Origin Mismatch

An Origin Mismatch error in CORS happens when the browser detects that the request’s Origin header does not match the allowed origins defined by the server. Here’s how you can debug and fix it:

Possible Causes & Fixes

1. Check the `Origin` Header in the Request

Before anything, open the Network tab in your browser’s Developer Tools and inspect the request headers. You’ll see something like this:

Origin: http://localhost:3000

If your frontend is running on http://localhost:3000, but your backend only allows http://localhost:3001, it will reject the request.
To fix this, update your CORS configuration to include the correct origin.

2. Fix CORS Configuration in the Backend

Right now, you have:

cors({ origin: ['*'] })

But origin: ['*'] is not valid in CORS middleware. Instead, use:

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

app.use(cors({
  origin: '*', // Allow all origins
  methods: ['GET', 'POST', 'OPTIONS'],
  allowedHeaders: ['Content-Type', 'Authorization']
}));

app.use(express.json());

Alternatively, to allow only your frontend:

app.use(cors({
  origin: 'http://localhost:3000', // Allow requests only from your frontend
  methods: ['GET', 'POST', 'OPTIONS'],
  allowedHeaders: ['Content-Type', 'Authorization']
}));

3. Allow Credentials if Needed

If your API requires credentials (cookies, sessions), update both frontend and backend:

Backend:

app.use(cors({
  origin: 'http://localhost:3000',
  credentials: true
}));

Frontend:

fetch('http://localhost:3001/cors/validator', {
  method: 'POST',
  credentials: 'include', // Send cookies if required
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify(payload)
});

4. Preflight Requests (OPTIONS Method)

Sometimes browsers send an OPTIONS request (preflight) before the actual request. Make sure your backend handles it:

app.options('*', cors());

Conclusion