location caching and distance checking

geoService.js

((angular, window) => {
  /**
   * GeoLocation Utility Service
   * @param $localStorage
   * @param $cordovaGeolocation
   * @param $q
   * @returns {{getCachedGeoLocation: getCachedGeoLocation, getDistance: getDistance}}
   * @constructor
   */
  var GeoService = function($localStorage, $cordovaGeolocation, $q) {
    var $storage = $localStorage,
      cachedLocation = $storage.currentLocation,
      isUS = navigator.language.match(/us/ig);

    // Was the last lookup more than 5 min ago and did it contain an error.
    var isLocationExpired = () =>
      angular.isUndefined(cachedLocation) // Is there a cachedLocation object
      || !cachedLocation.error // Error from device
      || cachedLocation.timestamp - Date.now() > 3E5; // Older than 300000ms(5 min)

    /**
     * Return the distance in miles if `navigator.language` is 'en-US' otherwise
     * returns distance in kilometers
     *
     * @param {number} distance
     * @returns {string} distance in either miles or km
     */
    var getLocaleDistance = (distance) =>
      isUS ? (distance / 1.609344).toFixed(1) + ' mi' : distance.toFixed(1) + ' km';

    /**
     * Calculates the distance between two latitude-longitude points
     *
     * Taken from {@link http://stackoverflow.com/a/7179026}
     * @param {number} lat1
     * @param {number} lon1
     * @param {number} lat2
     * @param {number} lon2
     * @returns {number}
     */
    var computeDistanceBetween = (lat1, lon1, lat2, lon2) => {
      var toRad = (val) => val * Math.PI / 180;

      var R = 6371 | 0, // Radius of the earth 6371 km
        dLat = toRad(lat2 - lat1),
        dLon = toRad(lon2 - lon1);

      var a = Math.sin(dLat / 2) * Math.sin(dLat / 2) + Math.cos(toRad(lat1)) * Math.cos(toRad(lat2)) * Math.sin(dLon / 2) * Math.sin(dLon / 2);

      var c = 2 * Math.atan(Math.sqrt(a), Math.sqrt(1 - a));
      return R * c;
    };

    /**
     * Storage for deferred variable.
     *
     * @type {null}
     */
    var deferred = null;

    /**
     * Get the user's last known location.
     * Checks cache(localStorage) before making call to device.
     *
     * @returns {external:Promise}
     */
    this.getCachedGeoLocation = function() {
      if (_.isNull(deferred)) {
        deferred = $q.defer();

        // Pull from cache
        if (!isLocationExpired()) {
          return $q.when(cachedLocation);

        // Pull from device
        } else {
          $cordovaGeolocation.getCurrentPosition().then((position) => {
            var currentPosition = {
              latitude: position.coords.latitude,
              longitude: position.coords.longitude,
              accuracy: position.coords.accuracy ,
              timestamp: +position.timestamp
            };

            $storage.currentLocation = currentPosition;
            deferred.resolve(currentPosition);
          }, (err) => {
            cachedLocation['error'] = err;
            deferred.reject(err)
          }).finally(() => {
            deferred = null;
          });
        }
      }

      return deferred.promise;
    };

    /**
     * Get the distance of the building from the cached geolocation of user
     *
     * @param {number} latitude
     * @param {number} longitude
     * @returns {string} prettyDistance
     */
    this.getDistance = function(latitude, longitude) {
      return getLocaleDistance(computeDistanceBetween(cachedLocation.latitude, cachedLocation.longitude, latitude, longitude));
    }
  };

  angular.module('iOffice.geo', ['ngCordova']).service('GeoService', GeoService);
})(angular, window);