CORS error on expressjs server with cors middleware already added

Question

I have a frontend and express js server hosted on aws lightsail that uses ubuntu with nginx.

The server is set as a reversed proxy. Here is the config

location /api/ { 
proxy_pass http://localhost:4000/api/;
proxy_http_version 1.1;
proxy_set_header Upgrade $http_upgrade;
proxy_set_header Connection 'upgrade';
proxy_set_header Host $host;
proxy_cache_bypass $http_upgrade;
}

I'm trying to make a post request (login) from client to express but I get this

Cross-Origin Request Blocked: The Same Origin Policy disallows reading the remote resource 
at http://localhost:4000/api/users/login. (Reason: CORS request did not succeed). Status 
code: (null).

but I have enabled cors on my server in multiple ways but with no success

const cors = require('cors');
const app = express();

const corsOption = {origin: "http://18.193.59.75:80"};

app.options("/*", function(req, res, next){ 
  res.header('Access-Control-Allow-Origin', '*'); 
  res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS');
  res.header('Access-Control-Allow-Headers', 
  'Content-Type, Authorization, Content-Length, X-Requested-With');
  res.send(200);});

//app.use(cors(corsOptions));
//app.options('*', cors(corsOptions));

Answer 1

Add the next() handler as given below and see if resolves your issue.

const cors = require('cors');
const app = express();

// const corsOption = {origin: "http://18.193.59.75:80"};

var corsOptions = function(req, res, next){ 
    res.header('Access-Control-Allow-Origin', '*'); 
    res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS');
    res.header('Access-Control-Allow-Headers', 
    'Content-Type, Authorization, Content-Length, X-Requested-With');
     next();
}

app.use(corsOptions);

Answer 2

I still had problems on mobile with cors and I erased all the config and did it all again just to realize that in my front-end I called 'http://localhost:4000' where my server was running but it was running on the server so to reach it I had to call it http://18.193.59.75/api/endpoint and the nginx will redirect to 'http://localhost:4000' with proxy pass.

here is my nginx config

server {
  listen 80;
  listen [::]:80;

  server_name _;
  location / {
     root /var/www/website/client;
     try_files $uri /index.html;
  }

  location /api { 
      proxy_pass http://localhost:4000;
  }

and my index.js from express

const express = require('express');
const cors = require('cors');
const app = express();

// var corsOptions = function(req, res, next){ 
//     res.header('Access-Control-Allow-Origin', '*'); 
//     res.header('Access-Control-Allow-Methods', 'GET,PUT,POST,DELETE,OPTIONS');
//     res.header('Access-Control-Allow-Headers', 
//     'Content-Type, Authorization, Content-Length, X-Requested-With');
//      next();
// }

// app.use(corsOptions);

app.use(cors());

app.use(aux);

app.use(express.json());
app.use(express.urlencoded({extended: false}))


app.use('/api/appointments', require('./api/appointmnets'));
app.use('/api/users', require('./api/users'));

app.get('/test', (req, res) => {
    res.json({'test' : 'successed'})
})

const PORT = process.env.PORT || 4000;

app.listen(PORT, () => {
    console.log(`App listens to port ${PORT}`);
})

and the domain that I use from front-end to call my express server

const domain = 'http://18.193.59.75:80/api/';
const domain = 'http://localhost:80/api';    //wronge