react-best-practices

name	react-best-practices
description	React performance optimization guidelines from Vercel Engineering. Contains 45 rules across 8 categories. Use when optimizing React components, bundle size, or data fetching patterns. IMPORTANT: This codebase uses Vite + TanStack Router (SPA), NOT Next.js. See applicability notes below for which rules apply.
license	MIT
metadata	{"author":"vercel","version":"1.0.0"}

Vercel React Best Practices

Comprehensive performance optimization guide for React applications, maintained by Vercel. Contains 45 rules across 8 categories, prioritized by impact.

Applicability Warning

This codebase uses Vite + TanStack Router (client-side SPA), NOT Next.js.

Rules to SKIP (Not Applicable)

These rules reference Next.js-specific APIs or SSR patterns that don't exist in this SPA:

Rule	Why Skip
async-api-routes	Next.js API routes - we use Spring Boot backend
async-suspense-boundaries	RSC streaming - not applicable to CSR SPA
server-cache-react	React.cache() is RSC-only
server-cache-lru	Server-side caching - backend handles this
server-serialization	RSC serialization - not applicable
server-parallel-fetching	RSC data fetching - we use TanStack Query
server-after-nonblocking	next/server after() - Next.js only
rendering-hydration-no-flicker	SSR hydration - Vite is pure CSR
rerender-memo	React Compiler handles this automatically
bundle-dynamic-imports	Use React.lazy() instead of next/dynamic
client-swr-dedup	We use TanStack Query, not SWR

Rules to APPLY

Category	Applicable Rules
async-*	async-defer-await, async-parallel, async-dependencies
bundle-*	bundle-barrel-imports, bundle-defer-third-party, bundle-conditional, bundle-preload
client-*	client-event-listeners
rerender-*	All EXCEPT rerender-memo (compiler handles it)
rendering-*	rendering-animate-svg-wrapper, rendering-content-visibility, rendering-hoist-jsx, rendering-svg-precision, rendering-conditional-render
js-*	All rules fully applicable
advanced-*	All rules fully applicable

React Compiler Note

This codebase has React Compiler enabled (babel-plugin-react-compiler). Do NOT add manual memoization (useMemo, useCallback, React.memo) for new code. The compiler handles memoization automatically.

When to Apply

Reference these guidelines when:

• Writing new React components
• Implementing client-side data fetching
• Reviewing code for performance issues
• Optimizing bundle size or load times
• NOT when working with server-side patterns (this is a SPA)

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

Read individual rule files for detailed explanations and code examples:

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