Next.js App Router 2026

Version (2026)

• Next.js: 15.x (stable) / 16.x (latest with Cache Components, PPR)
• React: 19.x — Server Components stable
• Tailwind: v4
• TypeScript: required

npx create-next-app@latest dashboard --typescript --tailwind --app --src-dir

App Router — Core Mental Model

app/
├── layout.tsx          # Root layout (required — wraps all pages)
├── page.tsx            # Home page
├── (auth)/             # Route group — no URL segment
│   └── login/
│       └── page.tsx
├── dashboard/
│   ├── layout.tsx      # Nested layout (dashboard shell)
│   ├── page.tsx        # /dashboard
│   └── feed/
│       └── page.tsx    # /dashboard/feed
└── api/
    └── browser/
        └── route.ts    # Route handler (replaces pages/api/)

Rule: Folders = routes. Files = behavior.

• page.tsx — renders the route
• layout.tsx — persistent wrapper, doesn't re-render on child navigation
• loading.tsx — Suspense fallback while data loads
• error.tsx — error boundary
• route.ts — API endpoint (GET, POST, etc.)

Server vs Client Components

Server Components (default in 2026)

• Run on server. Zero JS sent to client.
• Can async/await directly — no useEffect, no useState
• Can access secrets, DB, filesystem
• Cannot: use hooks, event handlers, browser APIs

// app/dashboard/feed/page.tsx — Server Component by default
export default async function FeedPage() {
    // Direct async data fetch — no useEffect needed
    const feed = await fetch("http://api:8000/browser/feed", {
        cache: "no-store",  // always fresh (dynamic route)
    }).then(r => r.json())

    return (
        <div>
            {feed.map((post: any) => (
                <PostCard key={post.id} post={post} />
            ))}
        </div>
    )
}

Client Components

// 'use client' MUST be first line
"use client"

import { useState } from "react"

export function ApproveButton({ draftId }: { draftId: string }) {
    const [loading, setLoading] = useState(false)

    async function approve() {
        setLoading(true)
        await fetch(`/api/browser/approve/${draftId}`, { method: "POST" })
        setLoading(false)
    }

    return (
        <button onClick={approve} disabled={loading}>
            {loading ? "Posting..." : "Approve"}
        </button>
    )
}

Pattern: Server component renders the page, imports client components for interactive islands.

Fetch Caching (2026 — Explicit)

// Always fresh — for live data
const data = await fetch(url, { cache: "no-store" })

// Cached with revalidation every 60s — for semi-static
const data = await fetch(url, { next: { revalidate: 60 } })

// Fully static — for config/reference data
const data = await fetch(url)  // default: cached

Route Handlers (API endpoints)

// app/api/browser/approve/[id]/route.ts
import { NextRequest, NextResponse } from "next/server"

export async function POST(
    req: NextRequest,
    { params }: { params: { id: string } }
) {
    const res = await fetch(`http://api:8000/browser/approve/${params.id}`, {
        method: "POST",
    })
    const data = await res.json()
    return NextResponse.json(data)
}

Server Actions (form + mutation without API route)

// app/dashboard/compose/page.tsx
export default function ComposePage() {
    async function createDraft(formData: FormData) {
        "use server"   // marks this function as server action
        const content = formData.get("content") as string
        await fetch("http://api:8000/drafts", {
            method: "POST",
            body: JSON.stringify({ content }),
            headers: { "Content-Type": "application/json" },
        })
    }

    return (
        <form action={createDraft}>
            <textarea name="content" placeholder="What's on your mind?" />
            <button type="submit">Save Draft</button>
        </form>
    )
}

Layouts (Persistent Shell)

// app/dashboard/layout.tsx
export default function DashboardLayout({ children }: { children: React.ReactNode }) {
    return (
        <div className="flex h-screen">
            <nav className="w-64 bg-gray-900">
                {/* Sidebar — never re-renders between dashboard pages */}
            </nav>
            <main className="flex-1 overflow-auto">
                {children}
            </main>
        </div>
    )
}

Tailwind v4 (2026)

# v4 is CSS-first — no tailwind.config.js needed for basic use
uv add --dev @tailwindcss/vite  # or use Next.js built-in support

Key v4 changes vs v3:

• Config in CSS @theme block, not tailwind.config.js
• Automatic content detection (no content: [] array needed)
• @import "tailwindcss" replaces @tailwind base/components/utilities

/* app/globals.css */
@import "tailwindcss";

@theme {
    --color-brand: #6366f1;
    --font-sans: "Inter", sans-serif;
}

Environment Variables

# .env.local (server-only, never sent to client)
API_URL=http://api:8000

# .env.local (accessible in client — must prefix NEXT_PUBLIC_)
NEXT_PUBLIC_WS_URL=ws://your-vps:8000/ws/feed

// Server component — can use server-only env
const apiUrl = process.env.API_URL

// Client component — must use NEXT_PUBLIC_
const wsUrl = process.env.NEXT_PUBLIC_WS_URL

Anti-Patterns

// ❌ Pages Router — dead in 2026 for new projects
// pages/index.tsx
export async function getServerSideProps() { ... }  // NEVER

// ❌ useEffect for data fetching in 2026
useEffect(() => {
    fetch("/api/feed").then(...)  // use Server Component async fetch instead
}, [])

// ❌ 'use client' everywhere — kills perf
// Only add 'use client' when you actually need hooks/events

// ❌ API routes in pages/api/
// pages/api/browser.ts  — use app/api/ route handlers instead

WebSocket in Client Component

"use client"
import { useEffect, useState } from "react"

export function LiveFeed() {
    const [posts, setPosts] = useState<any[]>([])

    useEffect(() => {
        const ws = new WebSocket(process.env.NEXT_PUBLIC_WS_URL!)
        ws.onmessage = (e) => {
            const post = JSON.parse(e.data)
            setPosts(prev => [post, ...prev])
        }
        return () => ws.close()
    }, [])

    return <div>{posts.map(p => <div key={p.id}>{p.content}</div>)}</div>
}

References

• Next.js App Router docs: https://nextjs.org/docs/app
• Server/Client Components: https://nextjs.org/docs/app/getting-started/server-and-client-components
• Production checklist: https://nextjs.org/docs/app/guides/production-checklist
• Tailwind v4: https://tailwindcss.com/docs/v4-beta