Senior Frontend

my-app/
├── app/
│   ├── layout.tsx        # Root layout with fonts
│   ├── page.tsx          # Home page
│   ├── globals.css       # Tailwind + CSS variables
│   └── api/health/route.ts
├── components/
│   ├── ui/               # Button, Input, Card
│   └── layout/           # Header, Footer, Sidebar
├── hooks/                # useDebounce, useLocalStorage
├── lib/                  # utils (cn), constants
├── types/                # TypeScript interfaces
├── tailwind.config.ts
├── next.config.js
└── package.json

'use client';

import { useState } from 'react';
import { cn } from '@/lib/utils';

interface ButtonProps {
  className?: string;
  children?: React.ReactNode;
}

export function Button({ className, children }: ButtonProps) {
  return (
    <div className={cn('', className)}>
      {children}
    </div>
  );
}

const Tabs = ({ children }) => {
  const [active, setActive] = useState(0);
  return (
    <TabsContext.Provider value={{ active, setActive }}>
      {children}
    </TabsContext.Provider>
  );
};

Tabs.List = TabList;
Tabs.Panel = TabPanel;

// Usage
<Tabs>
  <Tabs.List>
    <Tabs.Tab>One</Tabs.Tab>
    <Tabs.Tab>Two</Tabs.Tab>
  </Tabs.List>
  <Tabs.Panel>Content 1</Tabs.Panel>
  <Tabs.Panel>Content 2</Tabs.Panel>
</Tabs>

function useDebounce<T>(value: T, delay = 500): T {
  const [debouncedValue, setDebouncedValue] = useState(value);

  useEffect(() => {
    const timer = setTimeout(() => setDebouncedValue(value), delay);
    return () => clearTimeout(timer);
  }, [value, delay]);

  return debouncedValue;
}

// Usage
const debouncedSearch = useDebounce(searchTerm, 300);

function DataFetcher({ url, render }) {
  const [data, setData] = useState(null);
  const [loading, setLoading] = useState(true);

  useEffect(() => {
    fetch(url).then(r => r.json()).then(setData).finally(() => setLoading(false));
  }, [url]);

  return render({ data, loading });
}

// Usage
<DataFetcher
  url="/api/users"
  render={({ data, loading }) =>
    loading ? <Spinner /> : <UserList users={data} />
  }
/>

// Server Component (default) - no 'use client'
async function ProductPage({ params }) {
  const product = await getProduct(params.id);  // Server-side fetch

  return (
    <div>
      <h1>{product.name}</h1>
      <AddToCartButton productId={product.id} />  {/* Client component */}
    </div>
  );
}

// Client Component
'use client';
function AddToCartButton({ productId }) {
  const [adding, setAdding] = useState(false);
  return <button onClick={() => addToCart(productId)}>Add</button>;
}

import Image from 'next/image';

// Above the fold - load immediately
<Image
  src="/hero.jpg"
  alt="Hero"
  width={1200}
  height={600}
  priority
/>

// Responsive image with fill
<div className="relative aspect-video">
  <Image
    src="/product.jpg"
    alt="Product"
    fill
    sizes="(max-width: 768px) 100vw, 50vw"
    className="object-cover"
  />
</div>

// Parallel fetching
async function Dashboard() {
  const [user, stats] = await Promise.all([
    getUser(),
    getStats()
  ]);
  return <div>...</div>;
}

// Streaming with Suspense
async function ProductPage({ params }) {
  return (
    <div>
      <ProductDetails id={params.id} />
      <Suspense fallback={<ReviewsSkeleton />}>
        <Reviews productId={params.id} />
      </Suspense>
    </div>
  );
}

// Accessible button
<button
  type="button"
  aria-label="Close dialog"
  onClick={onClose}
  className="focus-visible:ring-2 focus-visible:ring-blue-500"
>
  <XIcon aria-hidden="true" />
</button>

// Skip link for keyboard users
<a href="#main-content" className="sr-only focus:not-sr-only">
  Skip to main content
</a>

// Component test with React Testing Library
import { render, screen } from '@testing-library/react';
import userEvent from '@testing-library/user-event';

test('button triggers action on click', async () => {
  const onClick = vi.fn();
  render(<Button onClick={onClick}>Click me</Button>);

  await userEvent.click(screen.getByRole('button'));
  expect(onClick).toHaveBeenCalledTimes(1);
});

// Test accessibility
test('dialog is accessible', async () => {
  render(<Dialog open={true} title="Confirm" />);

  expect(screen.getByRole('dialog')).toBeInTheDocument();
  expect(screen.getByRole('dialog')).toHaveAttribute('aria-labelledby');
});

// next.config.js
const nextConfig = {
  images: {
    remotePatterns: [{ protocol: 'https', hostname: 'cdn.example.com' }],
    formats: ['image/avif', 'image/webp'],
  },
  experimental: {
    optimizePackageImports: ['lucide-react', '@heroicons/react'],
  },
};

// Conditional classes with cn()
import { cn } from '@/lib/utils';

<button className={cn(
  'px-4 py-2 rounded',
  variant === 'primary' && 'bg-blue-500 text-white',
  disabled && 'opacity-50 cursor-not-allowed'
)} />

// Props with children
interface CardProps {
  className?: string;
  children: React.ReactNode;
}

// Generic component
interface ListProps<T> {
  items: T[];
  renderItem: (item: T) => React.ReactNode;
}

function List<T>({ items, renderItem }: ListProps<T>) {
  return <ul>{items.map(renderItem)}</ul>;
}