What is CORS?

1. Definition

CORS (Cross-Origin Resource Sharing) is a security feature implemented by browsers that restricts web applications from making requests to a different domain (origin) than the one that served the webpage.

2. Why CORS Exists

Prevents unauthorized access
Protects user data
Avoids malicious cross-site requests

3. Example Problem

// Frontend (localhost:3000)
fetch("http://api.example.com/data")

// ❌ Error: Blocked by CORS policy

4. Solution (Enable CORS)

// Using cors package
const express = require("express");
const cors = require("cors");

const app = express();

app.use(cors());

app.get("/", (req, res) => {
  res.send("CORS enabled");
});

app.listen(3000);

5. How it Works

Browser sends request to another origin
Server responds with CORS headers
Browser allows or blocks request based on headers

6. Important Header

Access-Control-Allow-Origin: *

7. Advantages

Enhances security
Controls API access
Prevents unauthorized requests