Calculate the distance between two latitude/longitude coordinate pairs using the haversine formula.

haversineEarthDistance.js

/* By default this will return the distance in kilometers, but will return
 * miles when the inMiles argument is true.
 */

function haversineEarthDistance(lat1, lon1, lat2, lon2, inMiles) {
  function toRads(x) { return (x * Math.PI) / 180; }
  var EARTH_RADIUS_KM = 6373;
  var KM_PER_MI = 1.609344;

  var dLat = toRads(lat2 - lat1);
  var dLon = toRads(lon2 - lon1);

  var a =
    Math.sin(dLat / 2) * Math.sin(dLat / 2) +
    Math.cos(toRads(lat1)) *
    Math.cos(toRads(lat2)) *
    Math.sin(dLon / 2) *
    Math.sin(dLon / 2);
  var c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));

  if (inMiles) {
    return (EARTH_RADIUS_KM * c) / KM_PER_MI;
  } else {
    return EARTH_RADIUS_KM * c;
  }
}