What is the best way to fix a CORS error between a React frontend (localhost) and a Node.js backend (localhost)?

The most robust solution is to configure your Node.js backend to include the appropriate `Access-Control-Allow-Origin` header in its responses. Specifically, set `Access-Control-Allow-Origin` to `*` (for all origins - use cautiously in production) or to your React app's origin (e.g., `http://localhost:3000`). You might also need to include `Access-Control-Allow-Methods` (e.g., `GET, POST, PUT, DELETE`) and `Access-Control-Allow-Headers` (e.g., `Content-Type, Authorization`). Use the `cors` middleware package in Node.js for easy configuration.