Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Writing new React components or Next.js pages

• Implementing data fetching (client or server-side)

• Reviewing code for performance issues

• Refactoring existing React/Next.js code

• Optimizing bundle size or load times

Rule Categories by Priority

Priority

Category

Impact

Prefix

1

Eliminating Waterfalls

CRITICAL

async-

2

Bundle Size Optimization

CRITICAL

bundle-

3

Server-Side Performance

HIGH

server-

4

Client-Side Data Fetching

MEDIUM-HIGH

client-

5

Re-render Optimization

MEDIUM

rerender-

6

Rendering Performance

MEDIUM

rendering-

7

JavaScript Performance

LOW-MEDIUM

js-

8

Advanced Patterns

LOW

advanced-

Quick Reference

1. Eliminating Waterfalls (CRITICAL)

• async-defer-await - Move await into branches where actually used

• async-parallel - Use Promise.all() for independent operations

• async-dependencies - Use better-all for partial dependencies

• async-api-routes - Start promises early, await late in API routes

• async-suspense-boundaries - Use Suspense to stream content

2. Bundle Size Optimization (CRITICAL)

• bundle-barrel-imports - Import directly, avoid barrel files

• bundle-dynamic-imports - Use next/dynamic for heavy components

• bundle-defer-third-party - Load analytics/logging after hydration

• bundle-conditional - Load modules only when feature is activated

• bundle-preload - Preload on hover/focus for perceived speed

3. Server-Side Performance (HIGH)

• server-cache-react - Use React.cache() for per-request deduplication

• server-cache-lru - Use LRU cache for cross-request caching

• server-serialization - Minimize data passed to client components

• server-parallel-fetching - Restructure components to parallelize fetches

• server-after-nonblocking - Use after() for non-blocking operations

4. Client-Side Data Fetching (MEDIUM-HIGH)

• client-swr-dedup - Use SWR for automatic request deduplication

• client-event-listeners - Deduplicate global event listeners

5. Re-render Optimization (MEDIUM)

• rerender-defer-reads - Don't subscribe to state only used in callbacks

• rerender-memo - Extract expensive work into memoized components

• rerender-dependencies - Use primitive dependencies in effects

• rerender-derived-state - Subscribe to derived booleans, not raw values

• rerender-functional-setstate - Use functional setState for stable callbacks

• rerender-lazy-state-init - Pass function to useState for expensive values

• rerender-transitions - Use startTransition for non-urgent updates

6. Rendering Performance (MEDIUM)

• rendering-animate-svg-wrapper - Animate div wrapper, not SVG element

• rendering-content-visibility - Use content-visibility for long lists

• rendering-hoist-jsx - Extract static JSX outside components

• rendering-svg-precision - Reduce SVG coordinate precision

• rendering-hydration-no-flicker - Use inline script for client-only data

• rendering-activity - Use Activity component for show/hide

• rendering-conditional-render - Use ternary, not && for conditionals

7. JavaScript Performance (LOW-MEDIUM)

• js-batch-dom-css - Group CSS changes via classes or cssText

• js-index-maps - Build Map for repeated lookups

• js-cache-property-access - Cache object properties in loops

• js-cache-function-results - Cache function results in module-level Map

• js-cache-storage - Cache localStorage/sessionStorage reads

• js-combine-iterations - Combine multiple filter/map into one loop

• js-length-check-first - Check array length before expensive comparison

• js-early-exit - Return early from functions

• js-hoist-regexp - Hoist RegExp creation outside loops

• js-min-max-loop - Use loop for min/max instead of sort

• js-set-map-lookups - Use Set/Map for O(1) lookups

• js-tosorted-immutable - Use toSorted() for immutability

8. Advanced Patterns (LOW)

• advanced-event-handler-refs - Store event handlers in refs

• advanced-use-latest - useLatest for stable callback refs

How to Use

For detailed explanations and code examples, read the full compiled document: AGENTS.md

Each rule contains:

• Brief explanation of why it matters

• Incorrect code example with explanation

• Correct code example with explanation

• Additional context and references

Key Examples

Promise.all for Independent Operations (CRITICAL)

// ❌ Sequential: 3 round trips

const user = await fetchUser()

const posts = await fetchPosts()

const comments = await fetchComments()

// ✅ Parallel: 1 round trip

const [user, posts, comments] = await Promise.all([

  fetchUser(),

  fetchPosts(),

  fetchComments()

])

Avoid Barrel File Imports (CRITICAL)

// ❌ Imports entire library (200-800ms import cost)

import { Check, X, Menu } from 'lucide-react'

// ✅ Imports only what you need

import Check from 'lucide-react/dist/esm/icons/check'

import X from 'lucide-react/dist/esm/icons/x'

Dynamic Imports for Heavy Components (CRITICAL)

// ❌ Monaco bundles with main chunk ~300KB

import { MonacoEditor } from './monaco-editor'

// ✅ Monaco loads on demand

import dynamic from 'next/dynamic'

const MonacoEditor = dynamic(

  () => import('./monaco-editor').then(m => m.MonacoEditor),

  { ssr: false }

)

Use Functional setState (MEDIUM)

// ❌ Requires state as dependency, stale closure risk

const addItems = useCallback((newItems) => {

  setItems([...items, ...newItems])

}, [items])

// ✅ Stable callback, no stale closures

const addItems = useCallback((newItems) => {

  setItems(curr => [...curr, ...newItems])

}, [])

Constraints

Required Rules (MUST)

• Eliminate waterfalls: Use Promise.all and Suspense

• Optimize bundles: No barrel imports, use dynamic imports

• RSC boundaries: Serialize only the necessary data

Prohibited Patterns (MUST NOT)

• Sequential await: Do not execute independent fetches sequentially

• Array mutations: Use toSorted() instead of sort()

• Inline objects in React.cache: Causes cache misses

References

• React Documentation

• Next.js Documentation

• SWR

• better-all

• Vercel Blog: Optimizing Package Imports

• Vercel Blog: Dashboard Performance

Metadata

Version

• Current Version: 1.0.0

• Last Updated: 2026-01-22

• Compatible Platforms: Claude, ChatGPT, Gemini

• Original Source: vercel/agent-skills

Related Skills

• performance-optimization: General performance optimization

• state-management: State management

Tags

#React #Next.js #performance #optimization #vercel #waterfalls #bundle-size #RSC #frontend