---
name: react-best-practices
description: React performance optimization guidelines from Vercel Engineering. This skill should be used proactively when writing, reviewing, or refactoring React code to ensure optimal performance patterns. Triggers on tasks involving React components, bundle optimization, or performance improvements.
---

# React Best Practices

Comprehensive performance optimization guide for React applications, adapted from Vercel Engineering. Contains 30 rules across 7 categories, prioritized by impact to guide automated refactoring and code generation.

> **Note**: This version has been customized for pure React 19.2:
> - Server-side data fetching and SWR rules removed (project uses react-query)
> - Next.js-specific rules removed (next/dynamic, SSR hydration, etc.)
> - **React Compiler is enabled** - Memoization rules (`memo`, `useMemo`, `useCallback`) removed as the compiler handles this automatically

## When to Apply

Reference these guidelines when:
- Writing new React components
- Reviewing code for performance issues
- Refactoring existing React 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 | Client-Side Patterns | MEDIUM-HIGH | `client-` |
| 4 | Re-render Optimization | MEDIUM | `rerender-` |
| 5 | Rendering Performance | MEDIUM | `rendering-` |
| 6 | JavaScript Performance | LOW-MEDIUM | `js-` |
| 7 | 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-suspense-boundaries` - Use Suspense to stream content

### 2. Bundle Size Optimization (CRITICAL)

- `bundle-barrel-imports` - Import directly, avoid barrel files
- `bundle-conditional` - Load modules only when feature is activated
- `bundle-preload` - Preload on hover/focus for perceived speed

### 3. Client-Side Patterns (MEDIUM-HIGH)

- `client-localstorage-schema` - Type-safe localStorage access
- `client-passive-event-listeners` - Use passive event listeners for scroll/touch

### 4. Re-render Optimization (MEDIUM)

- `rerender-defer-reads` - Don't subscribe to state only used in callbacks
- `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

### 5. Rendering Performance (MEDIUM)

- `rendering-animate-svg-wrapper` - Animate div wrapper, not SVG element
- `rendering-content-visibility` - Use content-visibility for long lists
- `rendering-svg-precision` - Reduce SVG coordinate precision
- `rendering-activity` - Use Activity component for show/hide
- `rendering-conditional-render` - Use ternary, not && for conditionals

### 6. 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

### 7. Advanced Patterns (LOW)

- `advanced-event-handler-refs` - Store event handlers in 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`