var dPhi = Math.log(Math.tan(lat2/2+Math.PI/4)/Math.tan(lat1/2+Math.PI/4));
var q = (!isNaN(dLat/dPhi)) ? dLat/dPhi : Math.cos(lat1);  // E-W line gives dPhi=0

// if dLon over 180° take shorter rhumb across 180° meridian:
if (Math.abs(dLon) > Math.PI) {
  dLon = dLon>0 ? -(2*Math.PI-dLon) : (2*Math.PI+dLon);
}
var d = Math.sqrt(dLat*dLat + q*q*dLon*dLon) * R;
var brng = Math.atan2(dLon, dPhi);