index.js

// [START maps_geometry_headings]
// This example requires the Geometry library. Include the libraries=geometry
// parameter when you first load the API. For example:
// <script src="https://maps.googleapis.com/maps/api/js?key=YOUR_API_KEY&libraries=geometry">
let marker1, marker2;
let poly, geodesicPoly;

function initMap() {
  const map = new google.maps.Map(document.getElementById("map"), {
    zoom: 4,
    center: { lat: 34, lng: -40.605 },
  });
  map.controls[google.maps.ControlPosition.TOP_CENTER].push(
    document.getElementById("info")
  );
  marker1 = new google.maps.Marker({
    map,
    draggable: true,
    position: { lat: 40.714, lng: -74.006 },
  });
  marker2 = new google.maps.Marker({
    map,
    draggable: true,
    position: { lat: 48.857, lng: 2.352 },
  });
  const bounds = new google.maps.LatLngBounds(
    marker1.getPosition(),
    marker2.getPosition()
  );
  map.fitBounds(bounds);
  google.maps.event.addListener(marker1, "position_changed", update);
  google.maps.event.addListener(marker2, "position_changed", update);
  poly = new google.maps.Polyline({
    strokeColor: "#FF0000",
    strokeOpacity: 1.0,
    strokeWeight: 3,
    map: map,
  });
  geodesicPoly = new google.maps.Polyline({
    strokeColor: "#CC0099",
    strokeOpacity: 1.0,
    strokeWeight: 3,
    geodesic: true,
    map: map,
  });
  update();
}

function update() {
  const path = [marker1.getPosition(), marker2.getPosition()];
  poly.setPath(path);
  geodesicPoly.setPath(path);
  const heading = google.maps.geometry.spherical.computeHeading(
    path[0],
    path[1]
  );
  document.getElementById("heading").value = String(heading);
  document.getElementById("origin").value = String(path[0]);
  document.getElementById("destination").value = String(path[1]);
}
// [END maps_geometry_headings]