Vercel React Best Practices

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 57 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-defer-await` - Move await into branches where actually used
41- `async-parallel` - Use Promise.all() for independent operations
42- `async-dependencies` - Use better-all for partial dependencies
43- `async-api-routes` - Start promises early, await late in API routes
44- `async-suspense-boundaries` - Use Suspense to stream content
45 
46### 2. Bundle Size Optimization (CRITICAL)
47 
48- `bundle-barrel-imports` - Import directly, avoid barrel files
49- `bundle-dynamic-imports` - Use next/dynamic for heavy components
50- `bundle-defer-third-party` - Load analytics/logging after hydration
51- `bundle-conditional` - Load modules only when feature is activated
52- `bundle-preload` - Preload on hover/focus for perceived speed
53 
54### 3. Server-Side Performance (HIGH)
55 
56- `server-auth-actions` - Authenticate server actions like API routes
57- `server-cache-react` - Use React.cache() for per-request deduplication
58- `server-cache-lru` - Use LRU cache for cross-request caching
59- `server-dedup-props` - Avoid duplicate serialization in RSC props
60- `server-serialization` - Minimize data passed to client components
61- `server-parallel-fetching` - Restructure components to parallelize fetches
62- `server-after-nonblocking` - Use after() for non-blocking operations
63 
64### 4. Client-Side Data Fetching (MEDIUM-HIGH)
65 
66- `client-swr-dedup` - Use SWR for automatic request deduplication
67- `client-event-listeners` - Deduplicate global event listeners
68- `client-passive-event-listeners` - Use passive listeners for scroll
69- `client-localstorage-schema` - Version and minimize localStorage data
70 
71### 5. Re-render Optimization (MEDIUM)
72 
73- `rerender-defer-reads` - Don't subscribe to state only used in callbacks
74- `rerender-memo` - Extract expensive work into memoized components
75- `rerender-memo-with-default-value` - Hoist default non-primitive props
76- `rerender-dependencies` - Use primitive dependencies in effects
77- `rerender-derived-state` - Subscribe to derived booleans, not raw values
78- `rerender-derived-state-no-effect` - Derive state during render, not effects
79- `rerender-functional-setstate` - Use functional setState for stable callbacks
80- `rerender-lazy-state-init` - Pass function to useState for expensive values
81- `rerender-simple-expression-in-memo` - Avoid memo for simple primitives
82- `rerender-move-effect-to-event` - Put interaction logic in event handlers
83- `rerender-transitions` - Use startTransition for non-urgent updates
84- `rerender-use-ref-transient-values` - Use refs for transient frequent values
85 
86### 6. Rendering Performance (MEDIUM)
87 
88- `rendering-animate-svg-wrapper` - Animate div wrapper, not SVG element
89- `rendering-content-visibility` - Use content-visibility for long lists
90- `rendering-hoist-jsx` - Extract static JSX outside components
91- `rendering-svg-precision` - Reduce SVG coordinate precision
92- `rendering-hydration-no-flicker` - Use inline script for client-only data
93- `rendering-hydration-suppress-warning` - Suppress expected mismatches
94- `rendering-activity` - Use Activity component for show/hide
95- `rendering-conditional-render` - Use ternary, not && for conditionals
96- `rendering-usetransition-loading` - Prefer useTransition for loading state
97 
98### 7. JavaScript Performance (LOW-MEDIUM)
99 
100- `js-batch-dom-css` - Group CSS changes via classes or cssText
101- `js-index-maps` - Build Map for repeated lookups
102- `js-cache-property-access` - Cache object properties in loops
103- `js-cache-function-results` - Cache function results in module-level Map
104- `js-cache-storage` - Cache localStorage/sessionStorage reads
105- `js-combine-iterations` - Combine multiple filter/map into one loop
106- `js-length-check-first` - Check array length before expensive comparison
107- `js-early-exit` - Return early from functions
108- `js-hoist-regexp` - Hoist RegExp creation outside loops
109- `js-min-max-loop` - Use loop for min/max instead of sort
110- `js-set-map-lookups` - Use Set/Map for O(1) lookups
111- `js-tosorted-immutable` - Use toSorted() for immutability
112 
113### 8. Advanced Patterns (LOW)
114 
115- `advanced-event-handler-refs` - Store event handlers in refs
116- `advanced-init-once` - Initialize app once per app load
117- `advanced-use-latest` - useLatest for stable callback refs
118 
119## How to Use
120 
121Read individual rule files for detailed explanations and code examples:
122 
123```
124rules/async-parallel.md
125rules/bundle-barrel-imports.md
126```
127 
128Each rule file contains:
129- Brief explanation of why it matters
130- Incorrect code example with explanation
131- Correct code example with explanation
132- Additional context and references
133 
134## Full Compiled Document
135 
136For the complete guide with all rules expanded: `AGENTS.md`
137