frontend-excellence | claude-code-toolkit

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frontend-excellence | claude-code-toolkit

Frontend Excellence

React Server Components

Server Components run on the server and send rendered HTML to the client. They can directly access databases, filesystems, and internal APIs without exposing them to the browser.

// app/products/page.tsx (Server Component by default)
async function ProductsPage() {
  const products = await db.query("SELECT * FROM products WHERE active = true");
  return (
    <main>
      <h1>Products</h1>
      <ProductList products={products} />
      <AddToCartButton />  {/* Client Component */}
    </main>
  );
}

Rules:

Server Components cannot use useState, useEffect, or browser APIs
Mark interactive components with 'use client' at the top of the file
Pass serializable props from Server to Client Components (no functions, no classes)
Keep 'use client' boundary as deep in the tree as possible

Streaming SSR

import { Suspense } from 'react';

export default function Dashboard() {
  return (
    <div>
      <Header />  {/* renders immediately */}
      <Suspense fallback={<ChartSkeleton />}>
        <AnalyticsChart />  {/* streams when ready */}
      </Suspense>
      <Suspense fallback={<TableSkeleton />}>
        <RecentOrders />  {/* streams independently */}
      </Suspense>
    </div>
  );
}

Each Suspense boundary streams independently. Place boundaries around data-fetching components to avoid blocking the entire page.

Code Splitting

import dynamic from 'next/dynamic';

const HeavyEditor = dynamic(() => import('@/components/Editor'), {
  loading: () => <EditorSkeleton />,
  ssr: false,
});

const AdminPanel = dynamic(() => import('@/components/AdminPanel'));

Split on:

Route boundaries (automatic in Next.js App Router)
Conditionally rendered components (modals, drawers, admin panels)
Heavy libraries (chart libraries, rich text editors, maps)
Below-the-fold content

Bundle Optimization

// next.config.js
module.exports = {
  experimental: {
    optimizePackageImports: ['lucide-react', '@heroicons/react', 'lodash-es'],
  },
};

Checklist:

Run npx next build and review the output size per route
Use @next/bundle-analyzer to identify large dependencies
Replace moment with date-fns or dayjs (save ~200KB)
Import specific functions: import { debounce } from 'lodash-es/debounce'
Prefer CSS over JS for animations (no runtime cost)
Tree-shake icon libraries: import { Search } from 'lucide-react'

Core Web Vitals Targets

Metric	Good	Needs Work	Poor
LCP (Largest Contentful Paint)	<2.5s	2.5-4.0s	>4.0s
INP (Interaction to Next Paint)	<200ms	200-500ms	>500ms
CLS (Cumulative Layout Shift)	<0.1	0.1-0.25	>0.25

LCP Optimization

Preload hero images: <link rel="preload" as="image" href="..." />
Use priority prop on above-the-fold <Image> components
Inline critical CSS, defer non-critical stylesheets
Avoid client-side rendering for above-the-fold content
Set explicit width/height on images to prevent layout shifts

Image Optimization

import Image from 'next/image';

<Image
  src="/hero.jpg"
  alt="Descriptive alt text"
  width={1200}
  height={630}
  priority              // preload for LCP images
  sizes="(max-width: 768px) 100vw, 50vw"
  placeholder="blur"
  blurDataURL={base64}  // inline tiny placeholder
/>

Use next/image or equivalent (automatic WebP/AVIF, responsive srcset)
Set sizes attribute to avoid downloading oversized images
Use placeholder="blur" with a base64 data URL for perceived performance
Lazy load below-the-fold images (default behavior)

Font Loading Strategy

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

const inter = Inter({
  subsets: ['latin'],
  display: 'swap',       // show fallback font immediately
  preload: true,
  variable: '--font-inter',
});

export default function RootLayout({ children }) {
  return (
    <html className={inter.variable}>
      <body>{children}</body>
    </html>
  );
}

Use next/font for zero-CLS font loading with automatic subsetting
Set display: 'swap' to avoid invisible text during load
Self-host fonts instead of loading from Google CDN (saves DNS lookup)
Limit to 2 font families maximum

CLS Prevention

Always set width and height on images and videos
Use aspect-ratio CSS for responsive media containers
Reserve space for dynamic content (ads, embeds) with min-height
Avoid inserting content above existing content after load
Use CSS contain: layout for components that change size

Performance Monitoring

import { onCLS, onINP, onLCP } from 'web-vitals';

onCLS(console.log);
onINP(console.log);
onLCP(console.log);

Measure real user metrics (RUM), not just lab scores. Vercel Analytics and Google Search Console provide field data.