Jotai Testing

Learnings for writing unit tests against components/hooks that read or write Jotai atoms.

Sharing a store across renderHook calls in a single test

When a test needs to render a hook, unmount it, and then render the hook again (e.g., to verify state persists across an unmount/remount — the exact scenario for atoms that replace local useState), all renderHook calls must share the same Jotai store. Otherwise each renderHook's Provider wrapper creates its own isolated store and writes made by the first hook are invisible to the second.

Wrong — each call to makeWrapper() returns a component that creates a fresh <Provider> (no store prop), so every renderHook gets a new default store:

function makeWrapper() {
  return function Wrapper({ children }) {
    return <Provider>{children}</Provider>;
  };
}

Right — create one store per test and bind every renderHook in that test to it:

import { createStore, Provider } from "jotai";

function makeWrapper() {
  const store = createStore();
  return function Wrapper({ children }) {
    return <Provider store={store}>{children}</Provider>;
  };
}

// In the test:
const wrapper = makeWrapper();
const first = renderHook(() => useMyAtomHook(id), { wrapper });
// ... mutate state ...
first.unmount();
const second = renderHook(() => useMyAtomHook(id), { wrapper });
// second now sees state written by first

The symptom when you get this wrong is assertions like expected false to be true on the remounted hook's state, even though the setter clearly ran against the first hook.

See src/atoms/githubSyncAtoms.test.tsx for a complete example covering unmount/remount, cross-unmount completion, and per-key isolation.

StrictMode: use the reactStrictMode option, not a nested <StrictMode>

<StrictMode> only replays mount effects when it is the root of the rendered tree. A wrapper component of your own that renders <StrictMode> around children — which is what you end up writing as soon as you also need a Jotai Provider — does not replay: the mount effect runs once and a StrictMode-only bug passes the test. Measured with @testing-library/react v16:

This is not a renderHook quirk — render() behaves the same way. Pass the render option, which composes with whatever wrapper you already need: renderHook(() => useThing(), { wrapper, reactStrictMode: true }).

Worth testing because the app renders under <StrictMode> (src/renderer.tsx), where the dev mount/unmount/remount replay runs cleanup on a hook that is still mounted. A "am I still mounted" ref must therefore be re-armed in the effect body, not just at ref creation:

const mountedRef = useRef(true);
useEffect(() => {
  mountedRef.current = true; // without this, permanently false after the replay
  return () => {
    mountedRef.current = false;
  };
}, []);

No jest-dom matchers

The vitest setup does not register @testing-library/jest-dom, so matchers like toBeInTheDocument() or toBeDisabled() fail with Invalid Chai property: toBeInTheDocument. Use plain assertions instead: expect(screen.queryByTestId(...)).toBeNull() / .not.toBeNull() for presence, and expect((button as HTMLButtonElement).disabled).toBe(false) for disabled state.

act() hides passive-effect timing windows

React Testing Library wraps render/rerender/fireEvent in act(), which flushes passive effects synchronously before yielding to the microtask queue. A promise continuation therefore always runs after effects in tests, so bugs that live in the window between commit and passive-effect flush cannot be reproduced with RTL. Don't burn time writing a failing test for one — fix it (usually by moving the bookkeeping to useLayoutEffect) and say in the PR why coverage stops at the app-switch behavior.

Hooks That Indirectly Use React Query

If a renderHook test starts failing with No QueryClient set, use QueryClientProvider to set one, check whether the hook now calls another hook such as useSettings() or useAppVersion() that uses TanStack Query internally. Either wrap the test in a QueryClientProvider or mock the indirect hook when the test is only exercising Jotai/event behavior.

useStreamChat() has the same shape but a different error — useChatStreamManager requires ChatStreamProvider. Watch for it when hoisting a component out of a tab/route switch: PreviewPanel.test.tsx renders the panel bare and mocks each child, so anything newly rendered at panel level runs its hooks for real. Split the component in two — an outer one that reads only atoms and returns null when the feature is inactive, and an inner one holding the chat/query hooks — so the provider is only needed when the feature is actually on screen. Mock the new child in the parent's suite the way its siblings already are.

Preview postMessage Tests Need an App URL

Hooks that consume preview-iframe messages (e.g. useTestRecorder) validate event.origin against the running app's origin and fail closed when it isn't known. A renderHook test that only sets previewIframeRefAtom will therefore see every message silently dropped — with no error, because dropping a foreign message is the correct behavior.

The app URL is not a Jotai atom: appUrlByAppIdAtom was retired (see rules/jotai-state.md, and the retired-name guard in src/state_machines/boundaries.test.ts), and the URL now comes from useCurrentAppUrl in @/hooks/useAppRun. Mock that hook, backing it with an atom of your own so a test can also take the URL away mid-session — which is what the dev-server restart during isolation setup does:

const testAppUrlAtom = atom<AppUrlState>({
  appUrl: null,
  appId: null,
  originalUrl: null,
  mode: null,
});
vi.mock("@/hooks/useAppRun", () => ({
  useCurrentAppUrl: () => useAtomValue(testAppUrlAtom),
}));

store.set(testAppUrlAtom, {
  appUrl: "https://preview.test/",
  appId: 1,
  originalUrl: "https://preview.test/",
  mode: "host",
});

Then dispatch the MessageEvent with a matching origin:

const event = new MessageEvent("message", {
  data,
  origin: "https://preview.test",
});
Object.defineProperty(event, "source", { value: iframe.contentWindow }); // read-only on the prototype

Have the iframe stand-in record the target origin each outgoing postMessage was given, not just the payload — a hook that sends credentials into the preview pins them to the app's origin, and a fake with a one-argument postMessage cannot tell that apart from a wide-open "*". See src/hooks/useTestRecorder.test.tsx for the whole harness.

Partial jotai Mocks

When a component test mocks jotai, preserve the real module exports with importOriginal and override only the needed hooks. A full mock that only returns useAtomValue can fail during test collection with [vitest] No "atom" export is defined on the "jotai" mock once an indirectly imported atom module calls atom(...).

Seeding a running app URL

Don't try to make a component see a running dev server by writing preview-runtime atoms on a test store — @/atoms/previewRuntimeAtoms was deleted (symptom: Failed to resolve import "@/atoms/previewRuntimeAtoms"). Mock the hook instead, as PreviewPanel.test.tsx and TestsPanel.test.tsx do:

vi.mock("@/hooks/useAppRun", () => ({
  useCurrentAppUrl: () => ({
    appUrl: "http://localhost:32100",
    appId: 1,
    originalUrl: "http://localhost:32100",
    mode: "host" as const,
  }),
}));