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 buildand review the output size per route - Use
@next/bundle-analyzerto identify large dependencies - Replace
momentwithdate-fnsordayjs(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
priorityprop 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/heighton 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/imageor equivalent (automatic WebP/AVIF, responsive srcset) - Set
sizesattribute 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/fontfor 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
widthandheighton images and videos - Use
aspect-ratioCSS 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: layoutfor 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.