vl-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. > Portability note: this skill is self-contained (rules live in `rules/*.md` + `AGENTS.md` beside this file) and has NO VPS-only dependency — it runs in any Claude Code workspace. ## Single-voice craft (do NOT parallelize) Applying these guidelines is one coherent engineering pass, not a fan-out audit. Do NOT split rule categories across parallel agents — a real fix to a re-render often depends on a data-fetching or bundle change in the same file, and disjoint agents lose that cross-cutting context (R-SCOPE: one writer per file). Read the relevant rule files, hold the whole component in view, and refactor in a single voice. Craft upgrades when applying: 1. Priority-first, evidence-first. Walk categories top-down (Waterfalls -> Bundle -> Server -> Client -> Re-render -> Rendering -> JS -> Advanced). Before applying a rule, cite the exact `rules/<rule>.md` it comes from and the line in the user's code it targets — never invent a rule id (no-hallucination guardrail: every rule id MUST exist in the Quick Reference table or `rules/`). 2. Measure, don't guess. A "perf fix" that isn't on a real hot path is noise. Name the symptom (extra render, blocking await, oversized chunk) before the fix; if you can't name a measurable symptom, skip the rule rather than over-engineer (Karpathy: simplicity, surgical changes). 3. Surgical diffs only. Change the lines the rule targets; do not restyle, rename, or refactor adjacent code. Preserve existing behavior — a faster component that changes output is a regression. ## Output contract When this skill is applied, produce: - A short list of applied rules, each as `rule-id (rules/<file>.md) -> file:line` with a one-line rationale. - The surgical diff(s) — modified code only, behavior-preserving. - Any rule deliberately skipped, with the reason (not a hot path / would change behavior). ## Verify After applying: 1. Typecheck / build the affected package (e.g. `tsc --noEmit` or the project build) — green build is mandatory (L0/L1: runtime is the truth). 2. Confirm rendered output / behavior is unchanged (no new console errors, same DOM). 3. Re-read each applied rule's `rules/<file>.md` and confirm the code now matches the "Correct" example. Done = build green + behavior unchanged + every cited rule id verified to exist. ## 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`