getDistance calculate distance in km in javascript

gistfile1

function getDistance(lon1, lat1, lon2, lat2) {
  var R = 6371; // Radius of the earth in km
  var dLat = toRad(lat2-lat1);
  var dLon = toRad(lon2-lon1);
  var a = Math.sin(dLat/2) * Math.sin(dLat/2) +
          Math.cos(toRad(lat1)) * Math.cos(toRad(lat2)) * 
          Math.sin(dLon/2) * Math.sin(dLon/2); 
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a)); 
  return R * c; // Distance in km
}