[hmsimha]

I don't think I am.

If I'm testing a react app on localhost:3000, and it normally calls out to an api of mine also, I may want to feed it some dummy data. Let's say I use python http.serve on port 5000 in a directory with files in a subdirectory `api` with files `users` and `posts` (which contain JSON data representative of the API response). I then set my react app to use localhost:5000 as the api.

The react app will then make the requests to the api such as `localhost:5000/api/users` and `localhost:5000/api/posts`. The server will respond with the data; however, if the header `Access-Control-Allow-Origin: http://localhost:3000` isn't set, the browser will block this data (probably? unless there are exceptions for localhost)