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react-best-practices skill

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This skill helps you optimize React and Next.js performance by applying high-impact best practices across components, pages, and data fetching.

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---
name: react-best-practices
description: React and Next.js performance optimization guidelines from Vercel Engineering. Use when writing, reviewing, or refactoring React/Next.js code. 45 rules across 8 categories prioritized by impact.
---

# 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

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`

Overview

This skill provides Actionable React and Next.js performance optimization guidelines distilled from Vercel Engineering. It bundles 45 targeted rules across eight prioritized categories to guide writing, reviewing, and automated refactoring of React/Next.js code. Use it to reduce waterfalls, shrink bundles, and streamline server/client data flows for faster, more reliable apps.

How this skill works

The skill inspects code and flags patterns matched to high-impact rules such as eliminating async waterfalls, reducing bundle weight, and avoiding unnecessary re-renders. For each matched rule it offers a concise explanation, the performance impact, and a concrete corrective pattern or code transform. Rules are prioritized so automated fixes and reviewer prompts focus on changes with the largest runtime or build-time benefits first.

When to use it

Writing new React components or Next.js pages to follow performant defaults
Reviewing pull requests for regressions or performance anti-patterns
Refactoring legacy components to reduce bundle size and hydration cost
Optimizing server or client data fetching and caching strategies
Diagnosing slow renders, excessive network requests, or large bundles

Best practices

Eliminate serial awaits by running independent async work in parallel
Load heavy or third-party modules dynamically and defer analytics until after hydration
Minimize data sent to client components and use per-request or LRU caches on the server
Use memoization, primitive effect dependencies, and functional setState to avoid needless re-renders
Extract static JSX and heavy calculations out of render paths and prefer Suspense/streaming for progressive content

Example use cases

Convert sequential API calls in an API route to Promise.all for faster responses
Replace barrel imports with direct imports to reduce tree-shaking misses and smaller bundles
Refactor a complex form to memoize derived state and avoid subscribing to large objects
Use dynamic imports for a charting library so it loads only when a chart is displayed
Add server-side React.cache or an LRU cache to deduplicate repeated fetches across requests

FAQ

Which rules should I prioritize?

Start with Eliminating Waterfalls and Bundle Size Optimization rules—the largest, most immediate wins come from parallelizing async work and reducing shipped code.

Can I apply rules automatically?

Yes. Many rules map to deterministic transforms (e.g., Promise.all, dynamic imports, memo extraction) that can be automated or offered as review suggestions.