https://joshwright.com/tips/google-authentication-for-nodejs/

google-routes.js

// https://joshwright.com/tips/google-authentication-for-nodejs/

import { google } from 'googleapis';

const buildAuth = function () {
  return new google.auth.OAuth2(
    '0000000000000-xxxxxxxx.apps.googleusercontent.com', // clientId
    'XXXXXXXXXXXXXXXXXXX', // clientSecret
    'http://localhost:7777/google/callback', // redirect
  );
};

app.get('/google/login', async (req, res) => {
  const auth = buildAuth();
  const url = auth.generateAuthUrl({
    access_type: 'offline',
    prompt: 'consent',
    scope: [
      'https://www.googleapis.com/auth/userinfo.email',
      'profile',
    ]
  });
  res.redirect(url);
});

app.get('/google/callback', async (req, res) => {
  // get & set tokens
  const { code } = req.query;
  const auth = buildAuth();
  const { tokens } = await auth.getToken(code);
  auth.setCredentials(tokens);

  // fetch the user's email
  const peopleApi = google.people({version: 'v1', auth}).people;
  const me = await peopleApi.get({
    resourceName: 'people/me',
    personFields: 'emailAddresses',
  });
  const address = me.data.emailAddresses.find(e => e.metadata.primary);
  const email = address && address.value;

  // TODO: login with email

  res.redirect('/');
});