asymmetric-al/vercel-react-best-practices

Vercel React Best Practices

Performance optimization guide for React and Next.js applications (adapted from Vercel Engineering's agent skill; see references/upstream.md for attribution).

When to Apply

Use this skill when:

• Writing new React components or Next.js pages/layouts
• Implementing data fetching (server-side or client-side)
• Reviewing code for performance issues (slow TTFB, slow LCP, janky interactions)
• Refactoring React/Next.js code where perf regressions are likely
• Optimizing bundle size or load times

Do not use this skill when:

• The task is purely visual/styling with no behavioral change (use react-component-dev instead)
• The task is primarily caching/invalidation strategy (use cache-components instead)

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 (Rule IDs)

These are the rule IDs used by the upstream skill; use them as a vocabulary when discussing fixes.

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 partial dependency parallelism (upstream calls this "better-all")
• 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 the 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 an LRU cache for cross-request caching
• server-dedup-props - Avoid duplicate serialization in RSC props
• 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

6) Rendering Performance (MEDIUM)

• rendering-animate-svg-wrapper - Animate a div wrapper, not the 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 - Prefer ternary over && for conditionals
• rendering-usetransition-loading - Prefer useTransition for loading state

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 a 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

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

Workflow

1. Pick a measurable target (load time, TTFB, interaction jank, bundle size, server cost).
2. Start with waterfalls (async-*): restructure to parallelize independent work and stream via Suspense where appropriate.
3. Reduce bundle cost (bundle-*): avoid barrel imports; split heavy, rarely-used UI with next/dynamic; defer third-party scripts.
4. Fix server-to-client boundaries (server-*): avoid over-serializing props; keep heavy compute and secrets on the server; parallelize server fetches.
5. Fix client fetching (client-*): dedupe requests, avoid repeated global listeners, keep storage reads/versioning sane.
6. Remove rerender hotspots (rerender-*): prefer derived state, stable callbacks, and moving effects to events when possible.
7. Triage rendering + JS micro-opts (rendering-*, js-*): only after structural issues are addressed.
8. Verify with repo gates and any perf checks the task requires:
   • bun run format:check && bun run lint && bun run typecheck && bun run build && bun run test:unit
   • Optional: bun run test:perf (Playwright Web Vitals checks) when relevant

Checklist

• [ ] No obvious async waterfalls (parallelize independent work, stream with Suspense)
• [ ] No accidental bundle bloat (avoid barrel imports, consider next/dynamic for heavy UI)
• [ ] Server/client boundary is intentional (minimal props; no heavy objects serialized)
• [ ] Client data fetching is deduped; no repeated global listeners
• [ ] Rerenders are controlled (derived state, stable handlers, memo used only when warranted)
• [ ] Rendering issues addressed (long lists, hydration mismatches handled intentionally)
• [ ] Repo CI gates pass locally

Additional Resources

• Upstream source + attribution: references/upstream.md