react

Comprehensive React 19/19.2 best-practices guide for AI agents. Contains 49 rules across 9 categories, prioritized by impact from critical (concurrent rendering, server components) through to cross-cutting codebase hygiene (dedup, dead code, boundary coherence). Reflects React 19 headline changes: ref as a regular prop (forwardRef deprecated), native document metadata, resource preload APIs, useActionState, useOptimistic, use() hook, and <Context> as provider.

Rule files describe pattern shapes (not API names) and open with a "Shapes to recognize" section listing 2–4 syntactic disguises the same break can wear. Selected high-value rules (those whose disguises are most common in real codebases — form actions, ref-as-prop, derived state, context, the use() hook, useCallback/memo pairing) include an extra concrete "In disguise" incorrect/correct example pair to teach pattern detection beyond the grep-friendly cases.

When to Apply

• Writing new React components or refactoring existing ones
• Auditing or modernizing a directory, PR, or whole repository (see references/_review-algorithm.md for the required procedure)
• Migrating from React 18 to React 19 (forwardRef → ref-as-prop, <Context.Provider> → <Context>, useFormState → useActionState)
• Optimizing re-render performance or bundle size
• Using concurrent features (useTransition, useDeferredValue, Activity)
• Setting up Server Components or server/client boundaries
• Implementing form actions, optimistic updates, or data fetching
• Configuring React Compiler for automatic memoization
• Reviewing React code for common anti-patterns or outdated React 18 idioms
• Finding codebase-level issues that single-file rules can't see: duplicated logic across files, near-duplicate components, dead code, 'use client' files that don't need the client, prop-shape drift (see Category 9)

How to Review or Refactor a Codebase

When the user asks to review, refactor, modernize, or audit React code — single file or whole repo — follow references/_review-algorithm.md. Do not improvise.

Four non-negotiables from that doc:

1. Two modes — never refuse a whole-repo audit. Pick Mode A (scoped, ≤~20 files) or Mode B (whole-tree: inventory pass + targeted sweeps + full Category 9). The algorithm tells you how to handle "audit my codebase" without dumping 800 files into context.
2. Judgment over grep. Each rule names a pattern shape, not a syntactic marker. Read each rule's Shapes to recognize section before sweeping — grep finds the easy violations and misses the high-value ones (a manually-drilled callback ref because the author dodged forwardRef; an onSubmit doing the work of useActionState; a useState+useEffect shaped like derived state; a custom hook hiding the fetch dance). Grep is a trigger, never a verdict.
3. Category-major, not file-major — with forcing functions. Sweep one category at a time across all in-scope files in priority order (CRITICAL → … → CROSS-CUTTING). The algorithm requires a scope declaration, per-category progress lines, and a final coverage table (category × file, cells ∈ {clean, N findings, n/a}). A missing category in the output is immediately visible.
4. Codebase-level findings come from Category 9. Single-file rules can't tell you "these two components should be one" or "this hook is dead." Category 9 (Codebase Hygiene) sweeps the full inventory at the end and produces remove / dedup / reuse / consolidate findings.

Single-file ad-hoc questions ("is this hook OK?") can go straight to the relevant rule. The algorithm exists for the multi-file and whole-repo cases.

Rule Categories

| # | Category | Impact | Rules | Key Topics | |---|----------|--------|-------|------------| | 1 | Concurrent Rendering | CRITICAL | 6 | useTransition, useDeferredValue, Activity, batching, render purity | | 2 | Server Components | CRITICAL | 6 | RSC boundaries, data fetching, streaming, serializable props | | 3 | Actions & Forms | HIGH | 5 | Form actions, declarative form state, useOptimistic, server validation | | 4 | Data Fetching | HIGH | 7 | use() hook, cache(), Suspense, document metadata, resource hints | | 5 | State Management | MEDIUM-HIGH | 5 | Derived values, context split, functional updates, reducer | | 6 | Memoization & Performance | MEDIUM | 5 | React Compiler, useMemo, useCallback, React.memo | | 7 | Effects & Events | MEDIUM | 5 | useEffectEvent, cleanup, external stores, derived-state anti-pattern | | 8 | Component Patterns | LOW-MEDIUM | 5 | ref-as-prop, composition, controlled vs uncontrolled, key reset | | 9 | Codebase Hygiene | CROSS-CUTTING | 5 | Dedup, consolidation, dead code, boundary coherence, prop-shape drift |

Quick Reference

Critical patterns — get these right first:

• Fetch data in Server Components, not Client Components
• Push 'use client' boundaries as low as possible
• Use startTransition for expensive non-blocking updates
• Use <Activity> to preserve state across tab/page switches

React 19 modern idioms (do NOT generate React 18 patterns):

• function C({ ref, ...props }) — never wrap in forwardRef
• <MyContext value={v}> — never use <MyContext.Provider>
• useActionState — never use useFormState
• useRef<T>(null) — always pass an initial value
• Render <title>, <meta>, <link> inline — never reach for react-helmet
• preload/preconnect from react-dom — never hand-render <link rel="preload">

Common single-file mistakes — avoid these anti-patterns:

• Creating promises inside Client Components for use() (causes infinite loops)
• Memoizing everything (use React Compiler v1.0+ instead)
• Using effects for derived state, mutations, parent notifications, or app init
• Placing 'use client' too high in the component tree

Codebase-level patterns — surface these in Category 9 sweeps:

• Components/hooks/utilities with zero importers — delete
• 2+ files with the same effect/state shape — extract a shared hook
• 2+ structurally identical components with drift in labels/icons — consolidate with variants or composition
• 'use client' files whose hook usage doesn't require client execution — demote to Server Components (or split into server parent + client island)
• Same conceptual prop carried under different names across components — converge on a canonical name

Table of Contents

1. Concurrent Rendering — CRITICAL
   • 1.1 Preserve hidden subtree state across navigation instead of unmounting — HIGH (<Activity>)
   • 1.2 Keep previous content visible across navigation by wrapping the update in a transition — HIGH
   • 1.3 Trust automatic batching — don't reach for flushSync or unstable_batchedUpdates — HIGH
   • 1.4 Defer a value you don't own when it drives an expensive child re-render — CRITICAL (useDeferredValue)
   • 1.5 Mark expensive state updates as low-priority so input stays responsive — CRITICAL (useTransition)
   • 1.6 Keep render pure — never mutate, subscribe, or read external state during render — MEDIUM-HIGH
2. Server Components — CRITICAL
   • 2.1 Quarantine browser-API-dependent libraries inside Client Component wrappers — MEDIUM-HIGH
   • 2.2 Split slow async work behind its own Suspense boundary so fast content streams first — MEDIUM-HIGH
   • 2.3 Pull data on the server with async/await — never useEffect+fetch in a Client Component — CRITICAL
   • 2.4 Push the 'use client' boundary down to the interactive leaf, not up at the route — CRITICAL
   • 2.5 Only data that the RSC wire format can encode crosses the server→client boundary — HIGH
   • 2.6 Server content reaches inside a Client Component via children or named slots, not by being imported — HIGH
3. Actions & Forms — HIGH
   • 3.1 Wire form submission through the action prop, not a JS-only onSubmit handler — HIGH
   • 3.2 Lift imperative pending/error/submit bookkeeping into a single declarative form-state hook — HIGH (useActionState)
   • 3.3 Submit buttons read parent-form pending state from context, not from a prop drilled in — MEDIUM-HIGH (useFormStatus)
   • 3.4 Show the post-mutation outcome immediately with automatic rollback on failure — HIGH (useOptimistic)
   • 3.5 Treat validation as a server-action concern; client checks are an enhancement, never the only gate — MEDIUM
4. Data Fetching — HIGH
   • 4.1 Independent fetches run concurrently — sequential await is a waterfall — MEDIUM-HIGH
   • 4.2 Per-request memoize data fetchers so multiple components reading the same data don't re-fetch — HIGH (cache())
   • 4.3 Each Suspense boundary needs an Error Boundary wrapping it — failures must be containable — MEDIUM
   • 4.4 Loading states are declared as Suspense fallbacks, not assembled from if (loading) return … — HIGH
   • 4.5 Read promises and conditional context with use() instead of useEffect+useState plumbing — HIGH (use())
   • 4.6 Page metadata renders as <title>/<meta>/<link> inline — drop helmet-style head managers — MEDIUM
   • 4.7 Reach for the imperative resource-hint APIs from react-dom instead of hand-rendering <link rel="preload"> — MEDIUM
5. State Management — MEDIUM-HIGH
   • 5.1 Compute derived values in the render body — never mirror them into separate state — MEDIUM
   • 5.2 Each context holds one independently-changing piece of state — split fat contexts apart — MEDIUM
   • 5.3 When the next state depends on the previous, pass setX(prev => …) — not setX(x + 1) — MEDIUM-HIGH
   • 5.4 Pass a function to useState when the initial value is expensive — never compute it inline — MEDIUM-HIGH
   • 5.5 Coordinated multi-field state transitions live in a single reducer — not in N sibling useState cells — MEDIUM (useReducer)
6. Memoization & Performance — MEDIUM
   • 6.1 Memoize from a measured baseline — don't pre-wrap every value and callback in useMemo/useCallback — MEDIUM
   • 6.2 Adopt React Compiler v1.0 — let the build memoize, then remove the manual useMemo/useCallback noise — MEDIUM
   • 6.3 Wrap expensive pure components in memo() only when their props are actually stable — MEDIUM
   • 6.4 Stabilize a callback's identity only when something downstream depends on identity stability — MEDIUM (useCallback)
   • 6.5 Cache a computation between renders only when its inputs are stable and the work is measurably expensive — MEDIUM (useMemo)
7. Effects & Events — MEDIUM
   • 7.1 Every effect that subscribes, schedules, or connects must return a cleanup that tears it down — MEDIUM
   • 7.2 useEffect is for syncing with external systems — never for derived state, mutations, event logic, parent notification, or app init — HIGH
   • 7.3 Effect deps are reference-compared — pass primitives, not in-render-constructed objects or arrays — MEDIUM
   • 7.4 Read latest props/state inside an effect's callback without re-subscribing when those values change — MEDIUM (useEffectEvent)
   • 7.5 Subscribe to external mutable state through useSyncExternalStore, not useEffect + useState — MEDIUM
8. Component Patterns — LOW-MEDIUM
   • 8.1 Components receive ref as a normal destructured prop — drop the forwardRef wrapper, drop the drilling — MEDIUM-HIGH
   • 8.2 Reach for controlled state only when something reads the value on every change — otherwise prefer uncontrolled with <form action> — LOW-MEDIUM
   • 8.3 When prop count climbs past ~6 with many optional slots, take children instead and let callers compose — LOW-MEDIUM
   • 8.4 Tie a stateful child's identity to the entity it edits by setting key={entity.id} — let React tear down stale state for you — LOW-MEDIUM
   • 8.5 Use a render prop only when the parent needs to control rendering — for logic reuse, prefer a custom hook — LOW-MEDIUM
9. Codebase Hygiene — CROSS-CUTTING (multi-file findings; required for whole-repo audits)
   • 9.1 Extract duplicated effect/state shapes into a shared hook — HIGH
   • 9.2 Consolidate near-duplicate components into one with variants or composition — HIGH
   • 9.3 Delete components, hooks, and utilities with zero importers — MEDIUM-HIGH
   • 9.4 Demote 'use client' files whose hook usage doesn't require the client — HIGH
   • 9.5 Converge on canonical names when the same concept wears different prop names across components — MEDIUM-HIGH

References

1. https://react.dev
2. https://react.dev/blog/2024/04/25/react-19-upgrade-guide
3. https://react.dev/blog/2024/12/05/react-19
4. https://react.dev/blog/2025/10/01/react-19-2
5. https://react.dev/blog/2025/10/07/react-compiler-1
6. https://react.dev/learn/you-might-not-need-an-effect
7. https://github.com/facebook/react

Related Skills

• For Next.js 16 App Router, see nextjs-16-app-router skill
• For client-side form handling, see react-hook-form skill
• For data caching with TanStack Query, see tanstack-query skill