React

React Best Practices Audit

Focused audit layer for React and Next.js runtime/rendering issues. Start with the universal checks, then route to the reference that matches the user's specific question.

Universal Checks (no exceptions — run these every time)

These checks take seconds to scan for and catch common high-impact mistakes. Skip them only if you have already verified them in the same session.

1. Components defined inside other components They are re-created on every render; state resets and memoization is lost. Extract to module scope.

2. Array index as key in dynamic lists key is React's identity anchor. When lists reorder, insert, or delete, an index key reassigns identity to the wrong element. Use a stable ID from the data. Exception: truly static, never-reordered lists.

3. Derived state via useState + useEffect If state is only a calculation from props/state, compute it during render (optionally useMemo for expensive work) instead of syncing with an Effect.

4. Data fetching in useEffect without cleanup Slow responses can overwrite fresh data. Use cleanup/cancellation, TanStack Query, or Server Components so stale requests cannot win the race.

5. Unmemoized Context provider value New object/function references make every consumer re-render. Memoize the provider value or split data and API contexts.

6. Server Actions for client-side data reads Server Actions serialize calls and are for mutations. Reads should use RSC, route handlers, or a client data layer that can parallelize and dedupe.

7. React.FC and forwardRef as default patterns Prefer plain function components. React.FC adds noise and misleading children behavior; React 19 makes ref a normal prop for new code. Load the React 19 reference for version-specific migration details.

Ownership Boundary with Frontend

react owns React runtime/rendering issues: Effects, re-renders, Context, keys, Server Actions, hydration, React 19 APIs, waterfalls, and bundle/runtime performance. frontend owns broader TypeScript, UI accessibility, browser security, Next.js file boundaries, Tailwind/component API patterns, and frontend file structure.

Priority Checklist (run after universal checks)

Use this for full audits. The universal checks above catch the most common bugs; this checklist catches the rest.

CRITICAL — Eliminating Waterfalls:

• No sequential awaits that could run in parallel
• Promise.all for independent operations
• API routes don't chain awaits unnecessarily
• Strategic Suspense boundaries for streaming

CRITICAL — Bundle Size:

• Direct imports (no barrel files)
• Dynamic imports for heavy components
• Conditional module loading
• Non-critical libs deferred
• Preload based on user intent

HIGH — Server-Side Performance:

• Server Actions have authentication
• RSC props minimized (only needed data)
• Parallel data fetching via composition
• React.cache() for per-request dedup
• after() for non-blocking operations
• No duplicate serialization

MEDIUM-HIGH — Client Data Fetching:

• SWR/TanStack Query for automatic deduplication
• Passive event listeners for scroll
• Global event listeners deduplicated
• localStorage versioned and minimal

MEDIUM — Re-render Optimization:

• Functional setState updates (no stale closures)
• Lazy state initialization for expensive values
• Narrow effect dependencies (primitives > objects)
• useTransition for non-urgent updates
• Derived state calculated during render (not via useEffect)
• Interaction logic in event handlers (not state + effect)
• useRef for transient values (mouse trackers, intervals)
• Extract to memoized components when re-render scope is too broad
• No unnecessary useMemo wrapping (prefer React Compiler if available)

MEDIUM — Rendering Performance:

• content-visibility for long lists
• useTransition over manual loading states
• Explicit conditional rendering (not && with truthy non-booleans)
• suppressHydrationWarning for known server/client differences
• Prevent hydration mismatch without UI flicker

LOW-MEDIUM — JavaScript Performance:

• Set/Map for O(1) lookups
• Early returns in functions
• Cached repeated function calls
• toSorted() over sort() for immutability
• Avoid layout thrashing
• Hoist RegExp creation outside render

LOW — Advanced Patterns:

• Event handlers in refs for stable references
• useEffectEvent for stable callback refs
• App initialization with module-level guards (not useEffect)

Report Format

For each violation:

[PRIORITY] Rule Name
File: path/to/file.tsx:line
Issue: [description of the problem]
Fix: [code example showing correct pattern]

After the full checklist, provide:

1. Violation Count by Priority — CRITICAL: N · HIGH: N · MEDIUM: N · LOW: N
2. Top 3 Highest-Impact Fixes — brief description + expected improvement
3. Overall Assessment — Pass / Needs Work / Critical Issues

Stack Context — Determine Before Routing

Before loading references, determine which stack the user is on:

• Next.js App Router → RSC, Server Actions, streaming, and react-19.md action props / useActionState patterns apply
• Vite SPA (React Router / TanStack Router) → no RSC, TanStack Query is the primary data layer, tanstack-query.md is the first reference to reach for
• Both → re-renders.md, useeffect-antipatterns.md, portals-and-stacking-context.md, and bundle-and-perf-investigation.md are stack-agnostic

If unsure, look at the imports: next/ imports → App Router; react-router-dom or @tanstack/react-router → Vite SPA.

References — Priority Table

Load the reference that matches the issue. Higher-priority references fix more users / cause more damage when missed — resolve CRITICAL before MEDIUM.

When to load a reference

• Load the highest-priority matching reference first — fix CRITICAL issues before reaching for MEDIUM optimization references
• A rule-level question ("should I do X?") → load the reference for that category
• A conceptual question ("why does X happen?") → load the same reference; they include both explanation and the correct pattern
• When multiple references match, load in priority order — they don't overlap
• The universal checks + priority checklist above cover the most common violations without loading any reference; only load when depth is needed beyond the checklist