react.dev/tips/16-references-to-components.md

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If you're using React components in a larger non-React application or transitioning your code to React, you may need to keep references to components. React.renderComponent returns a reference to the mounted component:

/** @jsx React.DOM */

var myComponent = React.renderComponent(<MyComponent />, myContainer);

Keep in mind, however, that the "constructor" of a component doesn't return a component instance! It's just a descriptor: a lightweight representation that tells React what the mounted component should look like.

/** @jsx React.DOM */

var myComponent = <MyComponent />; // This is just a descriptor.

// Some code here...

myComponent = React.renderComponent(myComponent, myContainer);

Note:

This should only ever be used at the top level. Inside components, let your props and state handle communication with child components, and only reference components via refs.