Server returning "Error: Failed to serialize user into session" when attempting to use register user using Passport.js and AJAX - node.js

Question

Server returning "Error: Failed to serialize user into session" when attempting to use register user using Passport.js and AJAX. The code works when not using AJAX from the registration page, but I want to have a popup that processes the registration too.
passport.js
var passport = require('passport');
var LocalStrategy = require('passport-local').Strategy;
var User = require('../models/user');
passport.serializeUser((user, done) => {
done(null, user.id);
});
passport.deserializeUser((id, done) => {
User.findById(id, (err, user) => {
done(err, user);
});
});
passport.use('local.popsignup', new LocalStrategy({
usernameField: 'popUsername',
passwordField: 'popPassword',
passReqToCallback: true
}, (req, email, password, done) => {
User.findOne({'email':email}, (err, user) => {
if(err){
return done(err);
}
if(user){
return done(null, false, req.flash('error', 'Email already exists, please login.'))
}
var newUser = new User();
newUser.email = email;
newUser.password = newUser.encryptPassword(password);
newUser.preferences = preferences;
newUser.save((err) => {
return done(null, newUser);
});
});
}));
user router
module.exports = (app, passport) => {
app.post('/popover', function(req, res, next) {
passport.authenticate('local.popsignup', function(err, user, info) {
if (user !== undefined) {loggedIn = true} else {loggedIn = false};
var errors = req.flash('error')
req.login(user, function(err) {
if (err) return next(err);
res.status(200).send({errors: errors, loggedIn: loggedIn});
});
})(req, res, next);
});
}
view ajax js
$.ajax({
url: "/popover",
type: "POST",
data: $('#popform-tag').serialize(),
success: function(data){
if(data.loggedIn == false) {
$("#email_alert").text(data.errors[0]).show();
} else {
$('#popover').modal('hide')
localStorage.setItem('preferences', 1);
}
//console.log(data);
},
error: function(XMLHttpRequest, textStatus, errorThrown) {
console.log("Status: " + textStatus);
console.log("Error: " + errorThrown);
}
})

Answer 1

Before adding ajax, send some parameters via PostMan to check if your API request-response is working as expected. If yes, only then use Ajax based on API type (post, put etc..)
Also instead of serialize() try using Ajax in this way -
var data = {
someKey: 'someValue'
}
$.ajax({
url: '/popover', // route path
type: 'POST',
data: JSON.stringify(data),
headers: {
'Content-Type': "application/json",
},
'success': function (data, textStatus, request) {
// do something
},
'error': function (err) {
// do some error handling
}
});