// BAD: fetching in component
function Profile() {
  const [user, setUser] = useState(null);
  useEffect(() => {
    fetch("/api/user")
      .then((r) => r.json())
      .then(setUser);
  }, []);
  if (!user) return <Spinner />;
  return <div>{user.name}</div>;
}

// GOOD: fetch in loader
export async function loader({ request }: Route.LoaderArgs) {
  let user = await getUser(request);
  return data({ user });
}

export default function Component() {
  const { user } = useLoaderData<typeof loader>();
  return <div>{user.name}</div>;
}

import { data } from "react-router";

// Bad: sequential fetches (slow)
export async function loader({ request }: Route.LoaderArgs) {
  let user = await getUser(request);
  let posts = await getPosts(user.id);
  let comments = await getComments(user.id);
  return data({ user, posts, comments });
}

// Good: parallel fetches
export async function loader({ request }: Route.LoaderArgs) {
  let user = await getUser(request);
  let [posts, comments] = await Promise.all([
    getPosts(user.id),
    getComments(user.id),
  ]);
  return data({ user, posts, comments });
}

// Both loaders can call getUser - cached per request
export async function loader({ request, context }: Route.LoaderArgs) {
  let client = await authenticate(request, context);
  let user = await getUser(client); // Uses cached result if already fetched
  return data({ user });
}

const { revalidate } = useRevalidator();

useEffect(() => {
  if (visibilityState === "hidden") return; // Don't poll hidden tabs
  let id = setInterval(revalidate, 30000);
  return () => clearInterval(id);
}, [revalidate, visibilityState]);

// Good: typed loader with useLoaderData
import { data } from "react-router";
import { useLoaderData } from "react-router";

export async function loader({ request, params }: Route.LoaderArgs) {
  return data({ user: await getUser(params.id) });
}

export default function Component() {
  const { user } = useLoaderData<typeof loader>();
  return <div>{user.name}</div>;
}

// Good: validate params early
import { data } from "react-router";
import { z } from "zod";

export async function loader({ params }: Route.LoaderArgs) {
  let itemId = z.string().parse(params.itemId);
  return data({ item: await getItem(itemId) });
}

export async function loader({ request }: Route.LoaderArgs) {
  let response = await fetch(url, { signal: request.signal });
  return data(await response.json());
}

routes/
  _.projects/
    queries.server.ts  # All data fetching functions
    route.tsx          # Loader calls query functions
    components/        # Route-specific components

export const middleware: Route.MiddlewareFunction[] = [
  sessionMiddleware,
  authMiddleware,
];

export async function loader({ context }: Route.LoaderArgs) {
  authorize(context, { requireUser: true, onboardingComplete: true });
  return null;
}

export const middleware: Route.MiddlewareFunction[] = [sessionMiddleware];

export const middleware: Route.MiddlewareFunction[] = [contextStorageMiddleware];

let result = await getBatcher().batch("key", () => getData());

let requestId = getRequestID();

export const middleware: Route.MiddlewareFunction[] = [loggerMiddleware];

return getTimingCollector().measure("load", "Load data", () => getData());

let cache = getSingleton(context);

if (fetchSite(request) === "cross-site") throw new Response(null, { status: 403 });

<Form method="post">
  <HoneypotInputs />
</Form>

return await cors(request, data(await getData()));

return redirect(safeRedirect(redirectTo, "/"));

let typed = createTypedCookie({ cookie, schema });

let ip = getClientIPAddress(request);

// UI-only access - use useRouteLoaderData
export default function ChildRoute() {
  const { user } = useRouteLoaderData<typeof profileLoader>("routes/_layout");
  return <div>Welcome, {user.name}</div>;
}

// Loader needs data - fetch again (cached, no extra request)
export async function loader({ request }: Route.LoaderArgs) {
  let client = await authenticate(request);
  let user = await getUser(client); // Uses cached result
  let settings = await getSettings(client, user.id);
  return data({ settings });
}

// route.tsx - only place that calls useLoaderData
export default function ItemsRoute() {
  const { items } = useLoaderData<typeof loader>();
  return <ItemList items={items} />;
}

// components/item-list.tsx - receives data as props
export function ItemList({ items }: { items: Item[] }) {
  return (
    <ul>
      {items.map((item) => (
        <li key={item.id}>{item.name}</li>
      ))}
    </ul>
  );
}

// Good: schema validation with i18n error messages
export async function action({ request }: Route.ActionArgs) {
  let t = await i18n.getFixedT(request);
  let formData = await request.formData();

  try {
    const { amount } = z
      .object({
        amount: z.coerce
          .number()
          .min(
            minimumAmount,
            t("Amount must be at least {{min}}.", { min: minimumAmount }),
          ),
      })
      .parse({ amount: formData.get("amount") });

    await processAmount(amount);
    throw redirect("/success");
  } catch (error) {
    if (error instanceof z.ZodError) {
      return data(
        { errors: error.issues.map(({ message }) => message) },
        { status: 400 },
      );
    }
    throw error;
  }
}

// Good: proper error handling
export async function action({ request }: Route.ActionArgs) {
  try {
    // ... validation and mutation
    throw redirect("/success");
  } catch (error) {
    if (error instanceof z.ZodError) {
      return data(
        { errors: error.issues.map(({ message }) => message) },
        { status: 400 },
      );
    }
    if (error instanceof Error) {
      return data({ errors: [error.message] }, { status: 400 });
    }
    throw error; // Re-throw redirects and unknown errors
  }
}

// Good: redirect after mutation
export async function action({ request }: Route.ActionArgs) {
  await createItem(formData);
  throw redirect("/items"); // Use throw for redirect
}

const schema = z.object({
  // Trim and lowercase email
  email: z.string().trim().toLowerCase().pipe(z.string().email()),

  // Parse currency string to number
  amount: z
    .string()
    .transform((val) => parseFloat(val.replace(/[,$]/g, "")))
    .pipe(z.number().positive()),

  // Convert checkbox to boolean
  subscribe: z
    .string()
    .optional()
    .transform((val) => val === "on"),
});

export async function clientAction({
  request,
  serverAction,
}: Route.ClientActionArgs) {
  let formData = await request.formData();
  let result = schema.safeParse(Object.fromEntries(formData));

  if (!result.success) {
    return data(
      { errors: result.error.flatten().fieldErrors },
      { status: 400 },
    );
  }

  return serverAction<typeof action>(); // Validation passed, call server
}

// Good: useFetcher for in-place updates (no navigation)
function LikeButton({ postId }: { postId: string }) {
  let fetcher = useFetcher();
  return (
    <fetcher.Form method="post" action="/api/like">
      <input type="hidden" name="postId" value={postId} />
      <button type="submit">Like</button>
    </fetcher.Form>
  );
}

// Good: Form for navigation after submit
function CreatePostForm() {
  return (
    <Form method="post" action="/posts/new">
      <input name="title" />
      <button type="submit">Create</button>
    </Form>
  );
}

// Good: pending state with fetcher
function SubmitButton() {
  let fetcher = useFetcher();
  let isPending = fetcher.state !== "idle";

  return (
    <Button type="submit" isDisabled={isPending}>
      {isPending ? <Spinner /> : "Submit"}
    </Button>
  );
}

// Good: with useSpinDelay to avoid flicker
const isPending = useSpinDelay(fetcher.state !== "idle", { delay: 50 });

const formRef = useRef<HTMLFormElement>(null);
const fetcher = useFetcher<typeof action>();

useEffect(
  function resetFormOnSuccess() {
    if (fetcher.state === "idle" && fetcher.data?.ok) {
      formRef.current?.reset();
    }
  },
  [fetcher.state, fetcher.data],
);

return (
  <fetcher.Form method="post" ref={formRef}>
    ...
  </fetcher.Form>
);

// Action returns fields on error
export async function action({ request }: Route.ActionArgs) {
  let formData = await request.formData();
  let fields = { email: formData.get("email")?.toString() ?? "" };
  let result = schema.safeParse(fields);

  if (!result.success) {
    return data(
      { errors: result.error.flatten().fieldErrors, fields },
      { status: 400 },
    );
  }
  // ...
}

// Component uses defaultValue
<input name="email" defaultValue={actionData?.fields?.email} />;

import { setTimeout } from "node:timers/promises";

export async function clientLoader({
  request,
  serverLoader,
}: Route.ClientLoaderArgs) {
  // Debounce by 500ms - request.signal aborts if called again
  return await setTimeout(500, serverLoader, { signal: request.signal });
}

clientLoader.hydrate = true;

// Bad: old defer pattern
import { defer } from "react-router";

export async function loader({ request }: Route.LoaderArgs) {
  return defer({
    critical: await getCriticalData(),
    lazy: getLazyData(), // Promise
  });
}

// Good: Single Fetch with data() - promises auto-stream
import { data } from "react-router";

export async function loader({ request }: Route.LoaderArgs) {
  return data({
    critical: await getCriticalData(),
    lazy: getLazyData(), // Promise automatically streamed
  });
}

// Good: Await with Suspense fallback
import { Await, useLoaderData } from "react-router";
import { Suspense } from "react";

export default function Component() {
  const { critical, lazy } = useLoaderData<typeof loader>();

  return (
    <div>
      <div>{critical.name}</div>
      <Suspense fallback={<Skeleton />}>
        <Await resolve={lazy}>{(data) => <LazyContent data={data} />}</Await>
      </Suspense>
    </div>
  );
}

// Bad: jsonHash from remix-utils
import { jsonHash } from "remix-utils/json-hash";

export async function loader({ request }: Route.LoaderArgs) {
  return jsonHash({
    a: getDataA(),
    b: getDataB(),
  });
}

// Good: native Promise.all
export async function loader({ request }: Route.LoaderArgs) {
  const [a, b] = await Promise.all([getDataA(), getDataB()]);
  return data({ a, b });
}

// Good: data() with promises for streaming
import { data } from "react-router";

export async function loader({ request }: Route.LoaderArgs) {
  return data({
    a: getDataA(), // Streams automatically
    b: getDataB(),
  });
}