How to resolve req.flash is not a function in node js

Question

In my nodejs app it shows an error like:

req.flash is not a function

I have installed connection-flash npm and session and express-session but instead of that it is throwing an error like:

TypeError: req.flash is not a function

in line no. 42 in my register.js. I googled it several times but did not find any proper solution. How can I solve this? Thanks in advance.

This is my register.js:

var express = require('express');
var router = express.Router();
var User = require('../models/user');
var flash = require('connect-flash');
router.get('/', (req, res, next) => {
    res.render('register');
})


router.post('/', (req, res, next) => {
    var name = req.body.name;
    var username = req.body.username;
    var email = req.body.email;
    var password = req.body.password;
    var cpassword = req.body.cpassword;

    req.checkBody('name', 'Name is required').notEmpty();
    req.checkBody('username', 'User Name is required').notEmpty();
    req.checkBody('email', 'Email is required').notEmpty();
    req.checkBody('email', 'Email is not valid').isEmail();
    req.checkBody('password', 'Password is required').notEmpty();
    req.checkBody('cpassword', 'Password do not match').equals(req.body.password);

    var errors = req.validationErrors();
    if(errors){
        res.render('register', {
            errors:errors
        });
    }else{
        var newUser = new User({
            name: name,
            username: username,
            email: email,
            password: password
        });
        User.createUser(newUser, function(err, user){
            if(err) throw err;
            console.log(user)
        });

        req.flash('success_msg', 'Successfully Registered');
        res.redirect('/login');
    }
})
module.exports = router;

And this is my app.js:

var createError = require('http-errors');
var express = require('express');
var path = require('path');
var bodyParser = require('body-parser');
var cookieParser = require('cookie-parser');
var logger = require('morgan');
var expressValidator = require('express-validator');
var passport = require("passport");
var LocalStrategy = require('passport-local').Strategy;

var session = require('session');

var flash = require('connect-flash');
var mongoose = require('mongoose');



var indexRouter = require('./routes/index');
var usersRouter = require('./routes/users');
var loginRouter = require('./routes/login');
var registerRouter = require('./routes/register');

var app = express();

app.set('views', path.join(__dirname, 'views'));
app.set('view engine', 'twig');

app.use(expressValidator());

app.use(logger('dev'));
app.use(express.json());
app.use(express.urlencoded({ extended: false }));
app.use(cookieParser());
app.use(express.static(path.join(__dirname, 'public')));


app.use('/', indexRouter);
app.use('/users', usersRouter);
app.use('/login', loginRouter);
app.use('/register', registerRouter);


app.use(flash());

app.use(function(req, res, next) {
    res.locals.success_msg = req.flash('success_msg');
    res.locals.error_msg = req.flash('error_msg');
    res.locals.error = req.flash('error');
    res.locals.user = req.user || null;
    next();
})

app.use(passport.initialize());
app.use(passport.session());
app.use(function(req, res, next) {
  next(createError(404));
});



app.use(function(err, req, res, next) {
  res.locals.message = err.message;
  res.locals.error = req.app.get('env') === 'development' ? err : {};

  res.status(err.status || 500);
  res.render('error');
});

module.exports = app;

Answer 1

Do you try move app.use(flash()) before app.use('/register... ?

app.use(flash());

app.use('/', indexRouter);
app.use('/users', usersRouter);
app.use('/login', loginRouter);
app.use('/register', registerRouter);

and base of flash-connection documnet

Flash messages are stored in the session. First, setup sessions as usual by enabling cookieParser and session middleware. Then, use flash middleware provided by connect-flash.

Follow the document instruction to use flash properly