How to use the google-auth-library function in google-auth-library

//  if the login token is not verified, returns an error reason
//    CouldNotVerifyToken
//  if the user is not registered, return a reason NoUser

import util from 'util';
import GoogleAuth from 'google-auth-library';
import mongoose from 'mongoose';

import credentialsClient from '../credentials-client';
import { makeError }
  from '../_common/express-helpers';

let User;

const googleAuth = new GoogleAuth();

const client
      = new googleAuth.OAuth2(credentialsClient.googleClientId, '', '');

export default async function authUserGoogle(idToken, isProduction = true) {
  if (!User) {
    User = mongoose.model('User');
  }

  return new Promise((resolve, _) => {
    //  the only reference to the function verifyIdToken I could find is at
    //  https://developers.google.com/identity/sign-in/web/backend-auth
    client.verifyIdToken(
      idToken,
      credentialsClient.googleClientId,
      //  I assume e is for errors

export const getOauth2Client = ({ accessToken, refreshToken } = {}) => {
  const credentials = CREDENTIALS.web || CREDENTIALS.installed;

  if (!credentials) {
    throw new Error("client-secret.json doesn't exist in root folder!");
  }

  const clientId = credentials.client_id;
  const clientSecret = credentials.client_secret;
  const redirectUrl = credentials.redirect_uris[0];

  const auth = new googleAuth();

  const oauth2Client = new auth.OAuth2(clientId, clientSecret, redirectUrl);

  oauth2Client.credentials = {
    ...(accessToken ? { access_token: accessToken } : undefined),
    ...(refreshToken ? { refresh_token: refreshToken } : undefined)
  };

  return oauth2Client;
};

import googleAuth from 'google-auth-library';
import * as cache from './utils/cache';

let config;
try {
  config = require(path.join(process.cwd(), '.blessedstanduprc'));
} catch(e) {
  throw new Error(
    'blessed-standup requires a `.blessedstanduprc` file containing a Google ' +
    'OAuth `CLIENT_ID` and `CLIENT_SECRET`.'
  );
}

const PERMISSION_SCOPE = 'https://spreadsheets.google.com/feeds';

const OAuth2Client = new googleAuth().OAuth2;
const client = new OAuth2Client(
  config.CLIENT_ID,
  config.CLIENT_SECRET,
  'urn:ietf:wg:oauth:2.0:oob'
);

const url = client.generateAuthUrl({
  access_type: 'offline',
  scope: PERMISSION_SCOPE
});

class Login extends Component {
  constructor(props, context) {
    super(props, context);

    this._onFocus = this._onFocus.bind(this);

constructor(ssKey, authId, options) {
    this._ssKey = ssKey
    this._authId = authId
    this._googleAuth = null
    this._visibility = 'public'
    this._projection = 'values'
    this._authMode = 'anonymous'
    this._authClient = new GoogleAuth()

    this._options = options || {}

    this._xmlParser = new xml2js.Parser({
      explicitArray: false,
      explicitRoot: false
    })

    if (!ssKey) {
      throw new Error('Spreadsheet key not provided')
    }

    this._setAuthAndDependencies(authId)
  }