What are concurrent features in React, and how do they relate to Strict Mode?

Concurrent features let React prepare more than one version of the user interface at the same time, pausing and resuming work as needed, so the app stays responsive even while handling a large or slow update. Strict Mode helps prepare your code for this by catching patterns that would not work reliably if React decided to pause, restart, or throw away an in progress render.

// A component written safely for concurrent rendering, avoiding side effects during render
function ProductList({ products }) {
  const sorted = useMemo(() => [...products].sort(), [products]); // pure, safe to be called multiple times
  return <List items={sorted} />;
}