Add a circle overlay polygon in Mapbox

createGeoJSONCircle.js

var createGeoJSONCircle = function(center, radiusInKm, points) {
    if(!points) points = 64;

    var coords = {
        latitude: center[1],
        longitude: center[0]
    };

    var km = radiusInKm;

    var ret = [];
    var distanceX = km/(111.320*Math.cos(coords.latitude*Math.PI/180));
    var distanceY = km/110.574;

    var theta, x, y;
    for(var i=0; i<points; i++) {
        theta = (i/points)*(2*Math.PI);
        x = distanceX*Math.cos(theta);
        y = distanceY*Math.sin(theta);

        ret.push([coords.longitude+x, coords.latitude+y]);
    }
    ret.push(ret[0]);

    return {
        "type": "geojson",
        "data": {
            "type": "FeatureCollection",
            "features": [{
                "type": "Feature",
                "geometry": {
                    "type": "Polygon",
                    "coordinates": [ret]
                }
            }]
        }
    };
};

map.js

map.addSource("polygon", createGeoJSONCircle([-93.6248586, 41.58527859], 0.5));

map.addLayer({
    "id": "polygon",
    "type": "fill",
    "source": "polygon",
    "layout": {},
    "paint": {
        "fill-color": "blue",
        "fill-opacity": 0.6
    }
});

source.txt

https://stackoverflow.com/questions/37599561/drawing-a-circle-with-the-radius-in-miles-meters-with-mapbox-gl-js