Vercel React Best Practices

# 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.

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`