nextjs-patterns

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# Next.js App Router Patterns

---

## File Conventions

| File | Purpose |
|------|---------|
| `page.tsx` | Route UI, receives `params`/`searchParams` |
| `layout.tsx` | Shared UI, wraps children, persists across nav |
| `loading.tsx` | Suspense fallback for the segment |
| `error.tsx` | Error boundary — must be `'use client'` |
| `not-found.tsx` | 404 UI — call `notFound()` to trigger |
| `route.ts` | API endpoint (`GET`, `POST`, etc.) |

---

## Server Actions

```typescript
// ✅ CORRECT
"use server"
import { z } from "zod"

const schema = z.object({ name: z.string().min(1) })

export async function updateName(formData: FormData) {
  const result = schema.safeParse({ name: formData.get("name") })
  if (!result.success) {
    return { success: false, error: result.error.flatten().fieldErrors }
  }
  await db.update(result.data)
  revalidatePath("/profile")
  return { success: true }
}

// ❌ WRONG
export async function updateName(formData: FormData) {
  await db.update({ name: formData.get("name") as string })
}
```

**Rules:** Always `"use server"` · Always Zod validate · Always `{success, error}` shape · Never return sensitive data

---

## Route Handlers

```typescript
// ✅ CORRECT
export async function POST(req: NextRequest) {
  try {
    const { email } = bodySchema.parse(await req.json())
    await subscribe(email)
    return NextResponse.json({ success: true })
  } catch (err) {
    if (err instanceof z.ZodError) {
      return NextResponse.json({ error: err.flatten() }, { status: 400 })
    }
    return NextResponse.json({ error: "Internal error" }, { status: 500 })
  }
}
```

---

## Image Optimization

```tsx
// ✅ CORRECT — always next/image
import Image from "next/image"
<Image src="/hero.jpg" alt="Hero" width={1200} height={630} priority />

// ❌ WRONG
<img src="/hero.jpg" alt="Hero" />
```

`priority` on above-the-fold images (improves LCP). Always `width` + `height` to prevent CLS.

---

## Font Optimization

```tsx
// ✅ CORRECT — no FOUT, self-hosted, no privacy leak
import { Inter } from "next/font/google"
const inter = Inter({ subsets: ["latin"], display: "swap" })
<html lang="he" dir="rtl" className={inter.className}>
```

---

## `use client` Boundary Rules

```tsx
// ✅ Push to leaf components only
// page.tsx — Server Component
export default async function Page() {
  const data = await db.query()
  return <LikeButton />
}

// LikeButton.tsx
"use client"
export function LikeButton() {
  const [liked, setLiked] = useState(false)
  return <button onClick={() => setLiked(true)}>Like</button>
}

// ❌ WRONG — entire page becomes client
"use client"
export default function Page() { ... }
```

---

## Common Mistakes

From Nadav011/apex-skills, MIT.

More from Nadav011/apex-skills