Next.js App Router Patterns

Quick Guide: Server Components are the default and "use client" is opt-in, so keep client components small and at the leaves of the tree. Reads happen in Server Components; writes happen in Server Actions, which are public HTTP endpoints and revalidate the cache themselves. Two version facts change the answers below: from 15, params, searchParams, cookies() and headers() are Promises; in 16, middleware.ts became proxy.ts, revalidateTag() takes a cacheLife profile, and experimental_ppr became cacheComponents.

Detailed Resources:

• examples/core.md — file conventions, dynamic routes, layouts, error and not-found UI, server-only code
• examples/server-components.md — the Server/Client boundary, composition, streaming with Suspense
• examples/metadata.md — title templates, generateMetadata, Open Graph and Twitter cards
• examples/parallel-routes.md — @slot folders, intercepting routes, the modal pattern
• examples/route-groups.md — one layout per section without changing URLs
• examples/versions.md — PPR, Turbopack, typed routes, after(), instrumentation, and the v16 migration
• examples/server-actions.md — action definition, form actions, validation, authorization, pending states
• examples/mutations.md — running independent actions together
• examples/revalidation.md — path versus tag invalidation, tagging fetches
• examples/optimistic.md — useOptimistic for instant feedback
• examples/event-handlers.md — invoking actions from click handlers with useTransition
• examples/streaming.md — progress updates from long-running actions
• examples/cookies.md — reading and writing cookies inside actions
• reference.md — decision trees, route segment config, hook signatures, version-by-version changes

---

Which path applies

• On Next.js 16 — caching is opt-in and several APIs were renamed; read the migration half of examples/versions.md before copying any cache, middleware or parallel-route pattern.
• On the Pages Router (pages/) — a different set of conventions applies and none of the files above describe it.

---

<critical_requirements>

Before writing Next.js code

Write components as Server Components, and add "use client" only for state, effects or event handlers. The directive is what decides whether the code ships to the browser, so keeping it at the leaves keeps the bundle small and secrets on the server.

Import server-only in any module that touches a secret. The build then fails at the offending import instead of shipping the key to the browser.

Give data-heavy routes a loading.tsx, and independent sections their own <Suspense>. loading.tsx wraps the page in a boundary automatically; a boundary per section stops the slowest fetch blocking the fastest.

Set SEO through the metadata object or generateMetadata. Next.js deduplicates and orders those tags, which manual <head> markup does not.

Mark Server Actions with 'use server' — at the top of the file, or at the top of the async function. Without it the function is bundled for the client.

Validate the input and check authorization inside every Server Action. An action is a public HTTP endpoint reachable by its ID, so both the caller's identity and the payload's shape are unverified until the action verifies them.

Call revalidatePath() or revalidateTag() after a mutation, and before any redirect(). The cache is what the next render reads, and redirect() throws, so nothing after it runs.

Await params, searchParams, cookies() and headers(). All four are Promises from 15 onward.

</critical_requirements>

---

Auto-detection: page.tsx, layout.tsx, loading.tsx, error.tsx, not-found.tsx, default.tsx, template.tsx, "use client", "use server", "use cache", App Router, Server Components, Client Components, generateMetadata, generateStaticParams, parallel routes, intercepting routes, revalidatePath, revalidateTag, updateTag, useActionState, useFormStatus, useOptimistic, next/form, next/cache, next/navigation, cacheComponents, cacheLife, cacheTag, PPR, Turbopack, typedRoutes, instrumentation.ts, proxy.ts, after()

Applies to:

• Routing an app through the app/ directory, with layouts, loading states and error boundaries
• Deciding which components run on the server and which ship to the browser
• Streaming and progressive rendering with Suspense
• SEO through the Metadata API, and static generation with generateStaticParams
• Parallel and intercepting routes, including modals with shareable URLs
• Creating, updating and deleting data in Server Actions, and invalidating the cache afterwards

Handled elsewhere:

• Persistence — an action calls a data layer; neither the client nor the query shape is settled here
• Authentication and session retrieval — the action asks who the caller is and acts on the answer
• Schema validation — an action parses FormData through a schema; which schema library defines it is a separate choice
• Component styling — components take a className, and the styling approach is someone else's
• Client state that outlives a single component tree — Client Components receive data as props

---

<philosophy>

Philosophy

The App Router inverts React's default: components run on the server unless they say otherwise, and client-side JavaScript is opt-in. That removes the client-server fetch waterfall, because a component can await its own data, and it keeps the bundle proportional to the interactivity a page actually has.

Four consequences shape every pattern here:

1. Streaming — HTML reaches the browser as it becomes available rather than when the slowest query finishes
2. Colocation — data fetching, metadata and UI live in the same segment
3. Nested layouts — a layout persists across navigations inside its segment, so its state and its DOM survive
4. Server-side mutations — a Server Action runs on the server and revalidates the cache in the same round trip, so the UI and the data refresh together

Server Actions suit form submissions and mutations coupled to UI. Route Handlers suit external API consumers, webhooks, and work that has to run in parallel — actions invoked from the client queue.

useActionState, useFormStatus and useOptimistic are React 19 hooks rather than Next.js APIs; useActionState replaces the Canary-era ReactDOM.useFormState.

</philosophy>

---

<patterns>

Core patterns

Pattern 1: The Server/Client boundary

Fetch in a Server Component and pass the result down; the Client Component owns only the interaction.

// app/dashboard/page.tsx — a Server Component, no directive needed
export default async function DashboardPage() {
  const user = await getUser(); // runs on the server, ships no JavaScript
  return <UserProfile user={user} />; // Client Component, receives data as props
}

A Client Component can still wrap Server Components, as long as they arrive through children rather than being imported inside it.

Full code: examples/server-components.md

Pattern 2: Streaming with loading.tsx and Suspense

loading.tsx covers a whole route; a <Suspense> per section lets each one arrive on its own.

<Suspense fallback={<ChartSkeleton />}>
  <RevenueChart /> {/* slow query — streams in independently */}
</Suspense>
<Suspense fallback={<InvoicesSkeleton />}>
  <LatestInvoices />
</Suspense>

Full code: examples/server-components.md

Pattern 3: Route file conventions

Folders define routes; the special files define behaviour for the segment and everything under it.

app/dashboard/
├── layout.tsx      persists across navigations within the segment
├── page.tsx        the route itself — without it the URL is not reachable
├── loading.tsx     wraps page.tsx in a Suspense boundary automatically
├── error.tsx       "use client"; receives { error, reset }
└── not-found.tsx   rendered when the segment calls notFound()

Full code: examples/core.md. The complete file table is in reference.md.

Pattern 4: Dynamic segments

Bracket depth chooses between one segment, many, and optionally none.

// app/users/[id]/page.tsx       → /users/1          params: { id: string }
// app/docs/[...slug]/page.tsx   → /docs/a/b         params: { slug: string[] }
// app/shop/[[...slug]]/page.tsx → /shop, /shop/a    params: { slug?: string[] }

export default async function UserPage({
  params,
}: {
  params: Promise<{ id: string }>;
}) {
  const { id } = await params;
}

Pair with generateStaticParams to pre-render the known values at build time.

Full code: examples/core.md

Pattern 5: Route groups, parallel routes and interception

Three folder conventions that change layout and rendering without changing the URL.

app/
├── (marketing)/layout.tsx      route group — one layout per section, no URL segment
├── dashboard/@analytics/       parallel slot — arrives as a prop on dashboard/layout.tsx
├── @modal/(.)photo/[id]/       intercepts /photo/[id] on soft navigation only
└── @modal/default.tsx          what a slot renders when nothing matches

Every slot needs a default.tsx; on Next.js 16 the build fails without one.

Full code: examples/route-groups.md and examples/parallel-routes.md

Pattern 6: Metadata

Export a static object where the values are known, and a function where they come from the route's own data.

export const metadata: Metadata = {
  title: { template: "%s | Acme", default: "Acme" },
};

// or, when the title depends on the route's data:
export async function generateMetadata({
  params,
}: PageProps): Promise<Metadata> {
  const post = await getPost((await params).slug); // memoised with the page's own fetch
  return {
    title: post.title,
    openGraph: { title: post.title, type: "article" },
  };
}

Full code: examples/metadata.md

Pattern 7: Defining and invoking a Server Action

A file-level 'use server' marks every export as an action, which is what lets a Client Component import one.

// app/actions.ts
"use server";
export async function createPost(formData: FormData) {
  /* ... */
}

// app/posts/new/page.tsx — a plain form, so it submits without JavaScript
<form action={createPost}>...</form>;

action.bind(null, id) passes arguments beyond the FormData, and keeps the form working unenhanced. For a non-form trigger, call the action inside startTransition so the click does not block the UI.

Full code: examples/server-actions.md and examples/event-handlers.md

Pattern 8: Validation and authorization inside the action

Both checks belong in the action body, because that is the only code path an attacker cannot skip.

"use server";
export async function deletePost(postId: string) {
  const user = await getCurrentUser();
  if (!user) throw new Error("Unauthorized");

  const post = await getPost(postId);
  if (post?.authorId !== user.id) throw new Error("Forbidden");

  await deletePostRecord(postId);
  revalidatePath("/posts");
}

Return validation failures rather than throwing them — a throw reaches the error boundary and clears the form.

Full code: examples/server-actions.md

Pattern 9: Revalidation, then redirect

revalidatePath targets known routes; revalidateTag targets data wherever it is displayed.

"use server";
export async function createPost(formData: FormData) {
  // ...mutate
  revalidatePath("/posts"); // v16: revalidateTag("posts", "max")
  redirect("/posts"); // throws — nothing below this line runs
}

Full code: examples/revalidation.md

Pattern 10: Pending and optimistic state

Three React 19 hooks cover the wait: one for form state, one for a nested submit button, one for showing the result before it lands.

const [state, formAction, isPending] = useActionState(createPost, initialState);

// useFormStatus() reads the same pending flag, but only from a component *inside* the form
const [optimisticMessages, addOptimistic] = useOptimistic(
  messages,
  (state, content: string) => [
    ...state,
    { id: `temp-${Date.now()}`, content, pending: true },
  ],
);

Full code: examples/optimistic.md and examples/server-actions.md

</patterns>

---

<red_flags>

Red flags

Breaks at runtime:

• 'use server' missing from an action file — the function is bundled for the client, taking its imports and any secret they reach with it
• No authorization check inside an action — the action ID is discoverable, so anyone can invoke it
• No input validation inside an action — FormData arrives unparsed and untyped from an unauthenticated caller
• redirect() inside try/catch — it signals by throwing, so the catch swallows the navigation
• Reading params, searchParams, cookies() or headers() without await — all four are Promises from 15 onward
• error.tsx without "use client" — the reset callback needs a client boundary
• A secret read in a "use client" module — the value is in the browser bundle
• On Next.js 16: a parallel route slot with no default.tsx — the build fails
• On Next.js 16: revalidateTag("tag") with one argument — a cacheLife profile is now required

Surprising behaviour:

• useFormStatus() returns pending: false forever when called in the component that renders the form; it only reads a form it is nested inside
• Missing revalidatePath/revalidateTag after a mutation leaves the UI showing the pre-mutation cache with no error anywhere
• Server Actions invoked in parallel from the client queue and run one at a time
• Throwing for a validation failure clears the user's form — return the errors instead
• "use client" on page.tsx makes the whole subtree client-side, which is legal and quietly discards every server-rendering benefit
• From 15, fetch is not cached by default under dynamic rendering, and GET Route Handlers are not cached either
• Setting or deleting a cookie inside an action triggers a server re-render

More gotchas, the full anti-pattern set and the version-by-version changes are in reference.md.

</red_flags>