Troubleshooting CORS Errors

Cross-Origin Resource Sharing (CORS) errors occur when a script running on a web page tries to make a request to a different domain (origin) than the one that served the page, and the server hosting the resource doesn't explicitly permit it.

What is an "Origin"?

An origin is defined by the combination of protocol (http/https), domain (example.com), and port. If any of these differ, it's considered a cross-origin request.

Why CORS Errors Happen

Browsers implement the Same-Origin Policy for security. Without it, any website could potentially make requests to any other domain using your credentials. CORS provides a controlled way for servers to relax this policy.

How to Fix (If You Own the Backend/API Server)

• Set Access-Control-Allow-Origin: This is the most crucial header. Set its value to the specific origin(s) that should be allowed to access your API.
• Handle Preflight Requests (OPTIONS): Ensure your server responds correctly to OPTIONS requests for relevant routes.

Example (Node.js/Express using cors middleware):

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

const corsOptions = {
  origin: 'https://www.yourfrontend.com',
  methods: 'GET,POST,PUT,DELETE'
};

app.use(cors(corsOptions));