Senior frontend

Senior Frontend Engineer Skill

Expert guidance for modern React 19 + Next.js 15 + Tailwind CSS v4 development.

---

Core Stack

Layer	Technology
Framework	Next.js 15 (App Router)
UI Library	React 19
Styling	Tailwind CSS v4
Language	TypeScript 5.x (strict)
Bundler	Turbopack (dev), webpack (prod fallback)
Package Manager	pnpm or Bun

---

React 19 Patterns

New Hooks — Use These, Not the Old Equivalents

useActionState (replaces useFormState)

'use client'
import { useActionState } from 'react'
import { submitForm } from '@/app/actions'

type State = { error?: string; success?: boolean }

export function ContactForm() {
  const [state, formAction, isPending] = useActionState<State, FormData>(
    submitForm,
    { error: undefined, success: false }
  )

  return (
    <form action={formAction}>
      <input name="email" type="email" required />
      {state.error && <p className="text-red-500 text-sm">{state.error}</p>}
      <SubmitButton />
    </form>
  )
}

useOptimistic — Instant UI feedback

'use client'
import { useOptimistic, useTransition } from 'react'

type Todo = { id: string; text: string; done: boolean }

export function TodoList({ todos }: { todos: Todo[] }) {
  const [optimisticTodos, addOptimistic] = useOptimistic(
    todos,
    (state, newTodo: Todo) => [...state, newTodo]
  )
  const [isPending, startTransition] = useTransition()

  const handleAdd = (text: string) => {
    const tempTodo = { id: crypto.randomUUID(), text, done: false }
    startTransition(async () => {
      addOptimistic(tempTodo)
      await createTodo(text)
    })
  }

  return (
    <ul>
      {optimisticTodos.map(todo => (
        <li key={todo.id} className={todo.done ? 'line-through opacity-50' : ''}>
          {todo.text}
        </li>
      ))}
    </ul>
  )
}

useFormStatus — Submit button state

'use client'
import { useFormStatus } from 'react-dom'

export function SubmitButton({ label = 'Submit' }: { label?: string }) {
  const { pending } = useFormStatus()
  return (
    <button
      type="submit"
      disabled={pending}
      className="btn-primary disabled:opacity-60 disabled:cursor-not-allowed"
    >
      {pending ? 'Loading…' : label}
    </button>
  )
}

React 19 use() Hook

import { use, Suspense } from 'react'

// Pass a Promise directly to use()
function UserProfile({ userPromise }: { userPromise: Promise<User> }) {
  const user = use(userPromise) // suspends until resolved
  return <div>{user.name}</div>
}

// Usage: wrap in Suspense
export default function Page() {
  const userPromise = fetchUser(123)
  return (
    <Suspense fallback={<Skeleton />}>
      <UserProfile userPromise={userPromise} />
    </Suspense>
  )
}

---

Next.js 15 App Router Patterns

File Structure

app/
├── layout.tsx          # Root layout (server component)
├── page.tsx            # Home page
├── globals.css         # Tailwind CSS v4 entry
├── (marketing)/        # Route group (no URL segment)
│   ├── layout.tsx
│   └── about/page.tsx
├── (app)/              # Protected route group
│   ├── layout.tsx      # Auth guard here
│   └── dashboard/
│       ├── page.tsx
│       └── loading.tsx # Automatic Suspense boundary
├── api/
│   └── [...route]/
│       └── route.ts    # Route handlers
└── _components/        # Private folder (not routable)

Server vs Client Components

Rule: Default to Server Components. Add 'use client' only when needed.

Need	Use
Data fetching, DB access, secrets	Server Component
useState, useEffect, event handlers	Client Component
Browser APIs (window, localStorage)	Client Component
Animations, gesture handling	Client Component
Everything else	Server Component

// ✅ Server Component — no directive needed
export default async function ProductPage({ params }: { params: { id: string } }) {
  const product = await db.product.findUnique({ where: { id: params.id } })
  if (!product) notFound()

  return (
    <div>
      <h1>{product.name}</h1>
      {/* Pass serializable data to client */}
      <AddToCartButton productId={product.id} />
    </div>
  )
}

Data Fetching in Server Components

// Next.js 15: fetch() is no longer cached by default!
// Must explicitly opt-in to caching.

// No cache (default in Next.js 15)
const data = await fetch('https://api.example.com/data')

// Cache permanently (like old Next.js default)
const data = await fetch('https://api.example.com/data', {
  cache: 'force-cache'
})

// Revalidate every N seconds
const data = await fetch('https://api.example.com/data', {
  next: { revalidate: 60 }
})

// Tag-based revalidation
const data = await fetch('https://api.example.com/data', {
  next: { tags: ['products'] }
})
// Then call: revalidateTag('products') in a Server Action

Server Actions

// app/actions.ts
'use server'
import { revalidatePath, revalidateTag } from 'next/cache'
import { redirect } from 'next/navigation'
import { z } from 'zod'

const schema = z.object({
  title: z.string().min(1).max(200),
  content: z.string().min(10),
})

export async function createPost(prevState: unknown, formData: FormData) {
  const parsed = schema.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 })
    revalidateTag('posts')
    revalidatePath('/blog')
  } catch {
    return { error: { _form: ['Database error. Please try again.'] } }
  }

  redirect('/blog') // Throws internally — don't wrap in try/catch
}

Parallel Routes & Intercepting Routes

app/
├── @modal/           # Parallel slot
│   └── (.)photo/[id]/
│       └── page.tsx  # Intercepted route (modal)
├── layout.tsx        # Receives { children, modal }
└── photo/[id]/
    └── page.tsx      # Full page (on hard refresh)

// app/layout.tsx
export default function Layout({
  children,
  modal,
}: {
  children: React.ReactNode
  modal: React.ReactNode
}) {
  return (
    <>
      {children}
      {modal}
    </>
  )
}

---

Tailwind CSS v4

Setup — CSS-First Configuration

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

/* Define theme in CSS, not tailwind.config.js */
@theme {
  --color-brand-50: oklch(0.97 0.02 260);
  --color-brand-500: oklch(0.55 0.22 260);
  --color-brand-900: oklch(0.25 0.12 260);

  --font-sans: "Inter Variable", sans-serif;
  --font-mono: "JetBrains Mono", monospace;

  --radius-card: 0.75rem;
  --shadow-card: 0 4px 24px oklch(0 0 0 / 0.08);

  --animate-fade-in: fade-in 0.2s ease-out;
}

@keyframes fade-in {
  from { opacity: 0; transform: translateY(4px); }
  to   { opacity: 1; transform: translateY(0); }
}

v4 Breaking Changes to Know

// ❌ v3 — JIT config in tailwind.config.js
// ✅ v4 — everything in CSS via @theme

// ❌ v3
<div className="bg-opacity-50">
// ✅ v4 — use slash syntax
<div className="bg-black/50">

// ❌ v3 — ring-offset
<div className="ring-2 ring-offset-2">
// ✅ v4 — outline or shadow instead

// New: arbitrary values with CSS vars
<div className="text-[var(--color-brand-500)]">

Component Patterns with Tailwind v4

// Use cva for variant-based components
import { cva, type VariantProps } from 'class-variance-authority'
import { cn } from '@/lib/utils'

const buttonVariants = cva(
  'inline-flex items-center justify-center rounded-lg font-medium transition-colors focus-visible:outline-none focus-visible:ring-2 disabled:pointer-events-none disabled:opacity-50',
  {
    variants: {
      variant: {
        default: 'bg-brand-500 text-white hover:bg-brand-600',
        outline: 'border border-brand-500 text-brand-500 hover:bg-brand-50',
        ghost: 'hover:bg-brand-50 text-brand-500',
        destructive: 'bg-red-500 text-white hover:bg-red-600',
      },
      size: {
        sm: 'h-8 px-3 text-sm',
        md: 'h-10 px-4 text-sm',
        lg: 'h-12 px-6 text-base',
      },
    },
    defaultVariants: { variant: 'default', size: 'md' },
  }
)

interface ButtonProps
  extends React.ButtonHTMLAttributes<HTMLButtonElement>,
    VariantProps<typeof buttonVariants> {}

export function Button({ className, variant, size, ...props }: ButtonProps) {
  return (
    <button className={cn(buttonVariants({ variant, size }), className)} {...props} />
  )
}

---

Performance Optimization

Image Optimization

import Image from 'next/image'

// Always specify width/height or fill
<Image
  src="/hero.webp"
  alt="Hero image"
  width={1200}
  height={630}
  priority            // LCP image — load eagerly
  placeholder="blur"
  blurDataURL="data:image/jpeg;base64,..."
/>

// Dynamic images — use fill + sized container
<div className="relative aspect-video w-full">
  <Image
    src={post.coverImage}
    alt={post.title}
    fill
    className="object-cover rounded-lg"
    sizes="(max-width: 768px) 100vw, (max-width: 1200px) 50vw, 33vw"
  />
</div>

Bundle Optimization

// Dynamic imports for heavy components
import dynamic from 'next/dynamic'

const HeavyChart = dynamic(() => import('@/components/Chart'), {
  loading: () => <ChartSkeleton />,
  ssr: false, // Client-only components
})

const RichEditor = dynamic(() => import('@/components/RichEditor'), {
  loading: () => <EditorSkeleton />,
})

Font Optimization

// app/layout.tsx
import { Inter } from 'next/font/google'

const inter = Inter({
  subsets: ['latin'],
  display: 'swap',
  variable: '--font-inter',  // Use as CSS variable
})

export default function RootLayout({ children }: { children: React.ReactNode }) {
  return (
    <html lang="en" className={inter.variable}>
      <body className="font-sans">{children}</body>
    </html>
  )
}

Metadata & SEO

// app/blog/[slug]/page.tsx
import type { Metadata } from 'next'

export async function generateMetadata({ params }: Props): Promise<Metadata> {
  const post = await getPost(params.slug)
  return {
    title: post.title,
    description: post.excerpt,
    openGraph: {
      title: post.title,
      description: post.excerpt,
      images: [{ url: post.ogImage, width: 1200, height: 630 }],
    },
    alternates: {
      canonical: `https://example.com/blog/${params.slug}`,
    },
  }
}

export async function generateStaticParams() {
  const posts = await getAllPosts()
  return posts.map(post => ({ slug: post.slug }))
}

---

Error Handling Patterns

// error.tsx — must be 'use client'
'use client'
import { useEffect } from 'react'

export default function Error({
  error,
  reset,
}: {
  error: Error & { digest?: string }
  reset: () => void
}) {
  useEffect(() => {
    console.error(error)
    // Log to Sentry, Datadog, etc.
  }, [error])

  return (
    <div className="flex flex-col items-center gap-4 p-8">
      <h2 className="text-xl font-semibold">Something went wrong</h2>
      <p className="text-muted-foreground text-sm">{error.message}</p>
      <button onClick={reset} className="btn-primary">Try again</button>
    </div>
  )
}

---

Key Rules (Always Follow)

1. Never use any — use unknown and narrow types, or proper generics
2. Server Components by default — only 'use client' when necessary
3. Validate all Server Action inputs with Zod before touching DB
4. Never redirect() inside try/catch — it throws internally
5. Use next/image for all <img> tags
6. Use next/font — never <link> to Google Fonts directly
7. Suspense boundaries around async data fetching trees
8. loading.tsx for route-level skeletons (automatic Suspense)
9. error.tsx for route-level error recovery (must be 'use client')
10. not-found.tsx for 404 states — call notFound() from server components