CORS

Summary

Cross-Origin Resource Sharing — a browser security mechanism that blocks your React frontend from calling your Express backend unless the server explicitly allows that origin. The first thing that breaks when you first connect the two.

• 1Trigger a CORS error by calling your local backend from your local frontend on a different port.
• 2Configure CORS on your Express server to allow your frontend's origin.
• 3Learn the difference between simple requests and preflighted (OPTIONS) requests.