react-best-practices

**IMPORTANT**: Any change to React or Next.js code must read through this skill first. React and Next.js guidelines from Vercel Engineering covering visual i...

Security Scan

This is a documentation-only React/Next.js best-practices skill whose files and instructions match its stated purpose; it appears coherent and carries low technical risk, but provenance (author/hosting) is unclear so verify origin before trusting it as an authoritative policy.

Purpose & Capability

Name/description promise performance and visual-stability guidance; the package contains 45 rule files and a SKILL.md with examples and explanations that directly implement that purpose. There are no unrelated env vars, binaries, or install steps requested.

Instruction Scope

SKILL.md and rule files are documentation and code examples only. They do not instruct the agent to read or exfiltrate host files, access credentials, run remote endpoints, or execute system commands. Examples reference localStorage/cookies in code snippets (as usage examples) but do not instruct the agent to access the runtime environment.

Install Mechanism

No install spec and no packaged binaries — this is instruction-only and will not write code or binaries to disk during install, which is the lowest-risk install profile.

Credentials

No required environment variables, credentials, or config paths are declared. The examples mention browser APIs (localStorage, cookies) in the context of app code, which is appropriate for the stated purpose and not a credential request.

Persistence & Privilege

The skill is not forced-always and does not request elevated or persistent privileges. Autonomous invocation is allowed (platform default) but there is no additional persistent footprint or self-modifying install behavior in the package.

Assessment

This skill is a documentation bundle of React/Next.js best-practices and is internally consistent with its description. It is low-risk because it contains only text and example code and requests no credentials or installs. Two practical checks before use: (1) verify the author/provenance — the metadata lists "vercel" as author but the registry source/homepage are missing; confirm you trust this copy before treating it as official policy, and (2) when you apply examples that manipulate localStorage/cookies or inject inline scripts, review them for safety in your app context (avoid storing secrets in localStorage, sanitize any injected content). If you need an official Vercel guideline, fetch it from Vercel's verified website or repository to ensure you have the authoritative and up-to-date source.

Vercel React Best Practices

Comprehensive performance optimization guide for React and Next.js applications, maintained by Vercel. Contains 45 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Refactoring existing React/Next.js code
• Writing new React components or Next.js pages
• Fixing visual instability or layout shift (CLS) issues
• Preventing flickering when reading from localStorage or cookies
• Handling hydration mismatches with client-only data
• Optimizing font loading (FOIT/FOUT prevention)
• Adding proper dimensions to images and dynamic content

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-cache-react - Use React.cache() for per-request deduplication
• server-cache-lru - Use LRU cache for cross-request caching
• 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

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-dependencies - Use primitive dependencies in effects
• rerender-derived-state - Subscribe to derived booleans, not raw values
• rerender-functional-setstate - Use functional setState for stable callbacks
• rerender-lazy-state-init - Pass function to useState for expensive values
• rerender-transitions - Use startTransition for non-urgent updates

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-activity - Use Activity component for show/hide
• rendering-conditional-render - Use ternary, not && for conditionals

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

8. Advanced Patterns (LOW)

• advanced-event-handler-refs - Store event handlers in refs
• 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
rules/_sections.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