Maps API v3: Calculate bearing

You can use the Geometry Library.

You need to add it to the API call:

https://maps.googleapis.com/maps/api/js?libraries=geometry

Then use the computeHeading method:

var heading = google.maps.geometry.spherical.computeHeading(pointA, pointB);

where pointA and pointB are the two LatLng objects.

https://developers.google.com/maps/documentation/javascript/reference?csw=1#spherical