React Router Patterns

Quick Guide: This skill covers React Router v7 Data Mode — createBrowserRouter plus loaders, actions, fetchers and pending states, without a full framework. Four v7 facts change the answer: every export comes from "react-router", with the DOM-only ones — RouterProvider included — also on "react-router/dom", while react-router-dom survives v7 as a deprecated re-export and is deleted in v8; json() and defer() are removed so loaders return plain objects; form method values are uppercase ("POST"); and loaders skip revalidation after an action error unless shouldRevalidate opts back in.

Detailed Resources:

• examples/core.md — router setup, root layout, root error boundary, and the two v7 migration traps
• examples/data-loading.md — loaders, actions, <Form>, useFetcher, optimistic UI, shouldRevalidate, deferred data
• examples/navigation.md — Link, NavLink, useNavigate, redirect, useSearchParams wired to loaders
• examples/error-handling.md — error bubbling, thrown responses, pending UI, route.lazy
• examples/layouts.md — Outlet, useOutletContext, pathless auth layouts, useBlocker
• reference.md — route options, hook and component tables, createBrowserRouter options

---

Which path applies

React Router ships three modes, and only one of them has the data APIs.

• Data Mode — createBrowserRouter + <RouterProvider>. Loaders, actions, fetchers and errorElement all work. This is the skill; start at examples/core.md.
• Declarative Mode — <BrowserRouter> + <Routes>. URL matching and <Link> only. A loader prop here is silently ignored, so reach for it when there is no data to load and nothing in this skill's patterns applies.
• Framework Mode — file-based routes, SSR and streaming through the router's own bundler plugin. A separate surface with its own conventions, not covered here.

---

<critical_requirements>

Before writing React Router code

Import everything from "react-router", and RouterProvider from "react-router/dom" — that copy wires up react-dom's flushSync. react-router-dom still installs in v7 as a re-export, so a stale import fails no build and nothing tells you; v8 deletes the package.

Reach for createBrowserRouter + <RouterProvider> whenever a loader, action or fetcher is involved. Those are Data Mode features; under <BrowserRouter> the props are accepted and ignored, with no error to tell you.

Return plain objects from loaders, or a Response you built yourself. json() and defer() were removed in v7.

Use throw redirect() rather than return redirect(). Throwing unwinds the whole call stack, which is what makes a shared requireAuth() helper stop the loader that called it.

Give the root route an errorElement or ErrorBoundary. It is the last catch in the tree; without one, a single loader failure replaces the app with the router's built-in error dump.

</critical_requirements>

---

Auto-detection: createBrowserRouter, RouterProvider, useLoaderData, useActionData, useNavigation, useSearchParams, useFetcher, useRouteError, isRouteErrorResponse, useOutletContext, useRevalidator, useBlocker, Outlet, NavLink, errorElement, shouldRevalidate, route.lazy, HydrateFallback

Applies to:

• Route trees with data loading, form actions and pending states
• Nested layouts with persistent shared UI
• Route-level error boundaries and not-found handling
• URL search params as application state
• Non-navigating mutations — inline forms, toggles, auto-save
• Route-level code splitting

Handled elsewhere:

• Caching and deduplicating server data across routes — a loader fetches per navigation, and whatever owns data fetching decides what is cached between them.
• Client state that has no business in the URL — this skill settles URL-shaped state only.
• Styling active and pending links — NavLink hands its state to a className or style function, and what those return is not its concern.
• Server rendering, streaming HTML and file-based route generation.

---

<philosophy>

React Router v7 treats the router as a data layer rather than a URL matcher. A route declares what to load (loader), what mutations it accepts (action), and what catches failures (errorElement) — all before its component renders. That moves orchestration out of components and removes the fetch-on-mount waterfall.

• Routes own their data. Components receive it; they do not fetch it.
• The URL is the source of truth. Path params, search params and navigation state all live there.
• Errors bubble. An error rises to the nearest errorElement, leaving parent layouts on screen.
• Revalidation is automatic. A successful action re-runs every active loader, so there is no cache to invalidate by hand.

</philosophy>

---

<decision_framework>

Where the logic goes

What does this code do?
  +-- Fetch data the component needs?      -> loader (parallel with sibling loaders)
  +-- Handle a form submission?            -> action (reads FormData, returns errors or redirects)
  +-- Mutate without changing the URL?     -> useFetcher (independent state, runs concurrently)
  +-- Gate access to a subtree?            -> pathless layout route whose loader throws redirect()

How to navigate

Where are you?
  +-- In JSX, on something clickable?      -> <Link> or <NavLink>
  +-- In a handler, after a side effect?   -> useNavigate()
  +-- In a loader or action?               -> throw redirect()
  +-- Outside the React tree entirely?     -> router.navigate() on the router object

The last one is the only route out of a module that never renders — a fetch wrapper redirecting on a 401, for instance. The hooks all need a component.

Where to catch errors

Put errorElement on the root for a guaranteed catch, and on individual routes where the failure has its own UI — a missing post reads differently from a failed dashboard. Throw a Response from a loader (throw new Response("Not Found", { status: 404 })) to make the failure an HTTP error that isRouteErrorResponse can narrow.

</decision_framework>

---

<patterns>

Core patterns

Pattern 1: Data Mode setup

Routes are objects, defined outside React so the router can run loaders before anything mounts.

const router = createBrowserRouter([
  {
    path: "/",
    element: <RootLayout />,
    errorElement: <RootError />,
    children: [
      { index: true, element: <HomePage /> },
      { path: "posts", element: <PostsPage />, loader: postsLoader },
    ],
  },
]);

Full code: examples/core.md

---

Pattern 2: Loaders and actions

A loader runs before render; an action handles submissions. Both receive { request, params }, where request is a standard Web Request. Sibling loaders run in parallel, parent before child.

export async function postsLoader({ request }: LoaderFunctionArgs) {
  const response = await fetch("/api/posts");
  if (!response.ok) throw new Response("Failed to load", { status: 500 });
  return { posts: await response.json() };
}

export async function createPostAction({ request }: ActionFunctionArgs) {
  const formData = await request.formData();
  const post = await createPost({ title: formData.get("title") });
  throw redirect(`/posts/${post.id}`);
}

Full code: examples/data-loading.md

---

Pattern 3: Error boundaries with errorElement

errorElement catches failures from the route's loader, action and component alike. isRouteErrorResponse separates a thrown Response from an unexpected exception.

function RouteError() {
  const error = useRouteError();
  if (isRouteErrorResponse(error)) {
    return <p role="alert">{error.status}: {error.statusText}</p>;
  }
  return <p role="alert">{error instanceof Error ? error.message : "Unknown error"}</p>;
}

Full code: examples/error-handling.md

---

Pattern 4: Nested layouts with Outlet

A parent route renders <Outlet /> where its matched child goes. The layout survives child navigation, so sidebars and their state persist.

function DashboardLayout() {
  return (
    <div>
      <nav><NavLink to="/dashboard" end>Overview</NavLink></nav>
      <main><Outlet /></main>
    </div>
  );
}

Full code: examples/layouts.md

---

Pattern 5: Navigation state with useNavigation

navigation.state is "idle", "loading" (a loader is running) or "submitting" (an action is). One indicator in the root layout covers the whole app.

function GlobalSpinner() {
  const navigation = useNavigation();
  if (navigation.state === "idle") return null;
  return <div role="progressbar" aria-busy="true" />;
}

Full code: examples/error-handling.md

---

Pattern 6: Non-navigating mutations with useFetcher

fetcher.Form submits to an action without changing the URL. Fetchers carry independent state and data, several can be in flight at once, and active loaders still revalidate when each one finishes.

function DeleteButton({ postId }: { postId: string }) {
  const fetcher = useFetcher();
  return (
    <fetcher.Form method="POST" action={`/posts/${postId}/delete`}>
      <button type="submit" disabled={fetcher.state !== "idle"}>Delete</button>
    </fetcher.Form>
  );
}

Full code: examples/data-loading.md

---

Pattern 7: Code splitting with route.lazy

route.lazy defers a route module until navigation. The function form ships one chunk; the v7.5+ object form splits each property so the loader and the component download in parallel.

{ path: "admin", lazy: () => import("./pages/admin") }

{
  path: "admin",
  lazy: {
    loader: async () => (await import("./pages/admin.loader")).loader,
    Component: async () => (await import("./pages/admin.component")).AdminPage,
  },
}

Full code: examples/error-handling.md

</patterns>

---

<red_flags>

Red flags

Breaks at runtime:

• Importing from "react-router-dom" — deprecated in v7 and deleted in v8, and it still resolves today so nothing tells you — import from "react-router", or "react-router/dom" for RouterProvider.
• Calling json() or defer() — removed in v7 — return a plain object, or a promise for the deferred half and read it with <Await>.
• loader or action on a <Route> under <BrowserRouter> — accepted and ignored with no warning — switch the tree to createBrowserRouter.
• No errorElement anywhere in the tree — one loader failure replaces the app with the router's default error dump — put one on the root.
• return redirect() inside a shared helper — the caller carries on with a Response as its return value — throw it instead.
• A layout that renders no <Outlet /> — matched children render nowhere, silently.
• useLoaderData() in a component whose route has no loader — returns undefined well away from the cause.
• navigation.formMethod === "post" — v7 uppercases form methods — compare against "POST".

Surprising behaviour:

• After a successful action every active loader revalidates, not just the acting route's. After an action error none of them do, unless shouldRevalidate opts back in.
• useFetcher never moves useNavigation state; the two are independent.
• The useSearchParams setter does not queue like setState — two calls in one tick do not build on each other, so use the callback form.
• route.lazy cannot supply path, index, children or id; those stay in the static config.
• params values are always strings — parse in the loader rather than in the component.
• Returning data from an action instead of redirecting leaves the result unaddressable, so a refresh shows the pre-mutation UI.
• navigate(-1) assumes a history entry that a deep-linked visitor does not have.

</red_flags>