A simple utility for conditionally joining classNames together

>**Note:** >This is a legacy React addon, and is no longer maintained. > >We don't encourage using it in new code, but it exists for backwards compatibility. >The recommended migration path is to use [`React.PureComponent`](https://facebook.github.io/re

React is a JavaScript library for building user interfaces.

Brand checking of React Elements.

Utility function to create a shallow copy of an object which had dropped some fields.