Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 57 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

async-

bundle-

server-

client-

rerender-

rendering-

js-

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-auth-actions

  - Authenticate server actions like API routes

• server-cache-react

  - Use React.cache() for per-request deduplication

• server-cache-lru

  - Use LRU cache for cross-request caching

• server-dedup-props

  - Avoid duplicate serialization in RSC props

• 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

• client-passive-event-listeners

  - Use passive listeners for scroll

• client-localstorage-schema

  - Version and minimize localStorage data

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-memo-with-default-value

  - Hoist default non-primitive props

• rerender-dependencies

  - Use primitive dependencies in effects

• rerender-derived-state

  - Subscribe to derived booleans, not raw values

• rerender-derived-state-no-effect

  - Derive state during render, not effects

• rerender-functional-setstate

  - Use functional setState for stable callbacks

• rerender-lazy-state-init

  - Pass function to useState for expensive values

• rerender-simple-expression-in-memo

  - Avoid memo for simple primitives

• rerender-move-effect-to-event

  - Put interaction logic in event handlers

• rerender-transitions

  - Use startTransition for non-urgent updates

• rerender-use-ref-transient-values

  - Use refs for transient frequent values

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-hydration-suppress-warning

  - Suppress expected mismatches

• rendering-activity

  - Use Activity component for show/hide

• rendering-conditional-render

  - Use ternary, not && for conditionals

• rendering-usetransition-loading

  - Prefer useTransition for loading state

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-init-once

  - Initialize app once per app load

• advanced-use-latest

  - useLatest for stable callback refs

How to Use

Read individual rule files for detailed explanations and code examples:

rules/async-parallel.md
rules/bundle-barrel-imports.md

Each rule file contains:

• Brief explanation of why it matters
• Incorrect code example with explanation
• Correct code example with explanation
• Additional context and references

Full Compiled Document

For the complete guide with all rules expanded:

AGENTS.md