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