geodistance.js

function geodistance(lat1,lon1,lat2,lon2){
  const R = 6371e3; // in metres
  const φ1 = lat1 * Math.PI/180; // φ, λ in radians
  const φ2 = lat2 * Math.PI/180;
  const Δφ = (lat2-lat1) * Math.PI/180;
  const Δλ = (lon2-lon1) * Math.PI/180;

  const a = Math.sin(Δφ/2) * Math.sin(Δφ/2) +
            Math.cos(φ1) * Math.cos(φ2) *
            Math.sin(Δλ/2) * Math.sin(Δλ/2);
  const c = 2 * Math.atan2(a**.5, (1-a)**.5);

  const d = R * c; // in metres
  return d;
}