nextjs-expert

Next.js Expert

Server Components are the default; "use client" is a deliberate, leaf-level choice. Fetch on the server, cache explicitly, and keep secrets off the client.

When to Use

• Building an App Router app: routing, layouts, loading/error UI, route groups.
• Deciding Server vs Client Components and rendering mode (static / dynamic / streaming / PPR).
• Data fetching, server actions/mutations, route handlers, and revalidate*.
• Debugging caching, hydration mismatches, or middleware.

When NOT to Use

• Generic React hooks/component patterns → react-expert.
• Pure styling → tailwind-expert.
• A standalone (non-Next) API/server → nodejs-backend-expert.

Core Principles

1. The server/client boundary is the whole game

• Everything is a Server Component unless it has "use client". Add the directive only for state, effects, event handlers, or browser APIs — and keep it at the leaves.
• "use client" makes a component and its imported tree part of the client bundle. Pass server-fetched data down as props instead of pulling data into client components.
• Never import DB clients, secrets, or server-only SDKs into a client component. Guard with import "server-only".

2. Fetch on the server, control caching explicitly

• async/await directly in Server Components; co-locate the fetch with the component that needs it (React dedups identical requests in a render).
• Caching is opt-in and explicit (especially Next 15, where fetches are uncached by default): fetch(url, { cache: "force-cache" }), { next: { revalidate: N } } for ISR, or { cache: "no-store" } for always-fresh.
• Tag data (next: { tags: [...] }) and invalidate precisely with revalidateTag/revalidatePath after writes.

3. Mutations via server actions

• "use server" functions for forms/mutations. Validate and authorize on the server — actions are public endpoints. Revalidate affected paths/tags afterward and return typed results.

4. Rendering & UX

• loading.tsx + <Suspense> to stream slow parts; error.tsx for boundaries; generateStaticParams to statically render dynamic routes.
• Use the next/image, next/font, and <Link> primitives — they fix common perf/CLS/font issues for free.

Decision Guide

Need	Do
Read data for a page	async Server Component fetch
Always-fresh data	cache: "no-store" / dynamic
Rebuild every N seconds	next: { revalidate: N } (ISR)
Invalidate after a write	revalidateTag / revalidatePath
Interactivity (state/events)	small "use client" leaf
Mutation/form submit	server action ("use server")
Slow section shouldn't block page	<Suspense> + loading.tsx

Common Mistakes

• "use client" at the top of a big tree → ships everything to the browser. Push it to leaves.
• Assuming fetches are cached (Next 15 default is no-cache) → set caching explicitly.
• Secrets/DB access in a client component → leaks to the bundle; use server-only.
• Forgetting to revalidate after a server action → stale UI.
• useEffect data fetching in a page that could be a Server Component → lose SSR, caching, and SEO.
• Hydration mismatch from Date.now()/window/random in render → guard or move to an effect/client component.

Examples

Server Component fetch with ISR + tags

// app/posts/page.tsx
export default async function Posts() {
  const posts = await fetch("https://api.example.com/posts", {
    next: { revalidate: 60, tags: ["posts"] },
  }).then((r) => r.json());
  return <ul>{posts.map((p) => <li key={p.id}>{p.title}</li>)}</ul>;
}

Server action: validate, authorize, revalidate

// app/actions.ts
"use server";
import { revalidateTag } from "next/cache";
import { z } from "zod";

const Input = z.object({ title: z.string().min(1) });

export async function createPost(formData: FormData) {
  const { title } = Input.parse(Object.fromEntries(formData)); // validate
  const user = await requireUser();                            // authorize
  await db.post.create({ data: { title, authorId: user.id } });
  revalidateTag("posts");
}

See Also

• react-expert — patterns inside client components.
• typescript-expert — typing pages, actions, and params.
• api-design-expert — route-handler contracts. tailwind-expert — styling.