Vercel React Best Practices

Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel Engineering. Contains rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

Rule Categories by Priority

1. Eliminating Waterfalls (CRITICAL)

async-defer-await

Move await into the exact branches where the value is needed, not at the top of functions:

// ❌ WRONG: Blocks execution even if user branch is never hit
async function handleRequest(userId?: string) {
  const user = userId ? await fetchUser(userId) : null
  if (!user) return redirect('/login')
  return renderProfile(user)
}

// ✅ CORRECT: Start early or await inside condition
async function handleRequest(userId?: string) {
  if (!userId) return redirect('/login')
  const user = await fetchUser(userId)
  return renderProfile(user)
}

async-parallel

Use Promise.all() for independent asynchronous operations:

// ❌ WRONG: Sequential waterfall (takes T1 + T2)
const user = await fetchUser(id)
const posts = await fetchPosts(id)

// ✅ CORRECT: Concurrent fetches (takes max(T1, T2))
const [user, posts] = await Promise.all([
  fetchUser(id),
  fetchPosts(id),
])

async-suspense-boundaries

Stream slow components independently using React Suspense rather than awaiting data in parent layouts:

// ✅ Stream heavy content without blocking the main shell
export default function Page() {
  return (
    <div>
      <Header />
      <Suspense fallback={<AnalyticsSkeleton />}>
        <SlowAnalyticsDashboard />
      </Suspense>
    </div>
  )
}

2. Bundle Size Optimization (CRITICAL)

bundle-barrel-imports

Import directly from the module source; avoid large barrel files (index.ts re-exporting hundreds of icons or utilities), as barrel files defeat compiler tree-shaking and bloat JS bundles:

// ❌ WRONG: Imports entire icon library barrel file
import { ChevronRight, Settings } from 'lucide-react'

// ✅ PREFERRED in large bundles: Direct or optimized path
import ChevronRight from 'lucide-react/dist/esm/icons/chevron-right'

bundle-dynamic-imports

Dynamically import heavy components (charts, rich-text editors, maps) with next/dynamic so they are not included in the initial page chunk:

import dynamic from 'next/dynamic'

const HeavyChart = dynamic(() => import('@/components/AnalyticsChart'), {
  loading: () => <ChartSkeleton />,
  ssr: false, // only if chart requires window / client canvas
})

bundle-defer-third-party

Load non-critical analytics, customer support widgets, and logging scripts only after hydration using next/script with strategy="afterInteractive" or strategy="lazyOnload".

3. Server-Side Performance & Next.js 15 (HIGH)

Next.js 15 Default Caching Behavior

[!IMPORTANT] In Next.js 15, fetch requests and GET Route Handlers are uncached by default (cache: 'no-store'). If you want data to be cached, you must explicitly declare cache: 'force-cache' or next: { revalidate: 3600 }.

// ✅ Explicit caching when data is cacheable
const res = await fetch('https://api.example.com/products', {
  next: { revalidate: 60 }, // ISR: cached for 60s
})

server-cache-react

Use React.cache() for per-request function deduplication across Server Components:

import { cache } from 'react'

export const getOrganization = cache(async (orgId: string) => {
  return await db.organization.findUnique({ where: { id: orgId } })
})

server-after-nonblocking

In Next.js 15+, use after() from next/server to run non-critical side effects (analytics logging, cache warming, notifications) without delaying the HTTP response:

import { after } from 'next/server'

export async function POST(request: Request) {
  const data = await request.json()
  const result = await processOrder(data)

  // Runs in background after response is sent to client
  after(async () => {
    await sendAuditLog(result.id)
    await notifySlack(result)
  })

  return Response.json({ success: true, id: result.id })
}

4. Client-Side & React 19 Patterns

useActionState & useOptimistic (React 19)

Replace legacy useTransition + manual error states with React 19 Server Action hooks:

'use client'
import { useActionState, useOptimistic } from 'react'
import { updateNameAction } from '@/actions/profile'

export function ProfileForm({ initialName }: { initialName: string }) {
  const [state, formAction, isPending] = useActionState(updateNameAction, null)
  const [optimisticName, setOptimisticName] = useOptimistic(
    initialName,
    (_current, update: string) => update
  )

  return (
    <form action={async (formData) => {
      setOptimisticName(formData.get('name') as string)
      await formAction(formData)
    }}>
      <input name="name" defaultValue={optimisticName} />
      <button disabled={isPending}>Save</button>
    </form>
  )
}

No forwardRef in React 19

React 19 allows passing ref directly as a component prop. Do not wrap modern components in forwardRef().