The math behind
every map
you'll ever build.

function haversine(lat1, lon1, lat2, lon2) {
  const R = 6371, toRad = d => d * Math.PI / 180;
  const φ1 = toRad(lat1), φ2 = toRad(lat2);
  const Δφ = toRad(lat2 - lat1);
  const Δλ = toRad(lon2 - lon1);
  const a = Math.sin(Δφ/2)**2
          + Math.cos(φ1) * Math.cos(φ2)
          * Math.sin(Δλ/2)**2;
  return 2 * R * Math.asin(Math.sqrt(a));
}

function latLngToTile(lat, lon, z) {
  const n = 2 ** z;
  const x = Math.floor((lon + 180) / 360 * n);
  const r = lat * Math.PI / 180;
  const y = Math.floor(
    (1 - Math.log(Math.tan(r) + 1/Math.cos(r))
      / Math.PI) / 2 * n);
  return { z, x, y };
}