Nextjs App Router Expert

Next.js App Router Expert

Overview

Expert in Next.js 14/15 App Router architecture, React Server Components (RSC), Server Actions, and modern full-stack React development. Specializes in routing patterns, data fetching strategies, caching, streaming, and deployment optimization.

Decision Points

Route Pattern Selection

Is the UI requirement:
├── Simple nested layouts?
│   └── Use standard file-based routing (page.tsx, layout.tsx)
│       └── Check: Does each segment need its own loading/error states?
│           ├── Yes → Add loading.tsx/error.tsx per segment
│           └── No → Use parent boundaries only
├── Content that appears alongside main page?
│   └── Use parallel routes (@slot)
│       └── Check: Should content persist across navigation?
│           ├── Yes → Use default.tsx to maintain state
│           └── No → Let it unmount naturally
├── Modal/overlay that intercepts navigation?
│   └── Use intercepting routes (.)
│       └── Check: What's the fallback for direct access?
│           ├── Same content → Create both routes with shared component
│           └── Different UX → Create separate page implementations
└── Dynamic segments with complex patterns?
    └── Use catch-all routes ([...slug])
        └── Check: Are some segments optional?
            ├── Yes → Use [[...optional]] syntax
            └── No → Use [...required] syntax

Data Fetching Strategy

What's the data freshness requirement:
├── Static at build time?
│   └── Use generateStaticParams() + fetch with force-cache
├── Fresh on every request?
│   └── Use fetch with no-store or dynamic functions
├── Cached with periodic updates?
│   └── Use fetch with revalidate: seconds
└── User-specific but cacheable?
    └── Use fetch with cache + cookies() to make dynamic

Client/Server Component Boundary

Does the component need:
├── Browser APIs (localStorage, window)?
│   └── Use 'use client' at component level
├── Event handlers (onClick, onSubmit)?
│   └── Use 'use client' at component level
├── React hooks (useState, useEffect)?
│   └── Use 'use client' at component level
└── Only data fetching and rendering?
    └── Keep as Server Component
        └── Check: Do children need client features?
            ├── Yes → Pass server data as props to client children
            └── No → Keep entire tree as server components

Failure Modes

Hydration Mismatch Hell

Symptoms: Console errors "Text content did not match", layout shifts on page load Diagnosis: Server-rendered HTML differs from client-rendered HTML Fix:

• Use suppressHydrationWarning for unavoidable mismatches (dates, random IDs)
• Move dynamic content to useEffect with loading states
• Use consistent data sources between server and client

Client Boundary Creep

Symptoms: Large bundle sizes, slow initial page loads, excessive JavaScript Diagnosis: 'use client' placed too high in component tree, pulling server logic to client Fix:

• Move 'use client' to leaf components only
• Extract interactive parts into separate client components
• Pass server-computed data as props to client components

Cache Invalidation Chaos

Symptoms: Stale data after mutations, users see outdated content Diagnosis: Missing revalidatePath/revalidateTag after Server Actions Fix:

• Add revalidatePath('/specific/path') after mutations
• Use tags with revalidateTag('posts') for related content
• Set appropriate cache headers for external APIs

Waterfall Loading Paralysis

Symptoms: Slow page loads, sequential loading indicators, poor Core Web Vitals Diagnosis: Sequential data fetching instead of parallel, no Suspense boundaries Fix:

• Fetch data in parallel at component level, not in parent
• Wrap independent components in separate Suspense boundaries
• Use Promise.all() for related data that can load together

Route Organization Nightmare

Symptoms: Confusing URLs, components in wrong places, hard to navigate codebase Diagnosis: Mixing logical grouping with URL structure, missing route groups Fix:

• Use (auth), (dashboard) route groups for organization without URL impact
• Colocate related components in route directories
• Use intercepting routes for modals, not separate modal directories

Worked Examples

E-commerce Product Page with Reviews

// Decision: Product data is semi-static, reviews are dynamic
// Solution: ISR for product, streaming for reviews

// app/products/[id]/page.tsx
import { Suspense } from 'react';

// Static product data with 1-hour revalidation
async function getProduct(id: string) {
  const res = await fetch(`https://api.shop.com/products/${id}`, {
    next: { revalidate: 3600, tags: ['product'] }
  });
  if (!res.ok) throw new Error('Product not found');
  return res.json();
}

// Dynamic reviews, always fresh
async function getReviews(id: string) {
  const res = await fetch(`https://api.shop.com/products/${id}/reviews`, {
    cache: 'no-store'
  });
  return res.json();
}

export default async function ProductPage({ params }: { params: { id: string } }) {
  // Expert catches: Fetch product immediately, stream reviews
  // Novice misses: Would wait for all data before rendering
  
  const product = await getProduct(params.id);

  return (
    <main>
      <ProductDetails product={product} />
      <AddToCartForm productId={params.id} />
      
      {/* Stream reviews while showing product immediately */}
      <Suspense fallback={<ReviewsSkeleton />}>
        <ReviewsSection productId={params.id} />
      </Suspense>
    </main>
  );
}

async function ReviewsSection({ productId }: { productId: string }) {
  const reviews = await getReviews(productId);
  return <Reviews data={reviews} />;
}

// Server Action for cart - expert includes optimistic update
async function addToCart(formData: FormData) {
  'use server';
  
  const productId = formData.get('productId') as string;
  // ... save to database
  
  revalidateTag('cart'); // Invalidate cart count
  redirect('/cart');
}

Expert Decision Points Navigated:

• Product data: Semi-static → ISR with 1-hour cache
• Reviews: Always fresh → no-store cache
• UX: Show product immediately → Suspense boundary around reviews
• Cart action: Needs cache invalidation → revalidateTag for cart count

Quality Gates

• All async components are wrapped in Suspense or have loading.tsx
• Server Actions include revalidatePath/revalidateTag after mutations
• Client components use 'use client' at the lowest possible level
• Dynamic routes have proper error handling (try/catch, error.tsx)
• Metadata is generated for all public pages (SEO compliance)
• No hydration mismatches in browser console
• Bundle size stays under performance budget (check with @next/bundle-analyzer)
• All external data fetches have appropriate cache headers
• Form submissions work without JavaScript (progressive enhancement)
• Route groups are used for organization without affecting URLs

NOT-FOR Boundaries

Don't use this skill for:

• Simple React SPA without SSR needs → Use react-spa-development instead
• Next.js Pages Router projects → Use nextjs-pages-router instead
• Complex state management across routes → Use zustand-state-management instead
• Real-time features requiring WebSockets → Use websocket-integration instead
• Complex authentication flows → Use nextauth-integration instead
• Database schema design → Use postgresql-optimization instead
• Advanced deployment configurations → Use vercel-deployment or docker-deployment instead

Delegate to other skills when:

• Performance optimization beyond Next.js → react-performance-optimizer
• Complex form validation → react-hook-form-expert
• Advanced TypeScript patterns → typescript-advanced-patterns