Not sure but it's very easy to implement it yourself... Check this https://github.com/facebook/react/blob/2aeb8a2a6beb00617a4217f7f8284924fa2ad819/src/utils/jo

Not sure but it's very easy to implement it yourself... Check this https://github.com/facebook/react/blob/2aeb8a2a6beb00617a4217f7f8284924fa2ad819/src/utils/joinClasses.js https://github.com/facebook/react/blob/2aeb8a2a6beb00617a421... But I also miss it, yeah, very useful addon.