What does React.memo do, and what does it not prevent?

Direct answer

memo can skip a child render when its parent re-renders and the child's props are unchanged, but it does not block renders caused by the child's own state or context changes.

Row.jsx

const Row = memo(function Row({ item }) {
  return <li>{item.name}</li>;
});

Can skip: a parent-driven render when props compare equal.
Cannot skip: updates caused by the memoized component's own state.
Cannot skip: updates from context values that the component reads.
Requirement: rendering must already be pure; memo is an optimization, not a correctness tool.