Midpoint between two geographical points

middlePoint.js

//-- Define middle point function
const middlePoint = (lat1, lng1, lat2, lng2) => {
  //-- Define radius function
  if (typeof (Number.prototype.toRad) === "undefined") {
    Number.prototype.toRad = function () {
        return this * Math.PI / 180;
    }
  }

  //-- Define degrees function
  if (typeof (Number.prototype.toDeg) === "undefined") {
    Number.prototype.toDeg = function () {
        return this * (180 / Math.PI);
    }
  }
  //-- Longitude difference
  const dLng = (lng2 - lng1).toRad();

  //-- Convert to radians
  lat1 = lat1.toRad();
  lat2 = lat2.toRad();
  lng1 = lng1.toRad();

  const bX = Math.cos(lat2) * Math.cos(dLng);
  const bY = Math.cos(lat2) * Math.sin(dLng);
  const lat3 = Math.atan2(Math.sin(lat1) + Math.sin(lat2), Math.sqrt((Math.cos(lat1) + bX) * (Math.cos(lat1) + bX) + bY * bY));
  const lng3 = lng1 + Math.atan2(bY, Math.cos(lat1) + bX);

  //-- Return result
  return [lat3.toDeg(), lng3.toDeg()];
}