Passport local strategy doesnt return user - node.js

Question

My signup and login works completely fine until I want to return user to the front end. User ends up being undefined.
Here is my passport.js
const passport = require('passport');
const localStrategy = require('passport-local').Strategy;
const mongoose = require('mongoose');
const User = mongoose.model('users');
passport.serializeUser((user, done) => {
done(null, user._id)
})
passport.deserializeUser((id, done) =>
User.findById(id).then((user) => {
if (user) {
done(null, user)
} else {
}
})
);
passport.use(new localStrategy((username, password, done) => {
User.findOne({ username: username }, (err, user) => {
console.log(user)
if (err) { return done(err); }
if (!user) {
return done(null, false, { message: 'Username not found' });
}
if (!user.comparePassword(password, user.password)) {
return done(null, false, { message: 'Incorrect password.' });
}
return done(null, user);
});
}));
Here is my login route:
const passport = require('passport');
const mongoose = require('mongoose');
const User = mongoose.model('users');
module.exports = (app) => {
app.get('/test', (req, res) => {
res.send('elo')
})
app.post('/login',
passport.authenticate('local', {
successRedirect: '/loginSuccess',
failureRedirect: '/loginFailed',
})
);
app.get('/loginSuccess', (req, res) => {
console.log(req.user)
res.send({ success: true, test:'test', user: req.user })
})
app.get('/loginFailed', (req, res) => {
res.send({ success: false, error: "Incorrect credentials" })
})
};
In passport.js the user exists and should be returnet correctly. It's just getting lost somewhere along the way. Whats funnier, I used the exact same code in my other app few months ago and it worked properly.
Edit:
Turns out it works on postman, so I dont know whats going on now