vercel-react-best-practices

---
name: vercel-react-best-practices
description: React and Next.js performance optimization guidelines from Vercel Engineering. This skill should be used when writing, reviewing, or refactoring React/Next.js code to ensure optimal performance patterns. Triggers on tasks involving React components, Next.js pages, data fetching, bundle optimization, or performance improvements.
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 70 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-cheap-condition-before-await` - Check cheap sync conditions before awaiting flags or remote values
- `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-analyzable-paths` - Prefer statically analyzable import and file-system paths to avoid broad bundles and traces
- `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-no-shared-module-state` - Avoid module-level mutable request state in RSC/SSR
- `server-serialization` - Minimize data passed to client components
- `server-parallel-fetching` - Restructure components to parallelize fetches
- `server-parallel-nested-fetching` - Chain nested fetches per item in Promise.all
- `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-split-combined-hooks` - Split hooks with independent dependencies
- `rerender-move-effect-to-event` - Put interaction logic in event handlers
- `rerender-transitions` - Use startTransition for non-urgent updates
- `rerender-use-deferred-value` - Defer expensive renders to keep input responsive
- `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
- `js-request-idle-callback` - Defer non-critical work to browser idle time

### 8. Advanced Patterns (LOW)

- `advanced-effect-event-deps` - Don't put `useEffectEvent` results in effect deps
- `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`

From sanity-io/plugins at a80a997, shown as plain text under its license: MIT (permissive). The license text is in .agents/skills/vercel-react-best-practices/SKILL.md.

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• AGENTS.md108 KB · a2778854c9ef
• README.md3.4 KB · 3e655903c32e
• rules/
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• advanced-use-latest.md1.1 KB · c115760625fc
• async-api-routes.md1.1 KB · ef4dbb68c3b9
• async-cheap-condition-before-await.md1.2 KB · 03107c1f70a2
• async-defer-await.md2.2 KB · 071bb14f6107
• async-dependencies.md1.3 KB · b698f6e4a3c8
• async-parallel.md646 B · 012a30bd3447
• async-suspense-boundaries.md2.5 KB · 1a561dabbdc2
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• client-event-listeners.md2.0 KB · bc2eb03c03c7
• client-localstorage-schema.md2.0 KB · 1b51a076745f
• client-passive-event-listeners.md1.6 KB · 95539c981635
• client-swr-dedup.md1.2 KB · f02956c64e4e
• js-batch-dom-css.md3.2 KB · 0d2b8b8f66ca
• js-cache-function-results.md1.9 KB · 08d12e0b7340
• js-cache-property-access.md532 B · 73e47431e748
• js-cache-storage.md1.6 KB · cd1298e3492a
• js-combine-iterations.md759 B · b3ab8de50367
• js-early-exit.md1.1 KB · a5cef0219aa4
• js-flatmap-filter.md1.4 KB · ad6e12eb5c36
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• js-index-maps.md847 B · bd8acd340c4c
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• rendering-activity.md560 B · 5c3298cf75f7
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• rendering-conditional-render.md928 B · 9640534f3010
• rendering-content-visibility.md813 B · 0c32222d524e
• rendering-hoist-jsx.md989 B · 5a129602e93c
• rendering-hydration-no-flicker.md2.2 KB · fa396b26f8c0
• rendering-hydration-suppress-warning.md850 B · fb6274dc927f
• rendering-resource-hints.md2.7 KB · 3c5ddda33f65
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• rerender-dependencies.md824 B · 17eb5830956f
• rerender-derived-state-no-effect.md1.2 KB · cb11ec76f50a
• rerender-derived-state.md727 B · 3a0e1a6cfb6e
• rerender-functional-setstate.md3.0 KB · 31122857688c
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• rerender-memo-with-default-value.md1.2 KB · 97044a6c3097
• rerender-memo.md1.1 KB · 16a1d24c378b
• rerender-move-effect-to-event.md1.3 KB · abc2cbf167be
• rerender-no-inline-components.md2.1 KB · e6b9c6b7fc88
• rerender-simple-expression-in-memo.md1.0 KB · ca98973e64a6
• rerender-split-combined-hooks.md1.8 KB · 16bb5a81b25f
• rerender-transitions.md1.0 KB · cb75c614e337
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• rerender-use-ref-transient-values.md1.7 KB · f1a649af9d1d
• server-after-nonblocking.md2.0 KB · 7ddd06c3c228
• server-auth-actions.md2.6 KB · 1edc82d04e59
• server-cache-lru.md1.3 KB · bc65304e2539
• server-cache-react.md2.2 KB · 20ea6f4a155e
• server-dedup-props.md2.1 KB · 061cd37f0da2
• server-hoist-static-io.md4.4 KB · e81a56e48fe9
• server-no-shared-module-state.md1.8 KB · c50e91ed200e
• server-parallel-fetching.md1.6 KB · 701f53759d70
• server-parallel-nested-fetching.md993 B · 859fb12a10bb
• server-serialization.md992 B · 1093a2b11900

75 files, 228 KB in total. Each file has a sha256 in the pin's manifest; the first 12 characters are shown.