Distance between points simplified

function

getDistanceFromLatLonInM(lat1, lon1, lat2, lon2) {
  var a =
    Math.pow(Math.sin(deg2rad(lat2 - lat1) / 2), 2) +
    Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(lat2)) *
    Math.pow(Math.sin(deg2rad(lon2 - lon1) / 2), 2);
  return 12742000 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
}