Created by: gaearon
I think leaving it on was a mistake. It causes some issues (https://github.com/facebook/create-react-app/issues/5267), but more fundamentally, it's a leaky abstraction. It tries to make typeof behave to return 'symbol' for Symbols by hijacking every single typeof. This leads to slowing down all code for the benefit of edge cases that likely won't work 100% correctly anyway. It definitely makes React slower (I've measured this before).
I considered disabling it just for node_modules at first. But I don't see good reasons to leave it on in application code either. Code that relies on this is inherently fragile and can break in other ways.
We're early enough in 2.x cycle that I consider this a bugfix. It definitely will fix bugs for some libraries in node_modules. And arguably performance improvements in the application code are worth the edge case issues that this would uncover.