1---
2name: vercel-react-best-practices
3description: 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.
4license: MIT
5metadata:
6  author: vercel
7  version: "1.0.0"
8---
9 
10# Vercel React Best Practices
11 
12Comprehensive 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.
13 
14## When to Apply
15 
16Reference these guidelines when:
17- Writing new React components or Next.js pages
18- Implementing data fetching (client or server-side)
19- Reviewing code for performance issues
20- Refactoring existing React/Next.js code
21- Optimizing bundle size or load times
22 
23## Rule Categories by Priority
24 
25| Priority | Category | Impact | Prefix |
26|----------|----------|--------|--------|
27| 1 | Eliminating Waterfalls | CRITICAL | `async-` |
28| 2 | Bundle Size Optimization | CRITICAL | `bundle-` |
29| 3 | Server-Side Performance | HIGH | `server-` |
30| 4 | Client-Side Data Fetching | MEDIUM-HIGH | `client-` |
31| 5 | Re-render Optimization | MEDIUM | `rerender-` |
32| 6 | Rendering Performance | MEDIUM | `rendering-` |
33| 7 | JavaScript Performance | LOW-MEDIUM | `js-` |
34| 8 | Advanced Patterns | LOW | `advanced-` |
35 
36## Quick Reference
37 
38### 1. Eliminating Waterfalls (CRITICAL)
39 
40- `async-cheap-condition-before-await` - Check cheap sync conditions before awaiting flags or remote values
41- `async-defer-await` - Move await into branches where actually used
42- `async-parallel` - Use Promise.all() for independent operations
43- `async-dependencies` - Use better-all for partial dependencies
44- `async-api-routes` - Start promises early, await late in API routes
45- `async-suspense-boundaries` - Use Suspense to stream content
46 
47### 2. Bundle Size Optimization (CRITICAL)
48 
49- `bundle-barrel-imports` - Import directly, avoid barrel files
50- `bundle-analyzable-paths` - Prefer statically analyzable import and file-system paths to avoid broad bundles and traces
51- `bundle-dynamic-imports` - Use next/dynamic for heavy components
52- `bundle-defer-third-party` - Load analytics/logging after hydration
53- `bundle-conditional` - Load modules only when feature is activated
54- `bundle-preload` - Preload on hover/focus for perceived speed
55 
56### 3. Server-Side Performance (HIGH)
57 
58- `server-auth-actions` - Authenticate server actions like API routes
59- `server-cache-react` - Use React.cache() for per-request deduplication
60- `server-cache-lru` - Use LRU cache for cross-request caching
61- `server-dedup-props` - Avoid duplicate serialization in RSC props
62- `server-hoist-static-io` - Hoist static I/O (fonts, logos) to module level
63- `server-no-shared-module-state` - Avoid module-level mutable request state in RSC/SSR
64- `server-serialization` - Minimize data passed to client components
65- `server-parallel-fetching` - Restructure components to parallelize fetches
66- `server-parallel-nested-fetching` - Chain nested fetches per item in Promise.all
67- `server-after-nonblocking` - Use after() for non-blocking operations
68 
69### 4. Client-Side Data Fetching (MEDIUM-HIGH)
70 
71- `client-swr-dedup` - Use SWR for automatic request deduplication
72- `client-event-listeners` - Deduplicate global event listeners
73- `client-passive-event-listeners` - Use passive listeners for scroll
74- `client-localstorage-schema` - Version and minimize localStorage data
75 
76### 5. Re-render Optimization (MEDIUM)
77 
78- `rerender-defer-reads` - Don't subscribe to state only used in callbacks
79- `rerender-memo` - Extract expensive work into memoized components
80- `rerender-memo-with-default-value` - Hoist default non-primitive props
81- `rerender-dependencies` - Use primitive dependencies in effects
82- `rerender-derived-state` - Subscribe to derived booleans, not raw values
83- `rerender-derived-state-no-effect` - Derive state during render, not effects
84- `rerender-functional-setstate` - Use functional setState for stable callbacks
85- `rerender-lazy-state-init` - Pass function to useState for expensive values
86- `rerender-simple-expression-in-memo` - Avoid memo for simple primitives
87- `rerender-split-combined-hooks` - Split hooks with independent dependencies
88- `rerender-move-effect-to-event` - Put interaction logic in event handlers
89- `rerender-transitions` - Use startTransition for non-urgent updates
90- `rerender-use-deferred-value` - Defer expensive renders to keep input responsive
91- `rerender-use-ref-transient-values` - Use refs for transient frequent values
92- `rerender-no-inline-components` - Don't define components inside components
93 
94### 6. Rendering Performance (MEDIUM)
95 
96- `rendering-animate-svg-wrapper` - Animate div wrapper, not SVG element
97- `rendering-content-visibility` - Use content-visibility for long lists
98- `rendering-hoist-jsx` - Extract static JSX outside components
99- `rendering-svg-precision` - Reduce SVG coordinate precision
100- `rendering-hydration-no-flicker` - Use inline script for client-only data
101- `rendering-hydration-suppress-warning` - Suppress expected mismatches
102- `rendering-activity` - Use Activity component for show/hide
103- `rendering-conditional-render` - Use ternary, not && for conditionals
104- `rendering-usetransition-loading` - Prefer useTransition for loading state
105- `rendering-resource-hints` - Use React DOM resource hints for preloading
106- `rendering-script-defer-async` - Use defer or async on script tags
107 
108### 7. JavaScript Performance (LOW-MEDIUM)
109 
110- `js-batch-dom-css` - Group CSS changes via classes or cssText
111- `js-index-maps` - Build Map for repeated lookups
112- `js-cache-property-access` - Cache object properties in loops
113- `js-cache-function-results` - Cache function results in module-level Map
114- `js-cache-storage` - Cache localStorage/sessionStorage reads
115- `js-combine-iterations` - Combine multiple filter/map into one loop
116- `js-length-check-first` - Check array length before expensive comparison
117- `js-early-exit` - Return early from functions
118- `js-hoist-regexp` - Hoist RegExp creation outside loops
119- `js-min-max-loop` - Use loop for min/max instead of sort
120- `js-set-map-lookups` - Use Set/Map for O(1) lookups
121- `js-tosorted-immutable` - Use toSorted() for immutability
122- `js-flatmap-filter` - Use flatMap to map and filter in one pass
123- `js-request-idle-callback` - Defer non-critical work to browser idle time
124 
125### 8. Advanced Patterns (LOW)
126 
127- `advanced-effect-event-deps` - Don't put `useEffectEvent` results in effect deps
128- `advanced-event-handler-refs` - Store event handlers in refs
129- `advanced-init-once` - Initialize app once per app load
130- `advanced-use-latest` - useLatest for stable callback refs
131 
132## How to Use
133 
134Read individual rule files for detailed explanations and code examples:
135 
136```
137rules/async-parallel.md
138rules/bundle-barrel-imports.md
139```
140 
141Each rule file contains:
142- Brief explanation of why it matters
143- Incorrect code example with explanation
144- Correct code example with explanation
145- Additional context and references
146 
147## Full Compiled Document
148 
149For the complete guide with all rules expanded: `AGENTS.md`
150