What is XState and state machines in UI development?

XState is a JavaScript library for building finite state machines and statecharts — a formal, visual approach to modeling complex UI state. Define a machine: const authMachine = createMachine({ id: "auth", initial: "idle", states: { idle: { on: { LOGIN: "loading" } }, loading: { invoke: { src: "loginService", onDone: "authenticated", onError: "error" } }, authenticated: { on: { LOGOUT: "idle" } }, error: { on: { RETRY: "loading" } } } }). Use in React: const [state, send] = useMachine(authMachine); send("LOGIN"). Benefits: Impossible states prevented: in the loading state, LOGIN event is ignored (defined behavior, not a bug). Visual tooling: XState Visualizer shows the state chart graphically. Formal correctness: all transitions are explicit. Testability: machines are pure data — test without a browser. Use cases: complex forms with multi-step validation, auth flows, onboarding wizards, media players, collaborative editing states. XState adds complexity but eliminates entire classes of bugs related to invalid state combinations.