Google OAuth using domain wide delegation and service account

Answer:

You need to pass your Service Account private key obtained from the GCP console to your JWT Client, and specify which user you wish to impersonate as a subject.

Code:

After getting your private key, you need to pass this into your JWT Client before authorisation:

let google = require('googleapis');
let privateKey = require("./privatekey.json");

var jwtClient = new google.auth.JWT({
       email: privateKey.client_email,
       key: privateKey.private_key,
       scopes: ['https://www.googleapis.com/auth/drive'],
       subject: '[email protected]'
    });

jwtClient.authorize(function (error, tokens) {
  if (error) {
    console.log(error);
    return;
  } 
  else {
    console.log("Successfully connected!");
  }
});

Then you can do as you wish with the Drive API as the service account.