React 19 useOptimistic: Build Instant UI Without Waiting for the Server

The most underused React 19 hook is useOptimistic . Most teams still reach for local state + loading spinners when they could ship instant UI. Here's the hook, the patterns, and the edge cases that bite in production. What useOptimistic Solves // Old way — users see lag async function addTodo ( text : string ) { setLoading ( true ); const newTodo = await createTodo ( text ); // 200-800ms wait setTodos ( prev => [... prev , newTodo ]); setLoading ( false ); } With useOptimistic : const [ optimist