Created by: martinfrancois
As pointed out here: https://github.com/facebook/create-react-app/issues/9913#issuecomment-716879695
The comment in the code clearly states // which is why it's disabled by default. suggesting react-refresh should be disabled by default.
But if the environment variable FAST_REFRESH is not set at all, the condition process.env.FAST_REFRESH !== 'false' evaluates to true (as undefined !== 'false'), enabling react-refresh.