Fix CORS Error in 1 Line (Express.js)

Problem:

You are getting this error in frontend:

"Access to fetch at 'API_URL' from origin 'http://localhost:3000' has been blocked by CORS policy"

Fix (1 line):

Install CORS:
npm install cors

Enable it in your Express app:

const cors = require('cors');
app.use(cors());

That's it — your API will now accept requests from any origin.

---

More Secure Option (Recommended for production):

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

You can also allow multiple domains:

app.use(cors({
  origin: ['http://localhost:3000', 'https://yourdomain.com']
}));

---

Bonus Tip:

If you're using credentials (cookies/auth), enable:

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