API Working in Postman but Failing in Browser

You test your API in Postman — everything works fine.
Then you call it from your frontend — suddenly it fails.

Error:
Access to fetch at 'API_URL' from origin 'localhost' has been blocked by CORS policy

Why this happens:
Postman does not enforce browser security rules, but browsers do.

Root cause:
Your backend is not allowing cross-origin requests.

Fix in Node.js (Express):

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

Or configure specific origin:

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

After adding this, your API will work correctly in the browser.