react.dev/docs/10-addons.md

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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:

• TransitionGroup and CSSTransitionGroup, for dealing with animations and transitions that are usually not simple to implement, such as before a component's removal.
• LinkedStateMixin, to simplify the coordination between user's form input data and the component's state.
• cloneWithProps, to make shallow copies of React components and change their props.
• createFragment, to create a set of externally-keyed children.
• update, a helper function that makes dealing with immutable data in JavaScript easier.
• PureRenderMixin, a performance booster under certain situations.

The add-ons below are in the development (unminified) version of React only:

• TestUtils, simple helpers for writing test cases (unminified build only).
• Perf, for measuring performance and giving you hint where to optimize.

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

When using the react package from npm, simply require('react/addons') instead of require('react') to get React with all of the add-ons.