Tell me, what are portals used for in React?

React portals allow rendering child components into a DOM node that exists outside the hierarchy of the parent component. This is useful when you need to display an element above others, such as modal windows, tooltips, or pop-up menus, to avoid issues with CSS styles, z-index, or positioning.

Example of using a portal:

import React from 'react';
import ReactDOM from 'react-dom';

function Modal({ children }) {
  return ReactDOM.createPortal(
    <div className="modal">
      {children}
    </div>,
    document.getElementById('modal-root')
  );
}

Here, the content of the modal window will be rendered into the element with id "modal-root", which can be outside the main application root.