Next.js 15 Best Practices

Team standard for building Next.js 15 applications with App Router, Tailwind CSS, and TypeScript.

Decision Tree: Server vs Client Component

Need interactivity (onClick, onChange, state)?
├── YES → 'use client'
│   └── Keep it small. Push interactivity to leaf components.
└── NO → Server Component (default)
    ├── Need data? → async function + fetch/db directly
    ├── Need to show loading? → Add loading.tsx or <Suspense>
    └── Need client child? → Pass data as props (serializable only)

Key rule: Default to Server Components. Only add 'use client' at the lowest possible node.

Export Rule (MANDATORY)

Named exports everywhere. export default ONLY where Next.js requires it.

// WRONG
export default function BlogView({ blog }: BlogViewProps) { ... }
export default function Button({ ...props }: ButtonProps) { ... }
export default useSidebarStore

// CORRECT
export function BlogView({ blog }: BlogViewProps) { ... }
export function Button({ ...props }: ButtonProps) { ... }
export const useSidebarStore = create<SidebarStore>(...)

This applies to: view.tsx, all components/, hooks/, lib/, config/, constants/, context/, store/, validation/, actions/, types/.

Page/View Pattern (MANDATORY)

Every route that needs client interactivity MUST use the page.tsx + view.tsx split:

app/blog/[id]/
├── page.tsx    # Server Component — fetches data, passes as props
├── view.tsx    # 'use client' — thin composition layer, imports from components/
└── loading.tsx # Loading skeleton (optional)

components/blog/            # Sub-components for blog views
├── blog-header.tsx
├── blog-content.tsx
├── blog-comments.tsx
└── blog-like-button.tsx

view.tsx must stay thin. It receives props, manages top-level state, and composes sub-components from components/[feature]/. Never let view.tsx grow into a monolith.

// page.tsx — Server Component (data fetching ONLY)
import { db } from '@/config/db'
import { notFound } from 'next/navigation'
import { BlogDetailView } from './view'

export default async function BlogPage({ params }: { params: Promise<{ id: string }> }) {
  const { id } = await params
  const [blog, comments] = await Promise.all([
    db.blog.findUnique({ where: { id } }),
    db.comment.findMany({ where: { blogId: id } }),
  ])
  if (!blog) notFound()
  return <BlogDetailView blog={blog} comments={comments} />
}

// view.tsx — Thin composition layer
'use client'

import type { Blog, Comment } from '@/types'
import { BlogHeader } from '@/components/blog/blog-header'
import { BlogContent } from '@/components/blog/blog-content'
import { BlogComments } from '@/components/blog/blog-comments'

interface BlogDetailViewProps {
  blog: Blog
  comments: Comment[]
}

export function BlogDetailView({ blog, comments }: BlogDetailViewProps) {
  return (
    <article>
      <BlogHeader title={blog.title} author={blog.author} date={blog.createdAt} />
      <BlogContent content={blog.content} />
      <BlogComments blogId={blog.id} comments={comments} />
    </article>
  )
}

Mapping rule: app/[route]/view.tsx → components/[route]/**

Never use useEffect to fetch initial data. Fetch in page.tsx, pass via props.

No Props Hell (MANDATORY)

Only pass props that the child cannot obtain itself. If a child can access data via context, Zustand store, or hook — let the child do it. Don't fetch/subscribe in a parent just to pass down.

// BAD: Props hell — parent fetches state just to pass it
export function BlogDetailView({ blog, comments }: BlogDetailViewProps) {
  const { user } = useAuth()
  const { theme } = useTheme()
  const { isOpen } = useSidebarStore()

  return (
    <BlogHeader user={user} theme={theme} />          {/* user/theme are prop-drilled */}
    <BlogComments blogId={blog.id} comments={comments} isOpen={isOpen} />
  )
}

// GOOD: Each child fetches its own shared state
export function BlogDetailView({ blog, comments }: BlogDetailViewProps) {
  return (
    <BlogHeader />                                     {/* reads useAuth() + useTheme() itself */}
    <BlogComments blogId={blog.id} comments={comments} />  {/* reads useSidebarStore() itself */}
  )
}

Props are for: Data that comes from the server (page.tsx → view.tsx → child) or data the child has no way to access on its own.

Props are NOT for: Context values, store state, or hook results the child can call directly.

Decision: Should I pass this as a prop?
├── Child can call useAuth(), useTheme(), useSidebarStore(), etc.?
│   └── NO prop — let child access it directly
├── Data comes from server (page.tsx fetched it)?
│   └── YES prop — pass it down
└── Data is local to parent (parent's useState)?
    └── YES prop — pass it down (or lift to store/context if many children need it)

For detailed patterns and examples, see references/server-client-components.md.

useEffect vs useLayoutEffect (MANDATORY)

Don't spam useEffect everywhere. Analyze first — pick the right hook for the job.

Need to run a side effect?
├── Does it read or modify DOM layout (measure size, scroll position, focus)?
│   └── YES → useLayoutEffect (runs synchronously before browser paints)
├── Does it need to prevent visual flicker (tooltip position, element resize)?
│   └── YES → useLayoutEffect
└── Everything else (API calls, subscriptions, analytics, timers)
    └── useEffect (runs after paint, does not block rendering)

// BAD: useEffect causes flicker — DOM measurement happens after paint
useEffect(() => {
  const height = ref.current.getBoundingClientRect().height
  setHeight(height)  // user sees a layout jump
}, [])

// GOOD: useLayoutEffect — measures before paint, no flicker
useLayoutEffect(() => {
  const height = ref.current.getBoundingClientRect().height
  setHeight(height)  // seamless, user sees correct layout immediately
}, [])

Rule: If you're about to write useEffect, ask: "Does this touch the DOM or could it cause a visual flicker?" If yes → useLayoutEffect.

Decision Tree: Data Fetching

Where is the data needed?
├── Server Component → fetch() or direct DB call
│   ├── Static data? → cache: 'force-cache' (default)
│   ├── Dynamic data? → cache: 'no-store'
│   ├── Timed revalidation? → next: { revalidate: N }
│   └── On-demand revalidation? → revalidateTag() / revalidatePath()
├── Client Component (real-time / polling) → useSWR or TanStack Query
└── Mutation → Server Action

Key rule: Fetch data in Server Components, pass to Client Components as props. Use SWR/TanStack Query only for client-side polling, optimistic UI, or real-time data.

For detailed patterns, see references/data-fetching.md.

Decision Tree: Forms & Mutations

Form submission?
├── Simple form → Server Action with <form action={}>
│   └── Need pending state? → useActionState + useFormStatus
├── Complex form (multi-step, validation) → React Hook Form + Zod
│   └── Submit via Server Action or API route
└── Optimistic UI needed? → useOptimistic + Server Action

Key rule: Use Server Actions for mutations. Use React Hook Form + Zod for complex client-side validation. Validate on both client AND server.

For detailed patterns, see references/forms-and-mutations.md.

Project Structure

Standard folder layout — each folder has ONE purpose:

src/
├── app/            # Routes, layouts, pages + view.tsx files ONLY
├── components/     # Reusable UI components (ui/ + feature-specific/)
├── actions/        # Server Actions organized by domain
├── config/         # App config, env wrappers, third-party setup
├── constants/      # App-wide constant values
├── context/        # React context providers
├── hooks/          # Custom React hooks
├── lib/            # Utilities and reusable libraries (cn(), formatDate(), db client)
├── providers/      # App-wide providers (auth, theme, query client, Zustand, etc.)
├── store/          # Client state management (Zustand, etc.)
├── styles/         # Global styles, Tailwind config
├── types/          # Shared TypeScript types/interfaces
└── validation/     # Zod schemas for forms and Server Actions

Placement rules:

• Route files (page.tsx, view.tsx, layout.tsx, loading.tsx) → app/
• Anything reusable across routes → appropriate folder above
• Never put business logic in app/ — only data fetching and composition

Clean Layout Rule (MANDATORY)

Always use a single providers.tsx in the providers/ folder that composes all app-wide providers. The root layout.tsx stays clean — it only imports Providers and wraps {children}.

// providers/providers.tsx — Single entry point for all providers
'use client'

import { AuthProvider } from './auth-provider'
import { ThemeProvider } from './theme-provider'
import { QueryProvider } from './query-provider'
import type { User } from '@/types'

interface ProvidersProps {
  user: User | null
  children: React.ReactNode
}

export function Providers({ user, children }: ProvidersProps) {
  return (
    <QueryProvider>
      <AuthProvider user={user}>
        <ThemeProvider>
          {children}
        </ThemeProvider>
      </AuthProvider>
    </QueryProvider>
  )
}

// app/layout.tsx — Clean. Only structure + Providers.
import { Providers } from '@/providers/providers'
import { getCurrentUser } from '@/config/auth'
import '@/styles/globals.css'

export default async function RootLayout({ children }: { children: React.ReactNode }) {
  const user = await getCurrentUser()

  return (
    <html lang="en">
      <body>
        <Providers user={user}>
          {children}
        </Providers>
      </body>
    </html>
  )
}

When adding a new provider: Add it inside providers.tsx — never touch layout.tsx.

For detailed conventions and examples, see references/project-structure.md.

Performance Quick Reference

For full performance guide, see references/performance.md.

Code Review Checklist

Quick checks when reviewing Next.js code:

1. 'use client' only where needed? (not at page/layout level)
2. Data fetched in Server Components, not Client?
3. Server Actions validate input on server side?
4. No secrets/env vars leaked to client bundle?
5. loading.tsx or <Suspense> for async content?
6. Images use next/image, links use next/link?
7. Metadata exported from pages/layouts?
8. No unnecessary useEffect for data that could be server-fetched?

For full checklist, see references/code-review-checklist.md.