calculateDistance.js

const EARTH_RADIUS_IN_METERS = 6371000;
const toRad = value => value * Math.PI / 180;

const calculateDistance = (startWaypoint, endWaypoint) => {

  const dLat = toRad(endWaypoint.lat - startWaypoint.lat);
  const dLon = toRad(endWaypoint.lon - startWaypoint.lon);

  const startLat = toRad(startWaypoint.lat);
  const endLat = toRad(endWaypoint.lat);

  const a = Math.sin(dLat/2) * Math.sin(dLat/2) + 
            Math.sin(dLon/2) * Math.sin(dLon/2) * Math.cos(startLat) * Math.cos(endLat);

  const c = 2 * Math.atan2(Math.sqrt(a), Math.sqrt(1 - a));

  return EARTH_RADIUS_IN_METERS * c;
};