Nodemailer gmail, Cannot create property 'mailer' auf string 'SMTP'

Ich versuche zu senden, die Daten aus dem Formular habe ich erstellt, um mein gmail-Konto, wenn Sie auf den füllen-button, den ich immer die gleichen Fehler.
Ich fand eine Menge von Fragen über nodemailer, aber keiner von Ihnen scheinen das gleiche Problem wie ich erlebe.

Natürlich habe ich angegeben, meine clientId aber nur gelöscht, für diesen Beitrag.

TypeError: Cannot create property 'mailer' on string 'SMTP'
at Mail (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\nodemailer\lib\mailer\index.js:45:33)
   at Object.module.exports.createTransport (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\nodemailer\lib\nodemailer.js:46:14)
   at C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\src\app.js:39:26
   at Layer.handle [as handle_request] (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\layer.js:95:5)
   at next (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\route.js:131:13)
   at Route.dispatch (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\route.js:112:3)
   at Layer.handle [as handle_request] (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\layer.js:95:5)
   at C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\index.js:277:22
   at Function.process_params (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\index.js:330:12)
   at next (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\index.js:271:10)
   at serveStatic (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\node_modules\serve-static\index.js:75:16)
   at Layer.handle [as handle_request] (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\layer.js:95:5)
   at trim_prefix (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\index.js:312:13)
   at C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\index.js:280:7
   at Function.process_params (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\index.js:330:12)
   at next (C:\Users\snowr\Documents\dev\zorgkaartnl\zorgkaartnl\node_modules\express\lib\router\index.js:271:10)

dies ist mein app.js:

//require modules
const express = require('express');
const app = express();
const pug = require('pug');
const fs = require('fs')
const bodyParser = require('body-parser');
const pg = require('pg');
const nodemailer = require('nodemailer');
const xoauth2 = require('xoauth2');


//set view engine and views
app.set('views', 'src/views');
app.set('view engine', 'pug');

app.use(bodyParser.urlencoded({extended: false}));
app.use(express.static('./resources/'));





//display index page
app.get('/', function ( req, res ){
  console.log('Index is displayed on localhost');
    res.render('index');
});

app.post('/zorginstelling/ziekenhuis-olvg-locatie-west-voorheen-sint-lucas-andreas-ziekenhuis-amsterdam-109428/waardeer', function (req, res) {
  var mailOpts, smtpTrans;
  console.log('form word gepost')
  //Setup Nodemailer transport, I chose gmail. 
  smtpTrans = nodemailer.createTransport('SMTP', {
      service: 'Gmail',
      auth: {
        xoauth2: xoauth2.createXOAuth2Generator({
            user: '[email protected]',
            clientId: '-' ,
            clientSecret: '-' ,
            refreshToken: '-'
        })
       }
  });

   //Mail options
  mailOpts = {
      from: req.body.rating[name] + ' <' + req.body.rating[email][first] + '>', 
      to: '[email protected]',
      subject: 'Test',
      text: req.body.rating[comment] + req.body.rating[questions][behandeling] + req.body.rating[name]
  };

    smtpTrans.sendMail(mailOpts, function (error, response) {
      //Email not sent
      if (error) {
          console.log('There was a problem')
      }
      //Yay!! Email sent
      else {
          console.log('Email sent!')
      }
  });
});



var server = app.listen(4000, function () {
        console.log('Example app listening on port: ' + server.address().port);
    });