403 Forbidden message when calling the v3 Google Calendar API using a Service Account via OAuth 2.0

Question

This is a follow on from my thread about a 401 error when using the Google Calendar API and OAuth2, which can be found here

This contains details of the account setu

Answer 1

I was also getting the same error even after sharing the calendar as in the Justin's answer. After comparing with another working sample figured out that I have set

var SCOPES = ["https://www.googleapis.com/auth/calendar.readonly"];

instead of

var SCOPES = ["https://www.googleapis.com/auth/calendar"];

Hope this helps anyone who has done a similar mistake and getting this error.