How do you model async/loading state?

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Asynchronous operations (API calls) have multiple possible outcomes, so you must model **more than just the data** — you need to represent loading, success, and error states. Modeling this well prevents UI bugs like showing stale data or no feedback. ## The naive approach and its flaws ```js // ❌ separate booleans — can represent impossible combinations const [data, setData] = useState(null); const [loading, setLoading] = useState(false); const [error, setError] = useState(null); // What does { loading: true, error: "x", data: [...] } even mean? An invalid state. ``` Independent flags allow contradictory combinations (loading *and* error *and* data at once) that should be impossible — a source of confusing UI bugs. ## Better: a status enum (a small state machine) ```js // ✅ a single `status` makes states mutually exclusive const [state, setState] = useState({ status: "idle", data: null, error: null }); // status is one of: 'idle' | 'loading' | 'success' | 'error' async function load() { setState({ status: "loading", data: null, error: null }); try { const data = await api.get(); setState({ status: "success", data, error: null }); } catch (e) { setState({ status: "error", data: null, error: e.message }); } } ``` A single `status` field makes the states **mutually exclusive** — you can't be both loading and errored. Rendering becomes a clean switch: ```jsx switch (state.status) { case "loading": return ; case "error": return ; case "success": return ; default: return ; } ``` ## Don't forget the often-missed states ```text idle — hasn't started yet (vs loading) empty — succeeded but no results (show "No items" not a blank screen) refetching— has old data AND is loading new (show data + a subtle spinner) ``` ## The best approach for server data: use a library ```jsx // React Query / SWR model all of this for you const { data, isLoading, isError, error } = useQuery({ queryKey: ["x"], queryFn }); ``` Server-state libraries provide these states (plus caching, refetch, stale-while-revalidate) out of the box — usually better than hand-rolling. ## Why it matters Properly modeling async state — as a single status rather than loose booleans — eliminates a whole class of UI bugs (contradictory states, missing loading/error/empty feedback) and makes rendering logic clean and exhaustive. Recognizing the full set of states (idle, loading, success, error, empty, refetching) leads to better UX, and knowing that server-state libraries handle all of this is what keeps data-fetching code simple and robust.