Connect flash is not defiened

Oi! No módulo connec-flash, tenho um erro que não consigo identificar. Eu o revisei o código umas mil vezes. Não consigo prosseguir com meu projeto por causa disso. Modifiquei o escopo tentando identificar o erro, mas ele continua. Estou seguindo os passos de um livro e o problema parece não ser no código mas outro lugar.

Na página de registro:

ReferenceError: flash is not defined

        at module.exports (/controllers/newUser.js:3:9)

        at Layer.handle [as handle_request] (/node_modules/express/lib/router/layer.js:95:5)

        at next (/node_modules/express/lib/router/route.js:137:13)

        at module.exports (/middleware/redirectAuhenticationMiddleware.js:6:5)

        at Layer.handle [as handle_request] (/node_modules/express/lib/router/layer.js:95:5)

        at next (/node_modules/express/lib/router/route.js:137:13)

        at Route.dispatch (/node_modules/express/lib/router/route.js:112:3)

        at Layer.handle [as handle_request] (/node_modules/express/lib/router/layer.js:95:5)

        at /node_modules/express/lib/router/index.js:281:22

        at Function.process_params (/node_modules/express/lib/router/index.js:335:12)

        at next (/node_modules/express/lib/router/index.js:275:10)

        at /node_modules/connect-flash/lib/flash.js:21:5

        at Layer.handle [as handle_request] (/node_modules/express/lib/router/layer.js:95:5)

        at trim_prefix (/node_modules/express/lib/router/index.js:317:13)

        at /node_modules/express/lib/router/index.js:284:7

        at Function.process_params (/node_modules/express/lib/router/index.js:335:12) 

index.js:

const express = require('express')



    const path = require('path')

    const ejs = require('ejs')

    const mongoose = require('mongoose')

    const bodyParser = require('body-parser')

    const fileUpload = require('express-fileupload')

    const BlogPost = require('./models/BlogPost')

    const expressSession = require('express-session')

    const flash = require('connect-flash') 

newUser.js:

module.exports = (req, res) => {

        res.render('register', {errors:flash('validationErrors')}) 

}

storeUser.js:

 const User = require('../models/User')

    const path = require('path')





    module.exports = (req, res) => {

        User.create(req.body, (error, user) => {

            if (error){

             const validationErrors =  Object.keys(error.errors).map(key => error.errors[key].message)

             req.flash('validationErrors', validationErrors)

             //req.session.validationErrors = validationErrors   

             return res.redirect('/auth/register')

            }

            res.redirect('/')

        })

    } 

Alguém pode ajudar?

também configurei para ele ser usado globalmente:
index.js

const express = require('express')



const path = require('path')

const ejs = require('ejs')

const mongoose = require('mongoose')

const bodyParser = require('body-parser')

const fileUpload = require('express-fileupload')

const BlogPost = require('./models/BlogPost')

const expressSession = require('express-session')

const flash = require('connect-flash')



//middlewares



const homeController = require('./controllers/home')



const newPostController = require('./controllers/newpost')



const aboutController =  require('./controllers/about')



const contactController = require('./controllers/contact')



const blogController = require('./controllers/blog')



const getPostsController = require('./controllers/getpost')



const storePostsController = require('./controllers/store')



const newUserController = require('./controllers/newUser')



const storeUserController = require('./controllers/storeUser')



const loginController = require('./controllers/login')



const loginUserController = require('./controllers/loginUser')



const authMiddleware = require('./middleware/authMiddleware')



const redirectAuthenticationMiddleware = require('./middleware/redirectAuhenticationMiddleware')



const logoutController = require('./controllers/logout')



const app = new express()

 

mongoose.connect('mongodb://localhost/my_database', {useNewUrlParser:true})



// configurações

app.set('view engine', 'ejs')

app.use(express.static('public'))

app.use(bodyParser.json())

app.use(bodyParser.urlencoded({extended:true}))

app.use(fileUpload())



app.use(expressSession({

    secret:'keyboard cat',

    saveUninitialized: true,

    cookie: { secure: true },

    resave: true,



}))  



global.loggedIn = null

app.use("*", (req, res, next) =>{

    loggedIn = req.session.userId

    next()

})

app.use(flash()); 





// rotas



app.get('/', homeController)



app.get('/blog', blogController)



app.get('/post/:id', getPostsController)



app.get('/about/', aboutController)



app.get('/contact', contactController)



app.get('/posts/new/', authMiddleware, newPostController)



app.get ('/auth/register', redirectAuthenticationMiddleware, newUserController)



app.get('/auth/login', redirectAuthenticationMiddleware, loginController)



app.get('/auth/logout', logoutController)



app.post('/posts/store', authMiddleware, storePostsController)



app.post('/users/register', redirectAuthenticationMiddleware, storeUserController)



app.post('/users/login', redirectAuthenticationMiddleware, loginUserController)



app.use((req,res) => res.render('notfound'))



app.listen(4000, 

    ()=>{

        console.log(`servidor rodando na porta 4000`)

    }

    )