module.exports = (lon1, lat1, lon2, lat2) => {
  // mean radius of earth's = 6,371km
  const R = 6371;
  // distance between latitude and longitude in radians
  const dLat = ((lat2 - lat1) * Math.PI) / 180;
  const dLon = ((lon2 - lon1) * Math.PI) / 180;
  // haversine’ formula to calculate distance
  const a =
    Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.cos((lat1 * Math.PI) / 180) *
      Math.cos((lat2 * Math.PI) / 180) *
      Math.sin(dLon / 2) *
      Math.sin(dLon / 2);
  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
  const d = R * c;
  // if (d > 1) return Math.round(d) + "km";
  // else if (d <= 1) return Math.round(d * 1000) + "m";
  return d;
};
