CSS-in-JS refers to a pattern where CSS is written with Javascript, then extracted into a stylesheet.
It is _not recommended_ to use CSS-in-JS instead you may define your styles in a separate `*.css` file or inline (using the `style` attribute). If you still prefer using CSS-in-JS go checkout various libraries below:
React can be used to power animations. See [React Transition Group](https://reactcommunity.org/react-transition-group/) and [React Motion](https://github.com/chenglou/react-motion), for example.