Find the distance between two locations with longitude/latitude points using the Haversine formula.

distance.js

function deg2rad(deg) {
  return deg * (Math.PI/180)
};

function getDistance(lat1, lon1, lat2, lon2) {
  var R = 3959; // Radius of the earth in miles
  var dLat = deg2rad(lat2-lat1);  // deg2rad below
  var dLon = deg2rad(lon2-lon1); 
  var a = 
    Math.sin(dLat/2) * Math.sin(dLat/2) +
    Math.cos(deg2rad(lat1)) * Math.cos(deg2rad(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 miles
  return d;
};