get distance between two coordinates in kilometers

geo-distance.js

var convertDegToRad = require('./convert-deg-to-rad');

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