Access from source "https://example.com" is blocked even if I allow https://example.com/

Question

I have an app made using React, Node.js and Socket.io

I deployed the Node backend to heroku and the frontend to Netlify

I know the CORS errors are related to the server, but no matter what I add, it doesn't resolve the error in the picture below.

I also added the proxy script as "proxy" in React's package.json: "https://googledocs-clone-sbayrak.herokuapp.com/"

This is my server.js file;

const mongoose = require('mongoose');
const Document = require('./Document');
const dotenv = require('dotenv');
const path = require('path');
const express = require('express');
const http = require('http');
const socketio = require('socket.io');
dotenv.config();

const app = express();
app.use(cors());
const server = http.createServer(app);
const io = socketio(server, {
  cors: {
    origin: 'https://googledocs-clone-sbayrak.netlify.app/',
    methods: ['GET', 'POST'],
  },
});

app.get('/', (req, res) => {
  res.status(200).send('hello!!');
});

const connectDB = async () => {
  try {
    const connect = await mongoose.connect(process.env.MONGODB_URI, {
      useUnifiedTopology: true,
      useNewUrlParser: true,
    });

    console.log('MongoDB Connected...');
  } catch (error) {
    console.error(`Error : ${error.message}`);
    process.exit(1);
  }
};

connectDB();


let defaultValue = '';

const findOrCreateDocument = async (id) => {
  if (id === null) return;

  const document = await Document.findById({ _id: id });

  if (document) return document;

  const result = await Document.create({ _id: id, data: defaultValue });
  return result;
};
io.on('connection', (socket) => {
  socket.on('get-document', async (documentId) => {
    const document = await findOrCreateDocument(documentId);
    socket.join(documentId);
    socket.emit('load-document', document.data);
    socket.on('send-changes', (delta) => {
      socket.broadcast.to(documentId).emit('receive-changes', delta);
    });

    socket.on('save-document', async (data) => {
      await Document.findByIdAndUpdate(documentId, { data });
    });
  });
  console.log('connected');
});

server.lis ten(process.env.PORT || 5000, () =>
  console.log(`Server has started.`)
);

This is where I make requests to the front end;

import Quill from 'quill';
import 'quill/dist/quill.snow.css';
import { useParams } from 'react-router-dom';
import { io } from 'socket.io-client';

const SAVE_INTERVAL_MS = 2000;


const TextEditor = () => {
  const [socket, setSocket] = useState();
  const [quill, setQuill] = useState();
  const { id: documentId } = useParams();

  useEffect(() => {
    const s = io('https://googledocs-clone-sbayrak.herokuapp.com/');
    setSocket(s);


    return () => {
      s.disconnect();
    };
  }, []);

 /* below other functions */
 /* below other functions */
 /* below other functions */
 }

Answer 1

It seems that you have not imported the cors package. Is it imported from somewhere else?

var cors = require('cors') // is missing

Answer 2

TL;DR

https://googledocs-clone-sbayrak.netlify.app/ is not origin. Remove the trailing slash.

More details about the problem

Origin Trailing slashes are not allowed in header values

According to the CORS protocol (specified in the Fetch standard ), the browser will never set the Origin request header to a value with a trailing slash . So if the page at https://googledocs-clone-sbayrak.netlify.app/whatever makes a cross-origin request, the request's Origin header will contain

https://googledocs-clone-sbayrak.netlify.app

without any trailing slash.

Server-side byte-by-byte comparison

You are using Socket.IO, which depends on the node.js cors package . If the origin of the request matches the origin value of your CORS configuration (https://googledocs-clone-sbayrak.netlify.app/).

Put them together

Obviously,

'https://googledocs-clone-sbayrak.netlify.app' ===
    'https://googledocs-clone-sbayrak.netlify.app/'

evaluates to false, which causes the cors package to not set any Access-Control-Allow-Origin headers in the response, which causes the browser to The CORS check failed, hence the CORS error you observed.

Get standard examples

Section 3.2.5 of the Fetch standard even provides an instructive example of this error ,

Access-Control-Allow-Origin: https://rabbit.invalid/

And explains why it causes CORS checks to fail: