What is a React portal, and when would you use one?

Direct answer

A portal renders React children into a different DOM node while keeping those children connected to their original React tree for context and event propagation.

Modal.jsx

function Modal({ children }) {
  return createPortal(
    <div className="modal">{children}</div>,
    document.body
  );
}

Typical uses: modals, dialogs, tooltips, overlays, and integration with DOM containers outside the normal parent element.
React relationship: the portal child still receives context from its React ancestors.
Physical DOM: only the DOM placement changes.