Enable Google API Offline Server Side access with server auth code provided by mobile app (https://developers.google.com/identity/sign-in/android/offline-access)

google-server-side-access-example.js

const { google } = require('googleapis')

module.exports = app => {
  const controller = {}

  controller.auth = async (req, res, next) => {
    try {

      const oauth2Client = new google.auth.OAuth2(
        'YOUR_CLIENT_ID',
        'YOUR_CLIENT_SECRET',
        '' // Leave empty string for offline access
      )

      google.options({ auth: oauth2Client })

      // Exchange server auth code for OAuth2 credentials
      const { tokens } = await oauth2Client.getToken(
        req.body.serverAuthCode
      )

      oauth2Client.setCredentials(tokens)

      const plus = google.plus({
        version: 'v1',
        auth: oauth2Client
      })

      const people = await plus.people.get({ userId: 'me' })

      res.send(people.data)
    } catch (ex) {
      next(ex)
    }
  }

  return controller
}