Sample Leaflet with GeoJSON by ThomasG77

index.html

<html>
<head>
  <title>A Leaflet map!</title>
  <link rel="stylesheet" href="https://unpkg.com/leaflet@1.7.1/dist/leaflet.css" integrity="sha512-xodZBNTC5n17Xt2atTPuE1HxjVMSvLVW9ocqUKLsCC5CXdbqCmblAshOMAS6/keqq/sMZMZ19scR4PsZChSR7A==" crossorigin="" />
  <script src="https://unpkg.com/leaflet@1.7.1/dist/leaflet.js" integrity="sha512-XQoYMqMTK8LvdxXYG3nZ448hOEQiglfqkJs1NOQV44cWnUrBc8PkAOcXy20w0vlaXaVUearIOBhiXZ5V3ynxwA==" crossorigin=""></script>
  <style>
    #map{ height: 100% }
  </style>
</head>
<body>

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

  <script>

  // initialize the map
  var map = L.map('map').setView([42.35, -71.08], 3);

  // load a tile layer
  L.tileLayer('//tiles.mapc.org/basemap/{z}/{x}/{y}.png',
    {
      attribution: 'Tiles by <a href="//mapc.org">MAPC</a>, Data by <a href="//mass.gov/mgis">MassGIS</a>',
      maxZoom: 17,
      minZoom: 9
    }).addTo(map);

  // load GeoJSON from an external file
  fetch("countries.geojson").then(res => res.json()).then(data => {
    // add GeoJSON layer to the map once the file is loaded
    L.geoJson(data).addTo(map);
  });

  </script>
</body>
</html>