Calculate Distance between two LatLngs

calculate_distance.js

var calculateDistance = function(a, b) {
  var R = 6371; // Radius of the earth in km
  var dLat = (b.lat - a.lat) * (Math.PI / 180); 
  var dLon = (b.lng - a.lng) * (Math.PI / 180);
  var a =
    Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.cos(a.lat * (Math.PI / 180)) * Math.cos(b.lat * (Math.PI / 180)) *
    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 * 1000;
}