Edit in JSFiddle

// Leaflet has native support for raster maps, So you can create a map with a few commands only!

// The Leaflet map Object
var map = L.map('my-map').setView([48.1500327, 11.5753989], 10);

// The API Key provided is restricted to JSFiddle website
// Get your own API Key on https://myprojects.geoapify.com
var myAPIKey = "6dc7fb95a3b246cfa0f3bcef5ce9ed9a";

// Retina displays require different mat tiles quality
var isRetina = L.Browser.retina;

var baseUrl = "https://maps.geoapify.com/v1/tile/osm-bright/{z}/{x}/{y}.png?apiKey={apiKey}";
var retinaUrl = "https://maps.geoapify.com/v1/tile/osm-bright/{z}/{x}/{y}@2x.png?apiKey={apiKey}";

// Add map tiles layer. Set 20 as the maximal zoom and provide map data attribution.
L.tileLayer(isRetina ? retinaUrl : baseUrl, {
  attribution: 'Powered by <a href="https://www.geoapify.com/" target="_blank">Geoapify</a> | © OpenStreetMap <a href="https://www.openstreetmap.org/copyright" target="_blank">contributors</a>',
  apiKey: myAPIKey,
  maxZoom: 20,
  id: 'osm-bright',
}).addTo(map);

//generate an 30 min transit isochrone from Munich center
fetch(`https://api.geoapify.com/v1/isoline?lat=48.13736145&lon=11.574172059316222&type=time&mode=transit&range=1800&apiKey=${myAPIKey}`).then(data => data.json()).then(geoJSONFeatures => {

  //get hole polygons from geoJSOnFeatures
  const holePolygons = geoJSONFeatures.features[0].geometry.coordinates.map(poly => poly[0]);
  const mapCover = [
    [-179.99, 89.99],
    [-179.99, 0],
    [-179.99, -89.99],
    [0, -89.99],
    [179.99, -89.99],
    [179.99, 0],
    [179.99, 89.99],
    [0, 89.99],
    [-179.99, 89.99]
  ];
  const geoJSONPolygon = {
    "type": "Feature",
    "geometry": {
      "type": "Polygon",
      "coordinates": [mapCover, ...holePolygons]
    }
  }

  L.geoJSON(geoJSONPolygon, {
    stroke: true,
    color: '#000',
    weight: 2,
    opacity: 0.7,
    fill: true,
    fillColor: '#000',
    fillOpacity: 0.3,
    smoothFactor: 0.5,
    interactive: false
  }).addTo(map);
});

<div id="my-map"></div>

html,
body,
#my-map {
  width: 100%;
  height: 100%;
  margin: 0;
}