A function that calculates the distance, gradient and bearing of a path between two points on the Earth. Requires the Math.js library because of the Number.prototype.* calls.

Points.js

function Points(lat1, lon1, alt1,
                lat2, lon2, alt2) {
  alt1 = alt1 / 1000 || 0;
  alt2 = alt2 / 1000 || 0;
  var dLat = (lat2 - lat1).degToRad(),
      dLon = (lon2 - lon1).degToRad(),
      dAlt = alt2 - alt1;
  lat1 = lat1.degToRad();
  lat2 = lat2.degToRad();
  lon1 = lon1.degToRad();
  lon2 = lon2.degToRad();
  var a = (dLat / 2).sin().pow() +
          (dLon / 2).sin().pow() * lat1.cos()
                                 * lat2.cos(),
      Bx = lat2.cos() * dLon.cos(),
      By = lat2.cos() * dLon.sin(),
      gDist = 12742 * a.rt().atan2((1 - a).rt());
  this.midPoint = [
    Math.atan2(lat1.sin() + lat2.sin(),
             ((lat1.cos() + Bx) *
              (lat1.cos() + Bx) + By.pow()).rt()).radToDeg(),
    (lon1 + By.atan2(lat1.cos() + Bx)).radToDeg(),
    Math.avg(alt1, alt2)
  ];
  this.distance = Math.sqrt(gDist.pow() + dAlt.pow());
  this.gradient = Math.acos(
    12742 * a.rt().atan2((1 - a).rt()) / this.distance
  ).radToDeg();
  this.initBearing = ((
    dLon.sin() * lat2.cos()
  ).atan2(
    lat1.cos() * lat2.sin() -
    lat1.sin() * lat2.cos() * dLon.cos()
  ).radToDeg()).degAdjust();
  this.finalBearing = ((
    (-dLon).sin() * lat1.cos()
  ).atan2(
    lat2.cos() * lat1.sin() -
    lat2.sin() * lat1.cos() * (-dLon).cos()
  ).radToDeg() + 180).degAdjust();
  lat1 = this.midPoint[0].degToRad();
  lat2 = lat2;
  dLon = (lon2.radToDeg() - this.midPoint[1]).degToRad();
  this.bearing = ((
    dLon.sin() * lat2.cos()
  ).atan2(
    lat1.cos() * lat2.sin() -
    lat1.sin() * lat2.cos() * dLon.cos()
  ).radToDeg()).degAdjust();
};