CalculateDistance.js

var distance = function(lat1, lon1, lat2, lon2){
  var R = 6371e3; // metres
  var φ1 = lat1 * Math.PI / 180;
  var φ2 = lat2 * Math.PI / 180;
  var Δφ = (lat2-lat1)* Math.PI / 180;
  var Δλ = (lon2-lon1)* Math.PI / 180;

  var a = Math.sin(Δφ/2) * Math.sin(Δφ/2) +
          Math.cos(φ1) * Math.cos(φ2) *
          Math.sin(Δλ/2) * Math.sin(Δλ/2);
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));

  //distance in meters
  var d = R * c;

  return d;
}