Too Long; Didn't Read
First of all, a brief overview of our use case. Let's say I have a spreadsheet on Google Sheets which is not public and I want to be able to read/modify programmatically through some batch process running on my local machine or some server. This is something I had to do recently with a Node.js application and I found the authentication part a bit tricky to understand. So I thought of sharing my solution and I hope it helps someone in need. There might be better ways of doing this but I am sharing what worked best for me.