No 'Access-Control-Allow-Origin' header is present on the requested resource (MERN) - authentication

Question

This error is never ending, I keep getting it and it's been days I've been trying to find a solution for this annoying error.
Here is what happens when I try to log in.
My app works perfectly fine in localhost but there are alot of issue when I uploaded it to heroku and it is really annoying.
Im using
Axios.defaults.withCredentials = true;
code on my every front end.
My backend
const express = require("express");
const cors = require("cors");
const mongoose = require("mongoose")
const app = express();
const bcrypt = require("bcryptjs")
const saltRounds = 10;
const bodyParser = require("body-parser")
const cookieParser = require("cookie-parser")
const session = require("express-session")
const voterModel = require('./modules/voters.js')
const presidentModel = require('./modules/president.js')
const viceModel = require('./modules/vice.js')
const treasurerModel = require('./modules/treasurer.js')
var MongoDBStore = require('connect-mongodb-session')(session);
app.use(express.json());
const corsOptions = {
origin: 'https://incomparable-speculoos-abdd5f.netlify.app',
//update: or "origin: true," if you don't wanna add a specific one
credentials: true,
};
app.use(cors(corsOptions));
app.options('*', cors());
app.use(cookieParser());
app.use(bodyParser.urlencoded({ extended: true }))
mongoose.connect("hidden",
{
useNewUrlParser: true,
useUnifiedTopology: true
}
)
var store = new MongoDBStore({
uri: 'hidden',
collection: 'sessions'
});
// Catch errors
store.on('error', function(error) {
console.log(error);
});
app.use(session({
secret: "hidden",
resave: false,
store: store,
saveUninitialized: false,
cookie: {
maxAge: 1000 * 60 * 60 * 24
}
}))
app.post('/login', async (req, res) => {
const email = req.body.email;
const password = req.body.password;
voterModel.find({email: email}, {"email":1}, async (err, result) => {
if (err) {
console.log(err)
} else {
if(result.length > 0) {
const user = await voterModel.findOne({email: email})
const pass = await user.comparePassword(password)
if (pass) {
req.session.user = user
} else {
console.log("NOT LOGGED IN")
res.send({ message: 'Invalid email or password!'})
}
} else {
console.log("NOT LOGGED IN")
res.send({ message: 'Invalid email or password!'})
}
}
})
})
app.post('/register', async (req, res) => {
const username = req.body.username;
const email = req.body.email;
const password = req.body.password;
// HASING PASSWORD
bcrypt.hash(password, saltRounds, async (err, hash) => {
if (err) {
console.log(err)
}
// INSERTING VALUES
const voters = await voterModel({email: email, username: username, password: hash, status: false})
// CHECKS IF EMAIL IS IN USE
const isNewEmail = await voterModel.isThisEmailInUse(email)
if (!isNewEmail) return res.send({ message: 'This email is already taken!'})
// SAVES THE INSERT DATA FOR VOTERS
await voters.save()
res.send({success: true})
})
})
app.post('/voted', async (req, res) => {
// FOR UPDATING THE VOTING STATUS
const email = req.body.email
// VARIABLES FOR CHOSEN CANDIDATES OF USER
const president = req.body.president
const vice = req.body.vice
const treasurer = req.body.treasurer
// SETS THE STATUS OF VOTER TO TRUE SO HE/SHE CAN ONLY VOTE ONCE
voterModel.updateOne({email: email}, {$set : {status: true}}, (err, result) => {
if (err) {
console.log(err)
} else {
console.log(result)
}
})
// BELOW ARE THE COMMANDS FOR INCREMENTING THE VOTE COUNT OF SELECTED CANDIDATES OF THE VOTER
presidentModel.updateOne({nickname: president}, {$inc : {votes: 1}}, (err, result) => {
if (err) {
console.log(err)
} else {
console.log(result)
}
})
viceModel.updateOne({nickname: vice}, {$inc : {votes: 1}}, (err, result) => {
if (err) {
console.log(err)
} else {
console.log(result)
}
})
treasurerModel.updateOne({nickname: treasurer}, {$inc : {votes: 1}}, (err, result) => {
if (err) {
console.log(err)
} else {
console.log(result)
}
})
})
app.get('/login', (req, res) => {
if (req.session.user) {
res.send({loggedIn: true, user: req.session.user})
} else {
res.send({loggedIn: false})
}
})
app.post('/checkVote', (req, res) => {
const email = req.body.email
const num = true;
voterModel.find({ $and : [{email: email}, {status : num}]},(err, result) => {
if (err) {
console.log(err)
} else {
console.log(result)
if (result.length > 0) {
res.send( {voted: true } )
} else {
res.send( {voted: false } )
}
}
})
})
app.get("/logout", (req, res) => {
req.session.destroy(err => {
if (err) return next(err)
res.status(200).send('logged out')
})
res.status(200).send('User has been logged out');
});
const PORT = process.env.PORT || 3001
app.listen(PORT, () => {
console.log('running on port 3001')
})