vercel-react-best-practices

name	vercel-react-best-practices
description	React and Next.js performance optimization guidelines from Vercel Engineering. Auto-trigger: when writing, reviewing, or refactoring React/Next.js components, pages, data fetching, or bundle optimization code.
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 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

Skip Conditions

• Skip if the project is not React or Next.js (check for react in package.json dependencies)
• Skip if the change is purely content/copy - no component or data flow changes
• Skip if the file being edited is a config file (next.config, tailwind.config, etc.) with no component logic

Priority Mode

When to use: Quick code review, single-component changes, or time-constrained work.

Quick mode: Check only CRITICAL rules (Categories 1-2: Waterfalls + Bundle Size). These catch the highest-impact issues. Skip Categories 3-8.

Full mode (default): Check all applicable categories based on what the code touches.

Code touches...	Check categories
async/await, fetch, data loading	1 (Waterfalls)
import, dynamic, third-party libs	2 (Bundle Size)
Server components, cache(), streaming	3 (Server-Side)
useSWR, client-side fetching	4 (Client Data)
useState, useEffect, useMemo	5 (Re-renders)
JSX, SVG, CSS-in-JS, animations	6 (Rendering)
Loops, lookups, data transforms	7 (JS Perf)
Refs, custom hooks, advanced patterns	8 (Advanced)

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

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

DO NOT

• Apply low-priority optimizations (Categories 5-8) before fixing CRITICAL issues (Categories 1-2) - waterfall elimination and bundle size have 10x more impact than re-render optimization
• Add useMemo/useCallback everywhere "just in case" - memoization has a cost; only apply when profiling shows a bottleneck or when passing callbacks to memoized children
• Convert working client components to server components without checking if they use hooks, event handlers, or browser APIs - these will break silently or require "use client" anyway
• Optimize code you haven't measured - "this looks slow" is not a finding; use React DevTools Profiler or browser performance tools first
• Apply React.cache() in client components - it only works in server components within a single request

Quality Self-Check

After applying optimizations or reviewing code against these rules:

1. No regressions - does the component still render correctly? (check with browser-use if available)
2. Rules matched to code - did you only flag rules relevant to what the code actually does?
3. Priority respected - are CRITICAL issues (Categories 1-2) addressed before MEDIUM/LOW?
4. No premature optimization - is every change addressing a real pattern, not a hypothetical?

Full Compiled Document

For the complete guide with all rules expanded: AGENTS.md