ES6 function to asynchronously get user region by IP.

getRegion.js

import Cookies from 'cookies-js';

export function getRegion() {
  return new Promise((resolve, reject) => {
    function deg2rad(deg) {
      return deg * (Math.PI / 180);
    }

    function getDistanceFromLatLonInKm(lat1, lon1, lat2, lon2) {
      const R = 6371; // Radius of the earth in km
      const dLat = deg2rad(lat2 - lat1);
      const dLon = deg2rad(lon2 - lon1);
      const 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);
      const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));
      const d = R * c; // Distance in km
      return d;
    }

    const provinces = {
      'Etela-Suomi': {
        lat: 60.3845675,
        long: 25.0254808,
      },

      'Keski-Suomi': {
        lat: 62.4576291,
        long: 27.0149535,
      },

      'Ita-Suomi': {
        lat: 61.2935055,
        long: 28.4981116,
      },

      'Lansi-Suomi': {
        lat: 60.8023989,
        long: 22.746071,
      },
    };

    let userRegion;

    if (window.location.pathname === '/') {
      userRegion = Cookies.get('userRegion');
    } else if (Object.keys(provinces).indexOf(window.location.pathname.substring(1)) > -1) {
      userRegion = window.location.pathname.substring(1);
    } else {
      userRegion = Cookies.get('userRegion');
    }


    if (userRegion) {
      resolve(userRegion);
    } else {
      const fallback = setTimeout(() => {
        reject('Unable to get location');
      }, 5000);

      fetch('//freegeoip.net/json/')
        .then(response => response.text())
        .then(body => {
          const response = JSON.parse(body);
          const lat = response.latitude;
          const long = response.longitude;

          Object.keys(provinces).forEach(key => {
            const pLat = provinces[key].lat;
            const pLong = provinces[key].long;

            provinces[key].distanceTo = getDistanceFromLatLonInKm(lat, long, pLat, pLong);
          });

          let distanceTo = 999999;

          Object.keys(provinces).forEach(key => {
            if (provinces[key].distanceTo < distanceTo) {
              distanceTo = provinces[key].distanceTo;
              userRegion = key;
            }
          });

          resolve(userRegion);
          Cookies.set('userRegion', userRegion);
          clearTimeout(fallback);
        });
    }
  });
}