-121.1575,39.5197],[-121.3054,39.5197],[-121.3328,39.4265],[-121.4040,39.3389],[-121.6121,39.3170],[-121.6230,39.2951],[-121.9078,39.3061],[-121.8914,39.3827],[-121.8585,39.5361],[-121.9955,39.5361],[-121.9900,39.6401],[-121.9681,39.6511],[-121.9955,39.6675],[-121.9407,39.6949],[-121.9462,39.7333],[-122.0448,39.7990],[-122.0667,39.8428],[-122.0448,39.8811],[-121.7764,39.8866],[-121.7052,39.9852],[-121.6449,39.9852]
    ]
], {
    "fill": "#e2e3e3",
    "stroke": "#333",
    "stroke-width": 5
});

var map = L.mapbox.map('map', 'faraday2.206936a8')
    .setView([39.66914219401813, -121.44561767578125], 9);

var buffer = turf.buffer(polygon, -8, "miles");

buffer.features[0].properties = {
    "fill": "#0000ff",
    "stroke": "#0000ff",
    "stroke-width": 5,
    "description": "8 miles inward"
}

L.mapbox.featureLayer().setGeoJSON(polygon).addTo(map);
L.mapbox.featureLayer().setGeoJSON(buffer).addTo(map);
</script>
</body>
</html>

Updated missing url https://rawgit.com/Turfjs/turf/master/turf.min.js to https://cdn.jsdelivr.net/gh/turfjs/turf/turf.min.js

https://rawgit.com/Turfjs/turf/master/turf.min.js

https://api.tiles.mapbox.com/mapbox.js/v2.0.1/mapbox.js