Vl React Best Practices

---
name: vl-react-best-practices
description: React and Next.js performance optimization guidelines from Vercel Engineering (62 prioritized rules across 8 categories). Use when writing, reviewing, or refactoring React/Next.js code to apply optimal performance patterns. EN triggers - "react performance", "next.js best practices", "optimize re-renders", "fix waterfalls", "bundle size", "review this component". FR triggers - "bonnes pratiques react", "optimiser le rendu", "perf next.js", "reduire le bundle", "refactor react", "revoir ce composant". Fires on React components, Next.js pages, data fetching, bundle optimization, or render-performance work.
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 62 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

> Portability note: this skill is self-contained (rules live in `rules/*.md` + `AGENTS.md` beside this file) and has NO VPS-only dependency — it runs in any Claude Code workspace.

## Single-voice craft (do NOT parallelize)

Applying these guidelines is one coherent engineering pass, not a fan-out audit. Do NOT split rule categories across parallel agents — a real fix to a re-render often depends on a data-fetching or bundle change in the same file, and disjoint agents lose that cross-cutting context (R-SCOPE: one writer per file). Read the relevant rule files, hold the whole component in view, and refactor in a single voice.

Craft upgrades when applying:
1. Priority-first, evidence-first. Walk categories top-down (Waterfalls -> Bundle -> Server -> Client -> Re-render -> Rendering -> JS -> Advanced). Before applying a rule, cite the exact `rules/<rule>.md` it comes from and the line in the user's code it targets — never invent a rule id (no-hallucination guardrail: every rule id MUST exist in the Quick Reference table or `rules/`).
2. Measure, don't guess. A "perf fix" that isn't on a real hot path is noise. Name the symptom (extra render, blocking await, oversized chunk) before the fix; if you can't name a measurable symptom, skip the rule rather than over-engineer (Karpathy: simplicity, surgical changes).
3. Surgical diffs only. Change the lines the rule targets; do not restyle, rename, or refactor adjacent code. Preserve existing behavior — a faster component that changes output is a regression.

## Output contract

When this skill is applied, produce:
- A short list of applied rules, each as `rule-id (rules/<file>.md) -> file:line` with a one-line rationale.
- The surgical diff(s) — modified code only, behavior-preserving.
- Any rule deliberately skipped, with the reason (not a hot path / would change behavior).

## Verify

After applying:
1. Typecheck / build the affected package (e.g. `tsc --noEmit` or the project build) — green build is mandatory (L0/L1: runtime is the truth).
2. Confirm rendered output / behavior is unchanged (no new console errors, same DOM).
3. Re-read each applied rule's `rules/<file>.md` and confirm the code now matches the "Correct" example.
Done = build green + behavior unchanged + every cited rule id verified to exist.

## 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-auth-actions` - Authenticate server actions like API routes
- `server-cache-react` - Use React.cache() for per-request deduplication
- `server-cache-lru` - Use LRU cache for cross-request caching
- `server-dedup-props` - Avoid duplicate serialization in RSC props
- `server-hoist-static-io` - Hoist static I/O (fonts, logos) to module level
- `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
- `client-passive-event-listeners` - Use passive listeners for scroll
- `client-localstorage-schema` - Version and minimize localStorage data

### 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-memo-with-default-value` - Hoist default non-primitive props
- `rerender-dependencies` - Use primitive dependencies in effects
- `rerender-derived-state` - Subscribe to derived booleans, not raw values
- `rerender-derived-state-no-effect` - Derive state during render, not effects
- `rerender-functional-setstate` - Use functional setState for stable callbacks
- `rerender-lazy-state-init` - Pass function to useState for expensive values
- `rerender-simple-expression-in-memo` - Avoid memo for simple primitives
- `rerender-move-effect-to-event` - Put interaction logic in event handlers
- `rerender-transitions` - Use startTransition for non-urgent updates
- `rerender-use-ref-transient-values` - Use refs for transient frequent values
- `rerender-no-inline-components` - Don't define components inside components

### 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-hydration-suppress-warning` - Suppress expected mismatches
- `rendering-activity` - Use Activity component for show/hide
- `rendering-conditional-render` - Use ternary, not && for conditionals
- `rendering-usetransition-loading` - Prefer useTransition for loading state
- `rendering-resource-hints` - Use React DOM resource hints for preloading
- `rendering-script-defer-async` - Use defer or async on script tags

### 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
- `js-flatmap-filter` - Use flatMap to map and filter in one pass

### 8. Advanced Patterns (LOW)

- `advanced-event-handler-refs` - Store event handlers in refs
- `advanced-init-once` - Initialize app once per app load
- `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
```

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`