Distance between two geographical points

distanceBetween.js

/**
 * Source http://www.movable-type.co.uk/scripts/latlong.html
 */
const distanceBetween  = (lat2, lat1, lng2, lng1) => {
  Number.prototype.toRad = function() {
    return this * Math.PI / 180;
  };
  // Haversine Formula
  const x1 = lat2-lat1;
  const dLat = x1.toRad();
  const x2 = lng2-lng1;
  const dLon = x2.toRad();
  const a = Math.sin(dLat/2) * Math.sin(dLat/2) +
            Math.cos(lat1.toRad()) * Math.cos(lat2.toRad()) *
            Math.sin(dLon/2) * Math.sin(dLon/2);
  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1-a));
  const distance = Math.round(((RADIUS * c) * FOOT) / 2);