vercel-react-best-practices

name	vercel-react-best-practices
description	React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React/Next.js code to ensure optimal performance patterns. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.
license	MIT
metadata	{"author":"vercel","version":"1.0.0"}

Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 58 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 — MUST move await into branches where actually used
• async-parallel — MUST use Promise.all() for independent operations
• async-dependencies — SHOULD use better-all for partial dependencies
• async-api-routes — MUST start promises early, await late in API routes
• async-suspense-boundaries — SHOULD use Suspense to stream content

2. Bundle Size Optimization (CRITICAL)

• bundle-barrel-imports — MUST import directly, avoid barrel files
• bundle-dynamic-imports — SHOULD use next/dynamic for heavy components
• bundle-defer-third-party — SHOULD load analytics/logging after hydration
• bundle-conditional — SHOULD load modules only when feature is activated
• bundle-preload — MAY preload on hover/focus for perceived speed

3. Server-Side Performance (HIGH)

• server-auth-actions — MUST authenticate server actions like API routes
• server-cache-react — SHOULD use React.cache() for per-request deduplication
• server-cache-lru — MAY use LRU cache for cross-request caching
• server-dedup-props — SHOULD avoid duplicate serialization in RSC props
• server-hoist-static-io — SHOULD hoist static I/O (fonts, logos) to module level
• server-serialization — SHOULD minimize data passed to client components
• server-parallel-fetching — SHOULD restructure components to parallelize fetches
• server-after-nonblocking — MAY use after() for non-blocking operations

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

• client-swr-dedup — SHOULD use SWR for automatic request deduplication
• client-event-listeners — SHOULD deduplicate global event listeners
• client-passive-event-listeners — SHOULD use passive listeners for scroll
• client-localstorage-schema — SHOULD version and minimize localStorage data

5. Re-render Optimization (MEDIUM)

• rerender-defer-reads — SHOULD NOT subscribe to state only used in callbacks
• rerender-memo — SHOULD extract expensive work into memoized components
• rerender-memo-with-default-value — SHOULD hoist default non-primitive props
• rerender-dependencies — SHOULD use primitive dependencies in effects
• rerender-derived-state — SHOULD subscribe to derived booleans, not raw values
• rerender-derived-state-no-effect — MUST derive state during render, not effects
• rerender-functional-setstate — SHOULD use functional setState for stable callbacks
• rerender-lazy-state-init — SHOULD pass function to useState for expensive values
• rerender-simple-expression-in-memo — SHOULD NOT use memo for simple primitives
• rerender-move-effect-to-event — SHOULD put interaction logic in event handlers
• rerender-transitions — MAY use startTransition for non-urgent updates
• rerender-use-ref-transient-values — SHOULD use refs for transient frequent values

6. Rendering Performance (MEDIUM)

• rendering-animate-svg-wrapper — SHOULD animate div wrapper, not SVG element
• rendering-content-visibility — MAY use content-visibility for long lists
• rendering-hoist-jsx — SHOULD extract static JSX outside components
• rendering-svg-precision — SHOULD reduce SVG coordinate precision
• rendering-hydration-no-flicker — SHOULD use inline script for client-only data
• rendering-hydration-suppress-warning — MAY suppress expected mismatches
• rendering-activity — MAY use Activity component for show/hide
• rendering-conditional-render — MUST use ternary, not && for conditionals
• rendering-usetransition-loading — SHOULD prefer useTransition for loading state

7. JavaScript Performance (LOW-MEDIUM)

• js-batch-dom-css — SHOULD group CSS changes via classes or cssText
• js-index-maps — SHOULD build Map for repeated lookups
• js-cache-property-access — SHOULD cache object properties in loops
• js-cache-function-results — MAY cache function results in module-level Map
• js-cache-storage — SHOULD cache localStorage/sessionStorage reads
• js-combine-iterations — SHOULD combine multiple filter/map into one loop
• js-length-check-first — SHOULD check array length before expensive comparison
• js-early-exit — SHOULD return early from functions
• js-hoist-regexp — SHOULD hoist RegExp creation outside loops
• js-min-max-loop — MAY use loop for min/max instead of sort
• js-set-map-lookups — SHOULD use Set/Map for O(1) lookups
• js-tosorted-immutable — SHOULD use toSorted() for immutability

8. Advanced Patterns (LOW)

• advanced-event-handler-refs — MAY store event handlers in refs
• advanced-init-once — SHOULD initialize app once per app load
• advanced-use-latest — MAY use 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