programming-react— agent skill

Overview

Build React interfaces with explicit state ownership, pure rendering, and dependable loading and mutation behavior. As of 2026-10: React 19.3 (2026-09-09) is the latest stable release and React Compiler 1.0 is stable. Inspect installed React, framework, renderer, and compiler configuration first; documentation can include Canary-only APIs. Keep existing routing, styling, and state libraries unless the task requires changing them.

Mental model

A render describes UI from a snapshot of props, state, and context; React may repeat or abandon it. Events cause user-directed changes, while effects synchronize committed UI with external systems. React's server component model requires supporting infrastructure; it is not automatically enabled by using React 19.

State and component boundaries

• Store the smallest authoritative state. Derive filtered lists, counts, and validation summaries during render; do not mirror props or derived values through effects.
• Keep state near its consumers. Lift it to the closest shared owner when necessary; use composition before introducing context or a global store. Shareable navigation state belongs in the URL.
• Use functional updates when the next value depends on pending previous state. Treat props/state as immutable snapshots; mutate neither during rendering.
• Model mutually exclusive UI states explicitly. Keep initial loading, refreshing existing data, empty results, and failures distinguishable; a failed fetch is not an empty list.
• Define components at module scope to preserve identity. Use stable domain keys for reorderable lists; change a subtree's key only when resetting its state is intended.
• Make props reflect behavior rather than internal switches. Follow framework export conventions; neither named exports nor one-component-per-file is universally required.

Effects and current APIs

• Hooks belong at the top level of components/custom hooks, before conditional returns. React's use(resource) is an exception: it permits conditions and loops, but still belongs inside a component or hook and cannot be wrapped in try/catch.
• Effects must list their reactive dependencies and clean up subscriptions, timers, and pending work. Strict Mode's extra development setup/cleanup cycle exposes lifecycle errors; do not hide it with a "run once" ref.
• React 19.2's useEffectEvent reads current values for non-reactive logic called from effects. It is not a dependency-avoidance trick, a UI event handler, or a callback to pass elsewhere.
• React 19 permits function components to receive ref as a prop. Use this for new 19-targeted APIs; preserve forwardRef where older consumers need it.
• React 19.2's Activity can hide UI while preserving state and cleaning up its effects. Use it when that lifecycle is needed; ordinary conditional rendering remains appropriate.
• React Compiler can memoize supported code when enabled in the build. Do not assume it is active because React is installed. Profile before adding manual memoization; memoization must never be required for correctness.
• Use the current eslint-plugin-react-hooks recommended preset; it includes compiler-derived rules (set-state-in-render, set-state-in-effect, refs) that work without the compiler installed and replace the separate eslint-plugin-react-compiler.
• React 19.3 stabilizes <ViewTransition> with addTransitionType, and Fragment refs. View transitions animate only updates inside a Transition (startTransition, Suspense reveal, useDeferredValue); keep them progressive and respect reduced-motion preferences.

Data, server boundaries, and forms

• Prefer existing framework loaders or cache infrastructure for shared server data. A small effect-based fetch is valid when it handles races, cancellation, and errors; a cache library is not mandatory.
• Suspense handles participating data sources, lazy components, and promises consumed through use; it does not detect arbitrary effect-based fetching. Reuse a stable promise rather than creating one every client render.
• Use Server Components only in a supporting framework. Keep secrets server-side, and authorize each server mutation. "use client" marks a module dependency boundary; Client Components can still be prerendered on the server.
• Use native form semantics and labels. Controlled inputs are useful when React must drive their value; uncontrolled inputs and FormData suit many ordinary forms.
• React 19 form actions and useActionState provide pending state and action results. useFormStatus observes its parent form, not a form rendered by the same component. Follow existing form infrastructure when it already meets the need.
• Show expected validation failures as action state. Let unexpected failures reach the appropriate error boundary; handle event-handler and unrelated asynchronous failures explicitly because boundaries do not catch every error.
• Keep optimistic state temporary and reconciled with authoritative results. Failed or conflicting mutations need recovery; avoid optimistic updates when the operation cannot be safely predicted.

Example

Derive visible content directly and retain stable item identity:

type Item = { id: string; title: string; archived: boolean };

function ItemList({ items, showArchived }: {
  items: readonly Item[];
  showArchived: boolean;
}) {
  const visible = items.filter(item => showArchived || !item.archived);
  return (
    <ul>
      {visible.map(item => <li key={item.id}>{item.title}</li>)}
    </ul>
  );
}

No effect or duplicate state is needed. The caller remains responsible for distinguishing loading, failure, and a successful empty result.

Checklist

• Confirm the installed stable APIs and the framework's server/client model.
• Give each value one owner; derive values without synchronization effects.
• Check hook ordering, effect dependencies, cleanup, and stable identity.
• Exercise keyboard interaction, labels, pending/error states, and mutation reconciliation.
• Use existing installed lint, type-check, component, and critical-flow tests when authorized. Report what was source-reviewed versus executed.

References

• Current React versions, React 19.3, and React 19.2
• React Compiler 1.0 and hooks lint rules
• Removing unnecessary effects
• use and promise ownership
• useEffectEvent constraints
• Server Components
• Form actions and useActionState
• React 19 APIs
• React Compiler