Remix v2 Data Flow

Quick Reference

Loader + typed read:

import { json, type LoaderFunctionArgs } from "@remix-run/node";
import { useLoaderData } from "@remix-run/react";

export async function loader({ request }: LoaderFunctionArgs) {
  const invoices = await db.invoice.findMany();
  return json({ invoices });
}

export default function Invoices() {
  // typeof loader is a type ANNOTATION (not assertion) — drives SerializeFrom<T>.
  const { invoices } = useLoaderData<typeof loader>();
  return <InvoiceList invoices={invoices} />;
}

Action + redirect-after-success (PRG):

import { json, redirect, type ActionFunctionArgs } from "@remix-run/node";
import { useActionData, Form } from "@remix-run/react";

export async function action({ request }: ActionFunctionArgs) {
  const form = await request.formData();
  const parsed = NewProject.safeParse(Object.fromEntries(form));
  if (!parsed.success) return json({ errors: parsed.error.flatten().fieldErrors }, { status: 400 });
  const project = await db.project.create({ data: parsed.data });
  return redirect(`/projects/${project.id}`);
}

Canonical APIs

Route modules export loader / action; components read results via useLoaderData<typeof loader>() and useActionData<typeof action>(). After every action, Remix automatically revalidates the loaders of all matching routes on the page, so the UI stays consistent with the server without manual cache invalidation.

Signatures:

• loader: ({ request, params, context }: LoaderFunctionArgs) => Response | Promise<Response> — server-only read, runs on SSR and on client navigations via fetch.
• action: ({ request, params, context }: ActionFunctionArgs) => Response | Promise<Response> — server-only handler for non-GET requests (POST/PUT/PATCH/DELETE).
• json(data, init?: number | ResponseInit): TypedResponse<typeof data> — ergonomic JSON Response wrapper with status/headers.
• redirect(url, init?: number | ResponseInit): TypedResponse<never> — 30x response; default 302.

Imports: @remix-run/node for server utilities (json, redirect, defer, type args) on Node; substitute @remix-run/cloudflare or @remix-run/deno for those targets. Hooks and components come from @remix-run/react.

Type Annotations, Not Assertions

useLoaderData<typeof loader>() is a type annotation, not a as-style assertion. The generic feeds SerializeFrom<typeof loader>, which models the wire-format transformation: Date becomes string, Map/Set collapse, undefined fields are stripped, class methods vanish. If you call data.createdAt.getFullYear() on a Date field, that's a runtime bug — the type already says string.

When json() Is Optional in v2

v2 did not change the underlying contract: loaders and actions must return a Response. json() is the ergonomic wrapper that sets application/json and lets you supply status / headers. Bare object returns work in v2 (Remix auto-wraps as json()), but json() is preferred for explicit status, headers, and clean TypedResponse<T> typing. Reach for json() whenever you need:

• A non-200 status code (e.g. { status: 400 } for validation errors).
• Custom headers (caching, Set-Cookie).
• An explicit TypedResponse<T> for clean useLoaderData<typeof loader>() inference.

Throwing for Short-Circuits

Throwing a Response from a loader or action exits the data function immediately. Use this for auth guards (throw redirect("/login")) and 404s (throw new Response("Not Found", { status: 404 }) or throw json({ message }, { status: 404 })). Throwing a plain Error will not be classified as a route response by useRouteError() / isRouteErrorResponse().

Streaming Rejections: <Await errorElement> + useAsyncError()

When a promise passed through defer() rejects, an <Await errorElement={...}> boundary catches it inline — without it, the rejection bubbles to the route's ErrorBoundary and tears down the whole page, defeating the streaming benefit. Inside the errorElement, call useAsyncError() (from @remix-run/react) to read the rejection value — this is the streaming analogue of useRouteError().

function ReviewsError() {
  const error = useAsyncError(); // typed as `unknown`
  return <p>Failed to load reviews: {String(error)}</p>;
}

<Suspense fallback={<ReviewsSkeleton />}>
  <Await resolve={reviews} errorElement={<ReviewsError />}>
    {(r) => <ReviewList reviews={r} />}
  </Await>
</Suspense>

Sensitive Data

Everything returned from a loader travels to the browser as JSON. Project to a safe DTO ({ id, email, name }) before returning; never return the full Prisma User, password hashes, API keys, or internal flags. Loaders execute server-only — but the return value is shipped to the client wholesale.

Mutations Belong in Actions

Loaders run on every GET navigation and may be invoked speculatively by prefetch; they also re-run during automatic revalidation. Anything that mutates persistent state must live in action, reached via <Form method="post"> or useFetcher. Calling fetch() directly from a component to hit a Remix route bypasses revalidation, pending state, and progressive enhancement — use useFetcher().submit() / useFetcher().load() instead.

Routing Gotchas to Remember

• Only the deepest matching action runs. Index routes nested under a layout collide unless you target them with <Form action="/things?index" method="post" />.
• useActionData is scoped to the current route. It cannot access action results from parent or child routes; to share, lift the action or use a useFetcher with key.
• Headers from the leaf loader win. Remix only uses the deepest matching headers export. Parent caching policies are ignored unless you explicitly merge parentHeaders.
• Default revalidation revalidates ALL routes on the page after an action — even those whose params didn't change. Use shouldRevalidate to opt out of expensive parent loaders.

Pending State (v1 → v2)

useTransition is removed in v2 — use useNavigation. The submission object is flattened directly onto the navigation in v2 (and the fetcher likewise; both nav.formData/nav.formMethod and fetcher.formData/fetcher.formMethod are flat). formMethod is now UPPERCASE in v2 ("POST", not "post"); comparisons like nav.formMethod === "post" silently never match. fetcher.type is also gone — branch on fetcher.state plus presence of fetcher.formData.

GET submissions go idle → loading → idle. POST flow goes idle → submitting → loading → idle. Spinners gated only on "submitting" will miss GET forms. GET submissions still populate nav.formData and nav.formMethod === "GET" during the loading phase, so filter-form pending UI should branch on formData presence, not on state === 'submitting'. For useFetcher, submitting applies to BOTH GET (<fetcher.Form method='get'> and fetcher.submit(..., {method:'get'})) and non-GET; only fetcher.load() skips submitting. This is the inverse of useNavigation, which skips submitting for GET.

Gates (decision sequencing)

Answer in order. Pass means the condition is true; pick the API on the same line and stop.

loader vs useEffect

1. Is the data needed for correct first render of this route (SSR, prefetch, automatic revalidation after actions)?
   • Pass → loader + useLoaderData<typeof loader>(). Stop.
   • Fail → Step 2.
2. Is the fetch driven by post-mount user interaction, timer, or subscription (not route entry)?
   • Pass → useEffect / event handlers. Stop.
   • Fail → Prefer loader + revalidation; do not mirror navigation inside an effect.

json() vs raw Response vs defer()

1. Do any returned fields need to stream (slow query, expensive aggregation) while the page renders fast?
   • Pass → defer({ critical: await…, slow: promiseWithoutAwait }) + <Suspense><Await>…</Await></Suspense>. Stop.
   • Fail → Step 2.
2. Do you need a custom status code, custom headers, or explicit TypedResponse<T> typing?
   • Pass → json(data, init) (or redirect(url, init) for 3xx). Stop.
   • Fail → Step 3.
3. Do you need a non-JSON body (binary, plain text, streamed file)?
   • Pass → Build a raw new Response(body, init). Stop.
   • Fail → Default to json(data) — it's the documented v2 contract for object payloads.

<Form> / route action vs useFetcher

1. Should the URL or history stack change (bookmark / share / back returns to prior screen)?
   • Pass → <Form method="post"> posting to a route action. Stop.
   • Fail → Step 2.
2. Mutation stays on the same route (inline edit, list-row toggle, popover, optimistic UI)?
   • Pass → useFetcher() / fetcher.Form / fetcher.submit(). Stop.

Additional Documentation

• Loaders: See references/loaders.md for loader signature, typed useLoaderData, json() vs raw Response, redirect(), throwing, sensitive-data filtering, params handling.
• Actions: See references/actions.md for action signature, FormData parsing, useActionData<typeof action>(), zod/valibot validation, redirect-after-success.
• Defer & Await: See references/defer-await.md for defer() + <Await> + <Suspense>, when streaming helps TTFB, error handling.
• Revalidation & Pending State: See references/revalidation.md for automatic revalidation, shouldRevalidate, useRevalidator, useNavigation (plus v1 useTransition rename).

v1 → v2 Quick Diff

Concern	v1	v2
Navigation hook	useTransition()	useNavigation()
Submission shape	transition.submission.formMethod	flat nav.formMethod / nav.formData
formMethod casing	"post"	"POST" (UPPERCASE)
Fetcher type field	fetcher.type === "actionSubmission"	branch on fetcher.state + fetcher.formData
Loader args type	LoaderArgs / ActionArgs	LoaderFunctionArgs / ActionFunctionArgs
Returning data	json(data) required	json(data) still the documented contract