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nextjs Next.js 15 App Router deep dive — React Server Components, Server Actions, Suspense streaming SSR, Error Boundaries, parallel and intercepting routes, data prefetching, edge functions, Vercel deployment patterns, and performance best practices. Absorbs server-components, server-actions, suspense, error-boundary, parallel-routes, intercepting-routes, prefetch, edge-functions, vercel, vercel-react-best-practices.
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name nextjs description Next.js 15 App Router deep dive — React Server Components, Server Actions, Suspense streaming SSR, Error Boundaries, parallel and intercepting routes, data prefetching, edge functions, Vercel deployment patterns, and performance best practices. Absorbs server-components, server-actions, suspense, error-boundary, parallel-routes, intercepting-routes, prefetch, edge-functions, vercel, vercel-react-best-practices. layer domain category frontend triggers ["@slot","ErrorBoundary","RSC","React.lazy","Suspense boundary","app router","bundle size","cloudflare worker","data preload","edge computing","edge config","edge function","edge middleware","edge runtime","edge-side","error boundary","error recovery","error.tsx","fallback ui","form action","generateStaticParams","independent loading","intercepted route","intercepting routes","lazy","link prefetch","loading fallback","middleware","modal route","next.js","next.js optimization","next.js performance","next/image","next/link","nextjs","parallel route","parallel routes","photo modal","prefetch","prefetching","preload","react best practices","react error boundary","react performance","react performance patterns","revalidate","revalidatePath","revalidateTag","route interception","route prefetch","rsc","rsc payload","server action","server actions","server client boundary","server component","server mutation","server-only","simultaneous rendering","slot convention","soft navigation modal","stale while revalidate","suspense","suspense boundary","use hook","use server","vercel","vercel best practices","vercel cron","vercel deploy","vercel edge","vercel kv","vercel middleware","vercel.json"]
nextjs
Next.js 15 App Router deep dive — React Server Components, Server Actions, Suspense streaming SSR, Error Boundaries, parallel and intercepting routes, data prefetching, edge functions, Vercel deployment patterns, and performance best practices. Absorbs server-components, server-actions, suspense, error-boundary, parallel-routes, intercepting-routes, prefetch, edge-functions, vercel, vercel-react-best-practices.
Absorbs
server-componentsserver-actionssuspenseerror-boundaryparallel-routesintercepting-routesprefetchedge-functionsvercelvercel-react-best-practices
Core
Next.js App Router & RSC Patterns
Purpose
Provide expert guidance on Next.js 14+ App Router architecture, React Server Components, data fetching strategies, caching, and production deployment patterns. This skill focuses on the App Router paradigm and modern Next.js conventions.
Key Patterns
File System Routing
Route Hierarchy:
app/
layout.tsx # Root layout (wraps everything)
page.tsx # Home route /
loading.tsx # Suspense fallback for /
error.tsx # Error boundary for /
not-found.tsx # 404 for /
global-error.tsx # Root error boundary (replaces layout)
dashboard/
layout.tsx # Nested layout for /dashboard/*
page.tsx # /dashboard
loading.tsx # Suspense for /dashboard
settings/
page.tsx # /dashboard/settings
[teamId]/
page.tsx # /dashboard/:teamId (dynamic)
[...slug]/
page.tsx # /dashboard/* (catch-all)
(marketing)/ # Route group (no URL impact)
about/
page.tsx # /about
pricing/
page.tsx # /pricing
@modal/ # Parallel route (named slot)
(.)photo/[id]/
page.tsx # Intercepted route
api/
route.ts # API route /api
Special Files Priority: layout > template > error > loading > not-found > page
Server vs Client Components
Default: Server Components. Only add 'use client' when you need:
Event handlers (onClick, onChange, etc.)
State or effects (
useState
useEffect
useReducer
Browser-only APIs (window, localStorage, IntersectionObserver)
Custom hooks that use the above
Third-party components that require client context Push client boundaries down:
import { getStats } from '@/lib/data' ;
import { StatsChart } from '@/components/stats-chart' ;
export default async function DashboardPage (const stats = await getStats ();
return (
<section className ="py-16" >
<h1 className ="text-2xl font-bold mb-8" > Dashboard</h1 >
{/* Pass serializable data to client component */}
<StatsChart data ={stats} />
</section > 'use client' ;
import { useState } from 'react' ;
export function StatsChart ({ data }: { data: StatsData } ) {
const [range, setRange] = useState<'week' | 'month' >('week' );
}
BAD: Making entire page client-side:
'use client' ;
export default function DashboardPage (const [stats, setStats] = useState (null );
useEffect (() => { fetch ('/api/stats' ).then () }, []);
}
Data Fetching Server Components — fetch directly:
async function ProductPage ({ params }: { params: Promise <{ id: string }> } ) {
const { id } = await params;
const product = await fetch (`https://api.example.com/products/${id} ` , {
next : { revalidate : 3600 },
}).then (r =>json ());
return <ProductDetail product ={product} /> Parallel Data Fetching — avoid waterfalls:
async function DashboardPage (const [user, stats, notifications] = await Promise .all ([
getUser (),
getStats (),
getNotifications (),
]);
return ();
}
async function DashboardPage (const userPromise = getUser ();
const statsPromise = getStats ();
return (
<div >
<Suspense fallback ={ <UserSkeleton /> }>
<UserSection userPromise ={userPromise} />
</Suspense >
<Suspense fallback ={ <StatsSkeleton /> }>
<StatsSection statsPromise ={statsPromise} />
</Suspense >
</div >
Caching & Revalidation Cache Hierarchy (Next.js 15+):
Layer Scope Default Control Request Memoization Single render pass ON React.cache()Data Cache Cross-request OFF (Next 15) fetch({ next: { revalidate } })Full Route Cache Static routes ON for static export const dynamicRouter Cache Client-side ON (reduced in 15) router.refresh()
export const revalidate = 3600 ;
'use server' ;
import { revalidatePath, revalidateTag } from 'next/cache' ;
export async function publishPost (id : string await db.post .update ({ where : { id }, data : { published : true } });
revalidatePath ('/blog' );
revalidateTag ('posts' );
}
const posts = await fetch ('https://api.example.com/posts' , {
next : { tags : ['posts' ], revalidate : 3600 },
});
export const dynamic = 'force-dynamic' ;
export const dynamic = 'force-static' ;
export async function generateStaticParams (const posts = await getPosts ();
return posts.map (post =>slug : post.slug }));
}
Server Actions
'use server' ;
import { z } from 'zod' ;
import { revalidatePath } from 'next/cache' ;
const CreatePostSchema = z.object ({
title : z.string ().min (1 ).max (200 ),
content : z.string ().min (1 ),
});
export async function createPost (prevState : ActionState , formData : FormData const parsed = CreatePostSchema .safeParse ({
title : formData.get ('title' ),
content : formData.get ('content' ),
});
if (!parsed.success ) {
return { error : parsed.error .flatten ().fieldErrors };
}
try {
await db.post .create ({ data : parsed.data });
revalidatePath ('/posts' );
return { success : true };
} catch (e) {
return { error : { _form : ['Failed to create post' ] } };
}
}
'use client' ;
import { useActionState } from 'react' ;
import { createPost } from '@/app/actions' ;
export default function NewPostPage (const [state, formAction, isPending] = useActionState (createPost, null );
return (
<form action ={formAction} className ="space-y-6 max-w-2xl" >
<div >
<label htmlFor ="title" className ="block text-base font-medium mb-2" > Title</label >
<input
id ="title"
name ="title"
className ="w-full px-4 py-3 rounded-lg border transition-all duration-200 focus-visible:ring-2 focus-visible:ring-offset-2"
/>
{state?.error?.title && (
<p className ="text-red-600 text-sm mt-1" > {state.error.title[0]}</p >
)}
</div >
<button
type ="submit"
disabled ={isPending}
className ="px-6 py-4 text-base rounded-lg bg-blue-600 text-white transition-all duration-200 hover:bg-blue-700 focus-visible:ring-2 focus-visible:ring-offset-2 disabled:opacity-50"
>
{isPending ? 'Creating...' : 'Create Post'}
</button >
</form >
Middleware
import { NextResponse } from 'next/server' ;
import type { NextRequest } from 'next/server' ;
export function middleware (request : NextRequest const token = request.cookies .get ('session' )?.value ;
if (!token && request.nextUrl .pathname .startsWith ('/dashboard' )) {
return NextResponse .redirect (new URL ('/login' , request.url ));
}
const response = NextResponse .next ();
response.headers .set ('x-pathname' , request.nextUrl .pathname );
return response;
}
export const config = {
matcher : [
'/((?!_next/static|_next/image|favicon.ico|api).*)' ,
],
};
Metadata & SEO
export const metadata : Metadata = {
title : 'My App' ,
description : 'Description here' ,
openGraph : { title : 'My App' , description : '...' },
};
export async function generateMetadata (
{ params }: { params: Promise <{ slug: string }> }
): Promise <Metadata > {
const { slug } = await params;
const post = await getPost (slug);
return {
title : post.title ,
description : post.excerpt ,
openGraph : {
title : post.title ,
images : [post.coverImage ],
},
};
}
Image & Font Optimization import Image from 'next/image' ;
import { Inter } from 'next/font/google' ;
const inter = Inter ({
subsets : ['latin' ],
display : 'swap' ,
variable : '--font-inter' ,
});
<Image
src ={product.image}
alt ={product.name}
width ={800}
height ={600}
sizes ="(max-width: 768px) 100vw, (max-width: 1200px) 50vw, 33vw"
className ="rounded-xl"
priority ={isAboveFold}
/>
Best Practices
Server Components by default — Only add 'use client' when browser APIs or interactivity are needed.Colocate data fetching — Fetch data in the component that needs it. Request deduplication handles the rest.Stream with Suspense — Wrap slow data sources in <Suspense> to stream HTML incrementally.Never fetch from client to your own API routes — Use Server Actions or direct database access in Server Components.Use loading.tsx — Provides instant loading states via Suspense.Validate Server Action inputs — Always validate with Zod or similar. Never trust client data.Use next/image — Automatic WebP/AVIF, lazy loading, and responsive sizing.Use next/font — Zero layout shift, self-hosted font files.Prefer revalidatePath/revalidateTag — Over router.refresh() for cache invalidation.Keep middleware lean — It runs on every matched request. No heavy computation or database calls.
Common Pitfalls Pitfall Problem Fix 'use client' at page levelEntire page loses SSR benefits Push client boundary to smallest component Fetching own API routes from RSC Unnecessary network hop Call database/service directly in RSC Missing loading.tsx White flash during navigation Add loading files for major route segments Sequential awaits in RSC Waterfall data fetching Use Promise.all() or parallel Suspense Huge client bundles Slow hydration Dynamic import heavy components, keep client islands small Not using sizes on Image Full-size image downloaded on mobile Always set responsive sizes prop Mutating in GET handlers Caching serves stale mutations Use POST/PUT/DELETE or Server Actions for mutations Accessing searchParams synchronously Next.js 15 made params async await params and await searchParamsNo error boundaries Entire page crashes on component error Use error.tsx per route segment Hardcoding secrets client-side Security leak Use NEXT_PUBLIC_ prefix only for truly public values
From server-components
React Server Components deep dive — RSC payload, streaming SSR, server/client boundaries, data patterns
React Server Components
Purpose Provide expert guidance on React Server Components (RSC) architecture, including server/client boundary decisions, data fetching patterns, streaming SSR, RSC payload optimization, and common patterns for building performant applications with the App Router in Next.js 14+.
Key Patterns
Server vs Client Components Default to Server Components. Only add "use client" when you need browser APIs, event handlers, or React state/effects.
Feature Server Component Client Component async/await for dataYes No (use hooks) Database/file access Yes No useState, useEffectNo Yes Event handlers (onClick) No Yes Browser APIs (window, localStorage) No Yes Bundle size impact Zero JS sent Adds to bundle Re-renders on state change No Yes
Does it need useState/useEffect/event handlers?
YES -> "use client"
NO -> Does it fetch data or access server resources?
YES -> Server Component (default)
NO -> Server Component (default, still zero JS)
Server/Client Boundary Architecture Push "use client" boundaries as deep as possible in the component tree. Keep data fetching and layout in Server Components, and only wrap interactive leaf components as Client Components.
import { Suspense } from "react" ;
import { ProductList } from "./product-list" ;
import { ProductFilters } from "./product-filters" ;
import { Skeleton } from "@/components/skeleton" ;
export default async function ProductsPage ({
searchParams,
}: {
searchParams: Promise <{ category?: string ; sort?: string }>;
} ) {
const params = await searchParams;
return (
<div className ="flex gap-8" >
{/* Client Component for interactive filters */}
<aside className ="w-64" >
<ProductFilters
initialCategory ={params.category}
initialSort ={params.sort}
/>
</aside >
{/* Server Component with streaming */}
<main className ="flex-1" >
<Suspense fallback ={ <Skeleton count ={12} /> }>
<ProductList
category ={params.category}
sort ={params.sort}
/>
</Suspense >
</main >
</div >
import { db } from "@/lib/db" ;
export async function ProductList ({
category,
sort,
}: {
category?: string ;
sort?: string ;
} ) {
const products = await db.query .products .findMany ({
where : category ? eq (products.category , category) : undefined ,
orderBy : sort === "price" ? asc (products.price ) : desc (products.createdAt ),
});
return (
<div className ="grid grid-cols-3 gap-6" >
{products.map((product) => (
<ProductCard key ={product.id} product ={product} />
))}
</div >
"use client" ;
import { useRouter, useSearchParams } from "next/navigation" ;
import { useTransition } from "react" ;
export function ProductFilters ({
initialCategory,
initialSort,
}: {
initialCategory?: string ;
initialSort?: string ;
} ) {
const router = useRouter ();
const searchParams = useSearchParams ();
const [isPending, startTransition] = useTransition ();
function updateFilter (key : string , value : string const params = new URLSearchParams (searchParams.toString ());
if (value) {
params.set (key, value);
} else {
params.delete (key);
}
startTransition (() => {
router.push (`/products?${params.toString()} ` );
});
}
return (
<div className ={ `p-6 rounded-xl shadow-sm ${isPending ? "opacity-60 " : ""}`}>
<select
defaultValue ={initialCategory}
onChange ={(e) => updateFilter("category", e.target.value)}
className="px-4 py-3 rounded-lg border w-full transition-all duration-200 focus-visible:ring-2 focus-visible:ring-offset-2"
>
<option value ="" > All Categories</option >
<option value ="electronics" > Electronics</option >
<option value ="clothing" > Clothing</option >
</select >
</div >
Composition Pattern: Server Components as Children Pass Server Components as children to Client Components to avoid pulling them into the client bundle.
import { AuthProvider } from "./auth-provider" ;
import { Sidebar } from "./sidebar" ;
export default function Layout ({ children }: { children: React.ReactNode } ) {
return (
<AuthProvider >
<div className ="flex" >
<Sidebar />
<main className ="flex-1" > {children}</main >
</div >
</AuthProvider >
"use client" ;
import { SessionProvider } from "next-auth/react" ;
export function AuthProvider ({ children }: { children: React.ReactNode } ) {
return <SessionProvider > {children}</SessionProvider >
Data Fetching Patterns Pattern 1: Direct database access in Server Components
import { db } from "@/lib/db" ;
async function UserProfile ({ userId }: { userId: string } ) {
const user = await db.query .users .findFirst ({
where : eq (users.id , userId),
with : { posts : { limit : 10 } },
});
if (!user) notFound ();
return (
<div className ="p-6 rounded-xl shadow-sm" >
<h1 className ="text-2xl font-bold" > {user.name}</h1 >
<p className ="text-base text-gray-600" > {user.bio}</p >
</div > Pattern 2: Parallel data fetching
async function Dashboard (const [stats, recentOrders, notifications] = await Promise .all ([
getStats (),
getRecentOrders (),
getNotifications (),
]);
return (
<div className ="grid grid-cols-3 gap-6" >
<StatsCard stats ={stats} />
<OrderList orders ={recentOrders} />
<NotificationList notifications ={notifications} />
</div > Pattern 3: Streaming with Suspense boundaries
async function Page (return (
<div >
{/* Fast: renders immediately */}
<h1 className ="text-2xl font-bold" > Dashboard</h1 >
{/* Streams in when ready */}
<Suspense fallback ={ <StatsSkeleton /> }>
<StatsSection />
</Suspense >
{/* Independent stream -- does not block above */}
<Suspense fallback ={ <OrdersSkeleton /> }>
<RecentOrders />
</Suspense >
{/* Slow query streams last */}
<Suspense fallback ={ <AnalyticsSkeleton /> }>
<AnalyticsChart />
</Suspense >
</div >
Server Actions Server-side mutations callable from Client Components.
"use server" ;
import { revalidatePath } from "next/cache" ;
import { redirect } from "next/navigation" ;
import { db } from "@/lib/db" ;
import { z } from "zod" ;
const createPostSchema = z.object ({
title : z.string ().min (1 ).max (200 ),
content : z.string ().min (1 ),
});
export async function createPost (formData : FormData const parsed = createPostSchema.safeParse ({
title : formData.get ("title" ),
content : formData.get ("content" ),
});
if (!parsed.success ) {
return { error : parsed.error .flatten ().fieldErrors };
}
const post = await db.insert (posts).values (parsed.data ).returning ();
revalidatePath ("/posts" );
redirect (`/posts/${post[0 ].id} ` );
}
"use client" ;
import { useActionState } from "react" ;
import { createPost } from "../actions" ;
export default function NewPostPage (const [state, formAction, isPending] = useActionState (createPost, null );
return (
<form action ={formAction} className ="space-y-6 max-w-2xl" >
<div >
<label htmlFor ="title" className ="block text-base font-medium mb-2" >
Title
</label >
<input
id ="title"
name ="title"
className ="px-4 py-3 rounded-lg border w-full transition-all duration-200 focus-visible:ring-2 focus-visible:ring-offset-2"
/>
{state?.error?.title && (
<p className ="text-red-600 text-sm mt-1" > {state.error.title}</p >
)}
</div >
<div >
<label htmlFor ="content" className ="block text-base font-medium mb-2" >
Content
</label >
<textarea
id ="content"
name ="content"
rows ={8}
className ="px-4 py-3 rounded-lg border w-full transition-all duration-200 focus-visible:ring-2 focus-visible:ring-offset-2"
/>
{state?.error?.content && (
<p className ="text-red-600 text-sm mt-1" > {state.error.content}</p >
)}
</div >
<button
type ="submit"
disabled ={isPending}
className ="px-6 py-4 text-base rounded-lg bg-blue-600 text-white transition-all duration-200 hover:bg-blue-700 focus-visible:ring-2 focus-visible:ring-offset-2 disabled:opacity-50"
>
{isPending ? "Publishing..." : "Publish"}
</button >
</form >
Protecting Server-Only Code Prevent accidental import of server code into Client Components.
import "server-only" ;
import { drizzle } from "drizzle-orm/neon-http" ;
export const db = drizzle (process.env .DATABASE_URL !);
import "server-only" ;
import { cookies } from "next/headers" ;
export async function getSession (const cookieStore = await cookies ();
const token = cookieStore.get ("session" )?.value ;
if (!token) return null ;
return verifyToken (token);
}
RSC Payload Optimization Keep the RSC payload small by avoiding passing large data through component props.
async function ProductPage ({ params }: { params: Promise <{ id: string }> } ) {
const { id } = await params;
const product = await getProduct (id);
return <ProductView product ={product} /> async function ProductPage ({ params }: { params: Promise <{ id: string }> } ) {
const { id } = await params;
const product = await getProduct (id);
return (
<ProductView
name ={product.name}
price ={product.price}
image ={product.image}
description ={product.description}
/> Avoid serializing non-serializable values:
<ClientComponent onClick={() => console .log ("click" )} />
<ClientComponent action ={serverAction} /> <ClientComponent productId ={product.id} />
Best Practices
Default to Server Components -- only add "use client" when you need interactivity, state, or browser APIs.Push client boundaries down -- wrap the smallest interactive subtree, not entire pages.Use server-only package to guard server code from accidental client import.Fetch data in Server Components directly -- no need for API routes or useEffect + fetch.Use Promise.all for parallel fetches -- do not await sequentially unless data depends on each other.Wrap slow data fetches in <Suspense> -- stream the shell immediately and fill in data as it resolves.Pass Server Components as children to Client Components to keep them out of the client bundle.Validate Server Action inputs with Zod -- never trust form data from the client.Use revalidatePath or revalidateTag after mutations to refresh cached data.
Common Pitfalls Pitfall Problem Fix Adding "use client" to a page component Entire page tree becomes client-rendered, losing RSC benefits Keep page as Server Component; extract interactive parts into Client child components Importing useState in a Server Component Build error: hooks only work in Client Components Move stateful logic to a "use client" child component Passing functions as props across boundary Serialization error: functions are not serializable Use Server Actions ("use server") or handle events inside the Client Component Sequential await in Server Components Waterfall: each fetch waits for the previous one Use Promise.all() for independent data or Suspense for parallel streaming Importing server-only modules in Client Components Build error (good) or runtime error (bad without server-only) Add import "server-only" to all files that use process.env, DB, cookies, etc. Huge RSC payload Slow initial load due to large serialized props Only pass necessary fields; avoid serializing entire database rows Not using Suspense boundaries Entire page blocked until slowest query resolves Wrap independent data sections in separate <Suspense> boundaries Mutating data without revalidation UI shows stale data after Server Action Call revalidatePath() or revalidateTag() at the end of every mutation
From server-actions
Next.js Server Actions — "use server", form actions, revalidation, optimistic updates with useOptimistic, progressive enhancement
Server Actions Skill
Purpose Server Actions are Next.js's RPC mechanism: async functions on the server, callable from client components. They replace API routes for mutations, provide progressive enhancement, and integrate with React's concurrent features.
Server Action with Validation
'use server' ;
import { revalidateTag } from 'next/cache' ;
import { redirect } from 'next/navigation' ;
import { z } from 'zod' ;
const createPostSchema = z.object ({
title : z.string ().min (1 , 'Title is required' ).max (200 ),
content : z.string ().min (10 , 'Content must be at least 10 characters' ),
});
export type ActionState = {
success : boolean ;
errors ?: Record <string , string []>;
message ?: string ;
};
export async function createPost (prev : ActionState , formData : FormData Promise <ActionState > {
const parsed = createPostSchema.safeParse (Object .fromEntries (formData));
if (!parsed.success ) return { success : false , errors : parsed.error .flatten ().fieldErrors };
const session = await getSession ();
if (!session?.user ) return { success : false , message : 'Unauthorized' };
try {
await db.post .create ({ data : { ...parsed.data , authorId : session.user .id } });
} catch {
return { success : false , message : 'Failed to create post' };
}
revalidateTag ('posts' );
redirect ('/posts' );
}
Form with useActionState 'use client' ;
import { useActionState } from 'react' ;
import { createPost, type ActionState } from '@/app/actions/posts' ;
export function CreatePostForm (const [state, formAction, isPending] = useActionState (createPost, { success : false });
return (
<form action ={formAction} className ="space-y-4" >
<div >
<label htmlFor ="title" > Title</label >
<input id ="title" name ="title" required className ="w-full rounded-lg border px-4 py-3" />
{state.errors?.title && <p className ="text-sm text-red-600" > {state.errors.title[0]}</p > }
</div >
<div >
<label htmlFor ="content" > Content</label >
<textarea id ="content" name ="content" required className ="w-full rounded-lg border px-4 py-3" />
{state.errors?.content && <p className ="text-sm text-red-600" > {state.errors.content[0]}</p > }
</div >
<button type ="submit" disabled ={isPending}
className ="rounded-lg bg-blue-600 px-6 py-4 text-white transition-all duration-200 hover:bg-blue-700 disabled:opacity-50" >
{isPending ? 'Creating...' : 'Create Post'}
</button >
</form >
Optimistic Updates 'use client' ;
import { useOptimistic, useTransition } from 'react' ;
import { toggleLike } from '@/app/actions/likes' ;
export function LikeButton ({ postId, liked, count }: LikeButtonProps ) {
const [isPending, startTransition] = useTransition ();
const [optimistic, setOptimistic] = useOptimistic (
{ liked, count },
(current, newLiked : boolean ) => ({
liked : newLiked,
count : current.count + (newLiked ? 1 : -1 ),
})
);
return (
<button onClick ={() => startTransition(async () => {
setOptimistic(!optimistic.liked);
await toggleLike(postId);
})} disabled={isPending}>
{optimistic.liked ? 'Unlike' : 'Like'} ({optimistic.count})
</button >
Revalidation Strategies 'use server' ;
import { revalidatePath, revalidateTag } from 'next/cache' ;
revalidatePath ('/posts' );
revalidatePath ('/posts/[slug]' , 'page' );
revalidatePath ('/' , 'layout' );
revalidateTag ('posts' );
revalidateTag (`post-${id} ` );
const posts = await fetch (url, { next : { tags : ['posts' ] } });
Best Practices
Always validate inputs with Zod — never trust client dataCheck auth inside the action , not just in middlewareReturn structured errors instead of throwing — enables field-level UI messagesUse revalidateTag over revalidatePath for precise invalidationDon't call redirect() inside try/catch — redirect throws internallyUse useActionState over manual useState + startTransition for formsKeep actions in separate 'use server' files for clear boundaries
From suspense
React Suspense patterns including Suspense boundaries, React.lazy, loading.tsx in Next.js, use() hook, data fetching with Suspense, and streaming SSR
React Suspense Skill
Purpose Suspense lets you declaratively specify loading states while components wait for async operations (code loading, data fetching). This skill covers Suspense boundaries, lazy loading, Next.js integration, and the use() hook for Suspense-compatible data fetching.
Key Patterns
Suspense Boundary Basics import { Suspense } from "react" ;
export default function Dashboard (return (
<div className ="p-6" >
<h1 className ="text-2xl font-bold" > Dashboard</h1 >
<Suspense fallback ={ <DashboardSkeleton /> }>
<DashboardContent />
</Suspense >
</div > function DashboardSkeleton (return (
<div className ="animate-pulse space-y-4 py-16" >
<div className ="h-8 w-1/3 rounded-lg bg-gray-200 dark:bg-gray-800" />
<div className ="h-32 rounded-xl bg-gray-200 dark:bg-gray-800" />
</div >
React.lazy for Code Splitting import { lazy, Suspense } from "react" ;
const HeavyChart = lazy (() => import ("@/components/heavy-chart" ));
const MarkdownEditor = lazy (() => import ("@/components/markdown-editor" ));
export default function AnalyticsPage (return (
<Suspense fallback ={ <div className ="h-64 animate-pulse rounded-xl bg-gray-100" /> }>
<HeavyChart />
</Suspense > const Settings = lazy (() =>
import ("@/components/settings" ).then ((mod ) => ({ default : mod.Settings }))
);
Next.js loading.tsx (Automatic Suspense)
export default function Loading (return (
<div className ="flex min-h-screen items-center justify-center" >
<div className ="h-8 w-8 animate-spin rounded-full border-4 border-gray-300 border-t-blue-600" />
</div >
Nested Suspense Boundaries
export default function Page (return (
<div className ="space-y-8 py-16" >
<Suspense fallback ={ <HeaderSkeleton /> }>
<UserHeader />
</Suspense >
<div className ="grid grid-cols-2 gap-6" >
<Suspense fallback ={ <CardSkeleton /> }>
<RecentActivity />
</Suspense >
<Suspense fallback ={ <CardSkeleton /> }>
<Stats />
</Suspense >
</div >
</div >
The use() Hook (React 19+)
import { use, Suspense } from "react" ;
async function fetchUser (id : string const res = await fetch (`/api/users/${id} ` );
if (!res.ok ) throw new Error ("Failed to fetch user" );
return res.json () as Promise <{ name : string ; email : string }>;
}
function UserProfile ({ userPromise }: { userPromise: Promise <{ name: string ; email: string }> } ) {
const user = use (userPromise);
return (
<div className ="rounded-xl p-6 shadow-sm" >
<h2 className ="text-xl font-semibold" > {user.name}</h2 >
<p className ="text-gray-600 dark:text-gray-400" > {user.email}</p >
</div > export default function Page ({ params }: { params: { id: string } } ) {
const userPromise = fetchUser (params.id );
return (
<Suspense fallback ={ <ProfileSkeleton /> }>
<UserProfile userPromise ={userPromise} />
</Suspense >
Streaming SSR with Suspense (Next.js App Router)
import { Suspense } from "react" ;
async function SlowData (const data = await fetch ("https://api.example.com/slow" , { cache : "no-store" });
const result = await data.json ();
return <pre > {JSON.stringify(result, null, 2)}</pre > export default function Page (return (
<main >
<h1 > Instant shell</h1 >
<Suspense fallback ={ <p > Loading data...</p > }>
<SlowData /> {/* Streams in when ready */}
</Suspense >
</main >
Best Practices
Pair with Error Boundaries : Every <Suspense> should have a sibling or parent <ErrorBoundary>.Granular boundaries : Wrap independent sections separately so fast content is not blocked by slow.Meaningful skeletons : Match the skeleton shape to the real content to reduce layout shift.Lift promises up : Create promises in parent, pass to child. This starts fetching before rendering.Avoid Suspense waterfalls : Do not nest async components sequentially. Fetch in parallel where possible.loading.tsx per route : In Next.js, each route segment can have its own loading state.
Common Pitfalls Pitfall Fix Entire page shows spinner Use nested Suspense boundaries for independent sections Waterfall fetches Start all fetches at the same level, pass promises down Missing error handling Add ErrorBoundary around Suspense boundaries Layout shift on load Use skeleton components that match final content dimensions Lazy component flickers Only lazy-load components that are large or conditionally rendered
From error-boundary
React Error Boundaries including class-based ErrorBoundary, Next.js App Router error.tsx, fallback UI design, error recovery, retry patterns, and error reporting integration
React Error Boundaries Skill
Purpose Prevent a single component crash from taking down the entire application. Error boundaries catch rendering errors, display fallback UI, enable recovery, and report errors to monitoring services.
Error Boundary Types Type Scope File Framework Class ErrorBoundary Any component tree Custom component React 16+ error.tsx Route segment app/[route]/error.tsxNext.js App Router global-error.tsx Entire app (root layout) app/global-error.tsxNext.js App Router Suspense + ErrorBoundary Async boundaries Composition React 18+
Key Patterns
1. Reusable Class-Based ErrorBoundary 'use client' ;
import { Component , type ErrorInfo , type ReactNode } from 'react' ;
interface ErrorBoundaryProps {
children : ReactNode ;
fallback ?: ReactNode | ((error : Error , reset : () => void ReactNode );
onError ?: (error : Error , errorInfo : ErrorInfo void ;
}
interface ErrorBoundaryState {
hasError : boolean ;
error : Error | null ;
}
export class ErrorBoundary extends Component <ErrorBoundaryProps , ErrorBoundaryState > {
constructor (props : ErrorBoundaryProps super (props);
this .state = { hasError : false , error : null };
}
static getDerivedStateFromError (error : Error ): ErrorBoundaryState {
return { hasError : true , error };
}
componentDidCatch (error : Error , errorInfo : ErrorInfo this .props .onError ?.(error, errorInfo);
}
reset = () => {
this .setState ({ hasError : false , error : null });
};
render (if (this .state .hasError && this .state .error ) {
if (typeof this .props .fallback === 'function' ) {
return this .props .fallback (this .state .error , this .reset );
}
return this .props .fallback ?? <DefaultFallback error ={this.state.error} reset ={this.reset} /> return this .props .children ;
}
}
function DefaultFallback ({ error, reset }: { error: Error ; reset: () => void } ) {
return (
<div role ="alert" className ="p-6 rounded-xl border border-red-200 bg-red-50" >
<h2 className ="text-lg font-semibold text-red-800" > Something went wrong</h2 >
<p className ="mt-2 text-red-600" > {error.message}</p >
<button
onClick ={reset}
className ="mt-4 px-6 py-4 text-base rounded-lg bg-red-600 text-white
hover:bg-red-700 transition-all duration-200
focus-visible:ring-2 focus-visible:ring-offset-2 focus-visible:ring-red-500"
>
Try again
</button >
</div >
2. Next.js App Router error.tsx
'use client' ;
export default function DashboardError ({
error,
reset,
}: {
error: Error & { digest?: string };
reset: () => void ;
} ) {
useEffect (() => {
reportError (error);
}, [error]);
return (
<div role ="alert" className ="p-8 rounded-2xl border shadow-sm max-w-lg mx-auto mt-16" >
<h2 className ="text-xl font-bold" > Dashboard Error</h2 >
<p className ="mt-2 text-gray-600" >
{error.digest
? 'An unexpected error occurred. Our team has been notified.'
: error.message}
</p >
<button
onClick ={reset}
className ="mt-6 px-6 py-4 text-base rounded-lg bg-blue-600 text-white
hover:bg-blue-700 transition-all duration-200
focus-visible:ring-2 focus-visible:ring-offset-2 focus-visible:ring-blue-500"
>
Try again
</button >
</div >
3. Global Error Handler (Root Layout Crash)
'use client' ;
export default function GlobalError ({
error,
reset,
}: {
error: Error & { digest?: string };
reset: () => void ;
} ) {
return (
<html >
<body >
<div className ="min-h-screen flex items-center justify-center p-8" >
<div className ="p-8 rounded-2xl border shadow-sm max-w-md text-center" >
<h1 className ="text-2xl font-bold" > Something went wrong</h1 >
<p className ="mt-4 text-gray-600" >
An unexpected error occurred. Please try refreshing the page.
</p >
<button
onClick ={reset}
className ="mt-6 px-6 py-4 text-base rounded-lg bg-blue-600 text-white
hover:bg-blue-700 transition-all duration-200
focus-visible:ring-2 focus-visible:ring-offset-2 focus-visible:ring-blue-500"
>
Try again
</button >
</div >
</div >
</body >
</html >
4. Retry with Exponential Backoff 'use client' ;
import { useCallback, useState } from 'react' ;
function useRetryableQuery<T>(fetcher : () => Promise <T>, maxRetries = 3 ) {
const [data, setData] = useState<T | null >(null );
const [error, setError] = useState<Error | null >(null );
const [retryCount, setRetryCount] = useState (0 );
const [isLoading, setIsLoading] = useState (false );
const execute = useCallback (async () => {
setIsLoading (true );
setError (null );
for (let attempt = 0 ; attempt <= maxRetries; attempt++) {
try {
const result = await fetcher ();
setData (result);
setRetryCount (0 );
setIsLoading (false );
return result;
} catch (err) {
if (attempt === maxRetries) {
setError (err instanceof Error ? err : new Error (String (err)));
setRetryCount (attempt);
setIsLoading (false );
throw err;
}
await new Promise ((r ) => setTimeout (r, 1000 * Math .pow (2 , attempt)));
}
}
}, [fetcher, maxRetries]);
return { data, error, isLoading, retryCount, execute };
}
5. Granular Boundaries with Suspense import { Suspense } from 'react' ;
import { ErrorBoundary } from './ErrorBoundary' ;
export default function Dashboard (return (
<div className ="grid grid-cols-2 gap-6 py-16" >
{/* Each widget isolated -- one crash does not affect others */}
<ErrorBoundary fallback ={(err, reset ) => <WidgetError error ={err} reset ={reset} /> }>
<Suspense fallback ={ <WidgetSkeleton /> }>
<RevenueChart />
</Suspense >
</ErrorBoundary >
<ErrorBoundary fallback ={(err, reset ) => <WidgetError error ={err} reset ={reset} /> }>
<Suspense fallback ={ <WidgetSkeleton /> }>
<UserStats />
</Suspense >
</ErrorBoundary >
<ErrorBoundary fallback ={(err, reset ) => <WidgetError error ={err} reset ={reset} /> }>
<Suspense fallback ={ <WidgetSkeleton /> }>
<RecentOrders />
</Suspense >
</ErrorBoundary >
</div >
Error Reporting Integration
export function reportError (error : Error , context ?: Record <string , unknown >if (typeof window !== 'undefined' && window .Sentry ) {
window .Sentry .captureException (error, { extra : context });
}
fetch ('/api/errors' , {
method : 'POST' ,
headers : { 'Content-Type' : 'application/json' },
body : JSON .stringify ({
message : error.message ,
stack : error.stack ,
context,
timestamp : new Date ().toISOString (),
url : typeof window !== 'undefined' ? window .location .href : undefined ,
}),
}).catch (() => {});
}
Best Practices
Wrap every independent UI section in its own ErrorBoundary -- sidebar, main content, widgetsAlways provide a reset/retry action -- do not show dead-end error screensUse error.digest in Next.js to distinguish server errors (hide details) from client errorsReport errors to monitoring in componentDidCatch or useEffect in error.tsxCombine ErrorBoundary + Suspense for async components -- catch both loading and error statesShow contextual fallbacks -- a chart error should show a chart-sized placeholder, not a full-page errorDo not catch errors in event handlers with ErrorBoundary -- use try/catch instead
Common Pitfalls Pitfall Impact Fix No ErrorBoundary at all One bad component crashes the entire app Add boundaries at route and widget level Only a global boundary Entire page replaced by error UI for a small widget crash Use granular boundaries per section ErrorBoundary around event handlers Errors in onClick/onSubmit are not caught Use try/catch inside event handlers Forgetting 'use client' on error.tsx Next.js build error error.tsx must be a Client Component No error reporting Errors happen silently in production Integrate Sentry or custom error endpoint
From parallel-routes
Next.js parallel routes — @slot conventions, simultaneous rendering, independent loading/error states, conditional slots, and modal patterns
Skill: parallel-routes
Layer: domain | Category: frontend
Triggers
parallel routes, @slot, simultaneous rendering, dashboard layout, split views, Next.js slots
Links
linksTo: intercepting-routes, nextjs-app-router
linkedFrom: nextjs-app-router
Overview Parallel routes render multiple pages simultaneously within the same layout using named slots .
Slots are defined by the @folder convention and passed as props to the parent layout.
Slot Convention A slot is a directory prefixed with @. It does NOT create a URL segment.
app/
layout.tsx # receives @analytics and @team as props
page.tsx
@analytics/
page.tsx
loading.tsx # independent loading state
error.tsx # independent error boundary
@team/
page.tsx
loading.tsx
export default function Layout ({
children,
analytics,
team,
}: {
children: React.ReactNode;
analytics: React.ReactNode;
team: React.ReactNode;
} ) {
return (
<div className ="grid grid-cols-2 gap-6 py-16" >
<main > {children}</main >
<aside > {analytics}</aside >
<section > {team}</section >
</div >
default.tsx — Unmatched Route Fallback When a slot has no matching route for the current URL, Next.js renders default.tsx.
Without it, unmatched slots return a 404 on hard navigation.
export default function AnalyticsDefault (return <p > Select a date range to view analytics.</p > Rule : every parallel slot should have a default.tsx unless you explicitly want a 404.
Independent Loading and Error States Each slot streams independently — one slot can show a skeleton while another is ready.
export default function AnalyticsLoading (return <div className ="animate-pulse h-64 rounded-xl bg-muted" /> "use client" ;
export default function AnalyticsError ({ reset }: { reset: () => void } ) {
return (
<div className ="p-6 rounded-xl border border-destructive" >
<p > Analytics failed to load.</p >
<button onClick ={reset} className ="px-6 py-4 text-base rounded-lg" >
Retry
</button >
</div >
Conditional Rendering Use auth or feature flags to swap which slot content renders.
import { auth } from "@/lib/auth" ;
export default async function Layout ({
children,
admin,
user,
}: {
children: React.ReactNode;
admin: React.ReactNode;
user: React.ReactNode;
} ) {
const session = await auth ();
return (
<div className ="py-16" >
{children}
{session?.role === "admin" ? admin : user}
</div >
Key Rules
Slots are not URL segments — @analytics/page.tsx maps to /, not /analytics.
Soft navigation preserves previously active slot state even if the URL doesn't match.
Hard navigation (full reload) requires default.tsx for unmatched slots.
Combine with intercepting routes for modal patterns that overlay parallel slots.
Each slot can define its own loading.tsx, error.tsx, not-found.tsx, and nested layouts.
From intercepting-routes
Next.js intercepting routes — (.) (..) (...) conventions, modal overlays, photo galleries, share-able modals, route interception patterns
Skill: intercepting-routes
Layer: domain | Category: frontend
Triggers
intercepting routes, modal route, photo gallery modal, (.) (..) (...), route interception, shareable modal URL
Links
linksTo: parallel-routes, nextjs-app-router
linkedFrom: parallel-routes, nextjs-app-router
Overview Intercepting routes let you load a route from another part of your app within the current layout.
The canonical use case: clicking a photo in a feed opens a modal, but navigating directly to
/photo/123 renders the full page. The URL is shareable either way.
Convention Syntax Pattern Matches Analogy (.)Same level ./(..)One level up ../(..)(..)Two levels up ../../(...)App root /
Important : levels are based on route segments, not filesystem directories.
File Structure — Modal Pattern app/
layout.tsx
@modal/
(.)photo/[id]/
page.tsx # intercepted: renders inside modal
default.tsx # renders nothing when no modal is active
feed/
page.tsx # photo grid
photo/[id]/
page.tsx # full page: direct navigation or refresh
Combining with Parallel Routes The @modal slot is a parallel route that intercepts /photo/[id].
export default function RootLayout ({
children,
modal,
}: {
children: React.ReactNode;
modal: React.ReactNode;
} ) {
return (
<html lang ="en" >
<body >
{children}
{modal}
</body >
</html >
export default function Default (return null ;
}
Intercepted Route (Modal View)
import { Modal } from "@/components/modal" ;
import { getPhoto } from "@/lib/data" ;
export default async function PhotoModal ({
params,
}: {
params: Promise <{ id: string }>;
} ) {
const { id } = await params;
const photo = await getPhoto (id);
return (
<Modal >
<img
src ={photo.url}
alt ={photo.alt}
className ="rounded-xl max-h-[80vh] object-contain"
/>
<p className ="p-6 text-base" > {photo.description}</p >
</Modal >
Full Page Route (Direct Navigation)
import { getPhoto } from "@/lib/data" ;
export default async function PhotoPage ({
params,
}: {
params: Promise <{ id: string }>;
} ) {
const { id } = await params;
const photo = await getPhoto (id);
return (
<main className ="py-16 max-w-4xl mx-auto" >
<img src ={photo.url} alt ={photo.alt} className ="rounded-xl w-full" />
<h1 className ="text-2xl font-bold mt-6" > {photo.title}</h1 >
<p className ="text-base mt-4" > {photo.description}</p >
</main >
Reusable Modal Component "use client" ;
import { useRouter } from "next/navigation" ;
export function Modal ({ children }: { children: React.ReactNode } ) {
const router = useRouter ();
return (
<div
className ="fixed inset-0 z-50 flex items-center justify-center bg-black/60"
onClick ={() => router.back()}
>
<div
className ="p-8 rounded-2xl bg-white shadow-xl max-w-2xl w-full"
onClick ={(e) => e.stopPropagation()}
>
{children}
</div >
</div >
Key Rules
Interception only works on soft navigation (client-side <Link>). Hard refresh loads the actual route.
Always pair with a parallel @slot so the modal overlays without replacing page content.
Provide default.tsx in the slot to avoid 404 on unmatched states.
The (..) depth counts route segments, not filesystem folders — @modal is not a segment.
Use router.back() to dismiss the modal and restore the previous URL.
From prefetch
Data prefetching patterns — Next.js Link prefetch, route prefetching, preloading data, React Query prefetch, stale-while-revalidate
Skill: prefetch
Layer: domain | Category: frontend
Triggers
prefetch, preload data, prefetchQuery, router.prefetch, hover preload, stale-while-revalidate, link prefetch
Links
linksTo: react-query, nextjs-app-router, server-components
linkedFrom: react-query, nextjs-app-router
Overview Data prefetching loads resources before the user needs them, eliminating perceived latency.
Strategies range from automatic link prefetching to manual hover-triggered data preloading.
Next.js Link Prefetch Next.js automatically prefetches <Link> routes when they enter the viewport.
import Link from "next/link" ;
<Link href ="/dashboard" > Dashboard</Link > <Link href ="/settings" prefetch ={false} > Settings</Link > <Link href ="/dashboard" prefetch ={true} > Dashboard</Link > prefetch valueBehavior undefinedPrefetches up to the nearest loading.tsx boundary truePrefetches the full page data falseNo prefetching — fetches on click
router.prefetch() Programmatically prefetch a route before navigation.
"use client" ;
import { useRouter } from "next/navigation" ;
export function ProjectCard ({ id }: { id: string } ) {
const router = useRouter ();
return (
<div
className ="p-6 rounded-xl shadow-sm border transition-all duration-200 hover:shadow-md"
onMouseEnter ={() => router.prefetch(`/project/${id}`)}
onClick={() => router.push(`/project/${id}`)}
>
<h3 className ="text-base font-semibold" > Project {id}</h3 >
</div >
React Query — prefetchQuery Prefetch data into the query cache so it is instantly available when the component mounts.
import { useQueryClient } from "@tanstack/react-query" ;
import { getProject } from "@/lib/api" ;
export function ProjectLink ({ id }: { id: string } ) {
const queryClient = useQueryClient ();
const prefetch = (prefetchQuery ({
queryKey : ["project" , id],
queryFn : () => getProject (id),
staleTime : 60_000 ,
});
};
return (
<a
href ={ `/project /${id }`}
onMouseEnter ={prefetch}
onFocus ={prefetch}
className ="px-6 py-4 text-base rounded-lg transition-all duration-200
hover:bg-muted focus-visible:ring-2 focus-visible:ring-offset-2"
>
View Project
</a >
Server Component Preloading Pattern Preload functions let you start fetching data at the top of a server component tree.
import { cache } from "react" ;
export const getUser = cache (async (id : string ) => {
const res = await fetch (`https://api.example.com/users/${id} ` );
return res.json ();
});
export const preloadUser = (id : string void getUser (id);
};
import { preloadUser, getUser } from "@/lib/data" ;
export default async function UserPage ({
params,
}: {
params: Promise <{ id: string }>;
} ) {
const { id } = await params;
preloadUser (id);
const user = await getUser (id);
return <h1 className ="text-2xl font-bold py-16" > {user.name}</h1 >
Stale-While-Revalidate Show cached data immediately while revalidating in the background.
const { data } = useQuery ({
queryKey : ["projects" ],
queryFn : fetchProjects,
staleTime : 30_000 ,
gcTime : 5 * 60_000 ,
refetchOnWindowFocus : true ,
});
Key Rules
Prefetch on hover and focus — not just hover — for keyboard accessibility.
Set staleTime when prefetching with React Query to avoid immediate refetch on mount.
Use React.cache() for server component data deduplication across the tree.
Avoid prefetching large payloads unconditionally — use viewport or hover triggers.
router.prefetch() caches the RSC payload; combine with data prefetching for full coverage.In production, Next.js <Link> prefetches automatically — disable for auth-gated routes.
From edge-functions
Edge computing — Vercel Edge Functions, Cloudflare Workers, edge middleware, geo-routing, edge config, runtime limitations
Edge Functions Skill
Purpose Edge functions run at CDN nodes closest to the user with sub-50ms cold starts. Use for auth checks, redirects, A/B testing, geo-routing, and header manipulation.
Edge vs Node.js Runtime Feature Edge Runtime Node.js Runtime Cold start < 50ms 250ms+ Node.js APIs Subset (no fs, net) Full Database HTTP clients only TCP + HTTP Max size 1-4 MB 50 MB+ npm packages Web-compatible only All
Vercel Edge API Route
export const runtime = 'edge' ;
export async function GET (request : Request const country = request.headers .get ('x-vercel-ip-country' ) ?? 'US' ;
const city = request.headers .get ('x-vercel-ip-city' ) ?? 'Unknown' ;
return Response .json ({ country, city, timestamp : Date .now () });
}
Edge Middleware
import { NextResponse } from 'next/server' ;
import type { NextRequest } from 'next/server' ;
export function middleware (request : NextRequest const { pathname } = request.nextUrl ;
const country = request.geo ?.country ?? 'US' ;
if (pathname === '/' && country === 'DE' ) {
return NextResponse .redirect (new URL ('/de' , request.url ));
}
if (!request.cookies .get ('ab-bucket' ) && pathname === '/pricing' ) {
const bucket = Math .random () < 0.5 ? 'control' : 'variant' ;
const res = NextResponse .rewrite (new URL (`/pricing/${bucket} ` , request.url ));
res.cookies .set ('ab-bucket' , bucket, { maxAge : 60 * 60 * 24 * 30 });
return res;
}
const response = NextResponse .next ();
response.headers .set ('X-Frame-Options' , 'DENY' );
response.headers .set ('X-Content-Type-Options' , 'nosniff' );
return response;
}
export const config = {
matcher : ['/((?!_next/static|_next/image|favicon.ico).*)' ],
};
Vercel Edge Config import { get } from '@vercel/edge-config' ;
export const runtime = 'edge' ;
export async function GET (const maintenance = await get<boolean >('maintenance_mode' );
const flags = await get<Record <string , boolean >>('feature_flags' );
if (maintenance) return new Response ('Under maintenance' , { status : 503 });
return Response .json ({ flags });
}
Edge-Compatible Database Access
import { neon } from '@neondatabase/serverless' ;
export const runtime = 'edge' ;
export async function GET (const sql = neon (process.env .DATABASE_URL !);
const posts = await sql`SELECT * FROM posts WHERE published = true LIMIT 20` ;
return Response .json (posts);
}
Cloudflare Workers export default {
async fetch (request : Request , env : Env ): Promise <Response > {
const country = (request as any ).cf ?.country ?? 'US' ;
const cached = await env.MY_KV .get ('config' , 'json' );
const result = await env.DB .prepare ('SELECT * FROM posts LIMIT 10' ).all ();
return Response .json ({ country, posts : result.results , config : cached });
},
};
Best Practices
Keep bundles small — 1-4 MB limits are strictUse HTTP database clients (Neon serverless, PlanetScale) not TCPUse Edge Config for feature flags (sub-ms reads)Scope middleware matchers to exclude _next/staticTest locally with vercel dev or wrangler devDon't assume global state — edge functions are stateless between requests
From vercel
Vercel deployment, Edge Config, KV, Cron Jobs, Middleware, and platform-specific optimization
Vercel Specialist
Purpose Optimize deployments, edge functions, middleware, cron jobs, and platform features on Vercel. This skill covers vercel.json configuration, Edge Config, Vercel KV, middleware patterns, preview deployments, and CI integration.
Key Patterns
vercel.json Configuration {
"$schema" : "https://openapi.vercel.sh/vercel.json" ,
"framework" : "nextjs" ,
"regions" : [ "iad1" , "sfo1" ] ,
"crons" : [
{
"path" : "/api/cron/cleanup" ,
"schedule" : "0 */6 * * *"
} ,
{
"path" : "/api/cron/sync" ,
"schedule" : "*/15 * * * *"
}
] ,
"headers" : [
{
"source" : "/api/(.*)" ,
"headers" : [
{ "key" : "Cache-Control" , "value" : "no-store" }
]
} ,
{
"source" : "/(.*)\\.(.*)$" ,
"headers" : [
{ "key" : "Cache-Control" , "value" : "public, max-age=31536000, immutable" }
]
}
] ,
"rewrites" : [
{ "source" : "/blog/:slug" , "destination" : "/api/blog/:slug" }
] ,
"redirects" : [
{ "source" : "/old-page" , "destination" : "/new-page" , "permanent" : true }
]
}
Middleware (Edge Runtime)
import { NextRequest , NextResponse } from "next/server" ;
export const config = {
matcher : [
"/((?!_next/static|_next/image|favicon.ico|public/).*)" ,
],
};
export function middleware (request : NextRequest const { pathname } = request.nextUrl ;
const country = request.geo ?.country || "US" ;
const bucket = request.cookies .get ("ab-bucket" )?.value ;
if (!bucket) {
const newBucket = Math .random () > 0.5 ? "a" : "b" ;
const response = NextResponse .next ();
response.cookies .set ("ab-bucket" , newBucket, {
httpOnly : true ,
maxAge : 60 * 60 * 24 * 30 ,
});
return response;
}
const ip = request.headers .get ("x-forwarded-for" ) || "unknown" ;
const ua = request.headers .get ("user-agent" ) || "" ;
if (isSuspiciousBot (ua)) {
return new NextResponse ("Forbidden" , { status : 403 });
}
const response = NextResponse .next ();
response.headers .set ("x-country" , country);
return response;
}
function isSuspiciousBot (ua : string boolean {
const suspiciousPatterns = [/curl/i , /python-requests/i , /scrapy/i ];
return suspiciousPatterns.some ((p ) => p.test (ua));
}
Vercel KV (Redis) Usage import { kv } from "@vercel/kv" ;
async function checkRateLimit (ip : string , limit : number , windowSec : number Promise <boolean > {
const key = `rate:${ip} ` ;
const count = await kv.incr (key);
if (count === 1 ) {
await kv.expire (key, windowSec);
}
return count <= limit;
}
async function getCachedData<T>(key : string , fetcher : () => Promise <T>, ttl : number ): Promise <T> {
const cached = await kv.get <T>(key);
if (cached) return cached;
const fresh = await fetcher ();
await kv.set (key, fresh, { ex : ttl });
return fresh;
}
async function setSession (sessionId : string , data : object await kv.set (`session:${sessionId} ` , data, { ex : 86400 });
}
Edge Config (Feature Flags) import { get } from "@vercel/edge-config" ;
export async function isFeatureEnabled (feature : string Promise <boolean > {
const flags = await get<Record <string , boolean >>("featureFlags" );
return flags?.[feature] ?? false ;
}
export async function middleware (request : NextRequest const maintenance = await get<boolean >("maintenance" );
if (maintenance) {
return NextResponse .rewrite (new URL ("/maintenance" , request.url ));
}
return NextResponse .next ();
}
Cron Job Handler
import { NextRequest , NextResponse } from "next/server" ;
export async function GET (request : NextRequest const authHeader = request.headers .get ("authorization" );
if (authHeader !== `Bearer ${process.env.CRON_SECRET} ` ) {
return NextResponse .json ({ error : "Unauthorized" }, { status : 401 });
}
try {
const deleted = await cleanupExpiredSessions ();
return NextResponse .json ({ success : true , deleted });
} catch (error) {
console .error ("Cron cleanup failed:" , error);
return NextResponse .json ({ error : "Cleanup failed" }, { status : 500 });
}
}
Best Practices
Deployment
Use preview deployments for every PR
Set up GitHub integration for automatic deploys
Use VERCEL_ENV to distinguish preview/production
Configure environment variables per environment (preview, production)
Use vercel --prod only from CI, not locally
Performance
Place compute in regions close to your database (set regions in vercel.json)
Use Edge Runtime for latency-sensitive endpoints
Use Edge Config for feature flags and config (reads in ~1ms)
Use ISR (Incremental Static Regeneration) for semi-static pages
Set appropriate Cache-Control headers for static assets
Middleware
Keep middleware lightweight (it runs on every matching request)
Use config.matcher to limit which paths trigger middleware
Avoid database calls in middleware; use Edge Config or KV instead
Return early with NextResponse.next() for passthrough
Cron Jobs
Always verify the CRON_SECRET authorization header
Keep cron handlers idempotent
Log outcomes for debugging
Use the Vercel dashboard to monitor cron execution history
Common Pitfalls Pitfall Fix Middleware running on all paths Use config.matcher to exclude static assets Slow middleware with DB calls Use Edge Config or Vercel KV instead Missing CRON_SECRET verification Always check auth header in cron endpoints Environment variable mismatch Set vars per environment in Vercel dashboard Large serverless function bundles Use dynamic imports, check with @vercel/nft Region mismatch with database Set regions in vercel.json near your DB
Examples
OG Image Generation (Edge)
import { ImageResponse } from "next/og" ;
export const runtime = "edge" ;
export async function GET (request : Request const { searchParams } = new URL (request.url );
const title = searchParams.get ("title" ) || "Default Title" ;
return new ImageResponse (
(
<div style ={{ display: "flex ", fontSize: 48 , background: "#000 ", color: "#fff ", width: "100 %", height: "100 %", alignItems: "center ", justifyContent: "center " }}>
{title}
</div > width : 1200 , height : 630 }
);
}
Vercel CLI Deployment Script #!/bin/bash
vercel
vercel --prod
vercel env pull .env.local
vercel link
From vercel-react-best-practices
React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React/Next.js code to ensure optimal performance patterns. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.
Vercel React Best Practices Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.
When to Apply Reference these guidelines when:
Writing new React components or Next.js pages
Implementing data fetching (client or server-side)
Reviewing code for performance issues
Refactoring existing React/Next.js code
Optimizing bundle size or load times
Rule Categories by Priority Priority Category Impact Prefix 1 Eliminating Waterfalls CRITICAL async-2 Bundle Size Optimization CRITICAL bundle-3 Server-Side Performance HIGH server-4 Client-Side Data Fetching MEDIUM-HIGH client-5 Re-render Optimization MEDIUM rerender-6 Rendering Performance MEDIUM rendering-7 JavaScript Performance LOW-MEDIUM js-8 Advanced Patterns LOW advanced-
Quick Reference
1. Eliminating Waterfalls (CRITICAL)
async-defer-await - Move await into branches where actually usedasync-parallel - Use Promise.all() for independent operationsasync-dependencies - Use better-all for partial dependenciesasync-api-routes - Start promises early, await late in API routesasync-suspense-boundaries - Use Suspense to stream content
2. Bundle Size Optimization (CRITICAL)
bundle-barrel-imports - Import directly, avoid barrel filesbundle-dynamic-imports - Use next/dynamic for heavy componentsbundle-defer-third-party - Load analytics/logging after hydrationbundle-conditional - Load modules only when feature is activatedbundle-preload - Preload on hover/focus for perceived speed
3. Server-Side Performance (HIGH)
server-cache-react - Use React.cache() for per-request deduplicationserver-cache-lru - Use LRU cache for cross-request cachingserver-serialization - Minimize data passed to client componentsserver-parallel-fetching - Restructure components to parallelize fetchesserver-after-nonblocking - Use after() for non-blocking operations
4. Client-Side Data Fetching (MEDIUM-HIGH)
client-swr-dedup - Use SWR for automatic request deduplicationclient-event-listeners - Deduplicate global event listeners
5. Re-render Optimization (MEDIUM)
rerender-defer-reads - Don't subscribe to state only used in callbacksrerender-memo - Extract expensive work into memoized componentsrerender-dependencies - Use primitive dependencies in effectsrerender-derived-state - Subscribe to derived booleans, not raw valuesrerender-functional-setstate - Use functional setState for stable callbacksrerender-lazy-state-init - Pass function to useState for expensive valuesrerender-transitions - Use startTransition for non-urgent updates
6. Rendering Performance (MEDIUM)
rendering-animate-svg-wrapper - Animate div wrapper, not SVG elementrendering-content-visibility - Use content-visibility for long listsrendering-hoist-jsx - Extract static JSX outside componentsrendering-svg-precision - Reduce SVG coordinate precisionrendering-hydration-no-flicker - Use inline script for client-only datarendering-activity - Use Activity component for show/hiderendering-conditional-render - Use ternary, not && for conditionals
7. JavaScript Performance (LOW-MEDIUM)
js-batch-dom-css - Group CSS changes via classes or cssTextjs-index-maps - Build Map for repeated lookupsjs-cache-property-access - Cache object properties in loopsjs-cache-function-results - Cache function results in module-level Mapjs-cache-storage - Cache localStorage/sessionStorage readsjs-combine-iterations - Combine multiple filter/map into one loopjs-length-check-first - Check array length before expensive comparisonjs-early-exit - Return early from functionsjs-hoist-regexp - Hoist RegExp creation outside loopsjs-min-max-loop - Use loop for min/max instead of sortjs-set-map-lookups - Use Set/Map for O(1) lookupsjs-tosorted-immutable - Use toSorted() for immutability
8. Advanced Patterns (LOW)
advanced-event-handler-refs - Store event handlers in refsadvanced-use-latest - useLatest for stable callback refs
How to Use Read individual rule files for detailed explanations and code examples:
rules/async-parallel.md
rules/bundle-barrel-imports.md
rules/_sections.md
Brief explanation of why it matters
Incorrect code example with explanation
Correct code example with explanation
Additional context and references
Full Compiled Document For the complete guide with all rules expanded: AGENTS.md
