You’ve built a React frontend. You’ve built a Node.js API. They work perfectly when you test them separately. You wire them together, make your first API call, and…

Access to fetch at 'http://localhost:3001/api/users' from origin 
'http://localhost:3000' has been blocked by CORS policy: 
No 'Access-Control-Allow-Origin' header is present on the requested resource.

Sound familiar? Every developer hits this wall. And most developers respond by Googling “how to fix CORS” and pasting app.use(cors()) without understanding what they just did.