Show drawn polygon area

Use mapbox-gl-draw to draw a polygon and Turf.js to calculate its area in square meters.
Mapbox GL unsupported
Mapbox GL requires WebGL support. Please check that you are using a supported browser and that WebGL is enabled.
<!DOCTYPE html>
<html>
<head>
<meta charset='utf-8' />
<title>Show drawn polygon area</title>
<meta name='viewport' content='initial-scale=1,maximum-scale=1,user-scalable=no' />
<script src='https://api.tiles.mapbox.com/mapbox-gl-js/v0.53.0/mapbox-gl.js'></script>
<link href='https://api.tiles.mapbox.com/mapbox-gl-js/v0.53.0/mapbox-gl.css' rel='stylesheet' />
<style>
body { margin:0; padding:0; }
#map { position:absolute; top:0; bottom:0; width:100%; }
</style>
</head>
<body>
 
<style>
.calculation-box {
height: 75px;
width: 150px;
position: absolute;
bottom: 40px;
left: 10px;
background-color: rgba(255, 255, 255, .9);
padding: 15px;
text-align: center;
}
 
p {
font-family: 'Open Sans';
margin: 0;
font-size: 13px;
}
</style>
 
<script src='https://api.tiles.mapbox.com/mapbox.js/plugins/turf/v3.0.11/turf.min.js'></script>
<script src='https://api.mapbox.com/mapbox-gl-js/plugins/mapbox-gl-draw/v1.0.9/mapbox-gl-draw.js'></script>
<link rel='stylesheet' href='https://api.mapbox.com/mapbox-gl-js/plugins/mapbox-gl-draw/v1.0.9/mapbox-gl-draw.css' type='text/css'/>
<div id='map'></div>
<div class='calculation-box'>
<p>Draw a polygon using the draw tools.</p>
<div id='calculated-area'></div>
</div>
 
<script>
mapboxgl.accessToken = '<your access token here>';
/* eslint-disable */
var map = new mapboxgl.Map({
container: 'map', // container id
style: 'mapbox://styles/mapbox/satellite-v9', //hosted style id
center: [-91.874, 42.760], // starting position
zoom: 12 // starting zoom
});
 
var draw = new MapboxDraw({
displayControlsDefault: false,
controls: {
polygon: true,
trash: true
}
});
map.addControl(draw);
 
map.on('draw.create', updateArea);
map.on('draw.delete', updateArea);
map.on('draw.update', updateArea);
 
function updateArea(e) {
var data = draw.getAll();
var answer = document.getElementById('calculated-area');
if (data.features.length > 0) {
var area = turf.area(data);
// restrict to area to 2 decimal points
var rounded_area = Math.round(area*100)/100;
answer.innerHTML = '<p><strong>' + rounded_area + '</strong></p><p>square meters</p>';
} else {
answer.innerHTML = '';
if (e.type !== 'draw.delete') alert("Use the draw tools to draw a polygon!");
}
}
 
</script>
 
</body>
</html>