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

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