react.dev/docs/09-addons.md

id	title	layout	permalink	prev	next
addons	Add-ons	docs	addons.html	tooling-integration.html	animation.html

React.addons is where we park some useful utilities for building React apps. These should be considered experimental but will eventually be rolled into core or a blessed utilities library:

• ReactTransitions, for dealing with animations and transitions that are usually not simple to implement, such as before a component's removal.
• ReactLink, to simplify the coordination between user's form input data and and the component's state.
• classSet, for manipulating the DOM class string a bit more cleanly.

To get the add-ons, use react-with-addons.js (and its minified counterpart) rather than the common react.js.