cc-react-19-patterns

React Modern Patterns Reference

Authoritative quick-reference for React 19–19.2 patterns. This skill is loaded automatically when the agent works on React code. It covers hooks, APIs, and conventions introduced or stabilised in React 19.0, 19.1, and 19.2.

Version baseline: Prefer latest React 19.x stable. For React Server Components packages, known fixed backports for the 2025/2026 advisories include 19.0.4, 19.1.5, and 19.2.4; flag earlier patch versions.

React Compiler: v1.0+ is stable (October 2025). Prefer compiler defaults for new code, but keep/remove existing manual memoization only with measurement and profiling.

Note: Code examples use vanilla React elements. When working in a design system context, substitute the corresponding design system components (e.g. FDS Input, Button, Select).

Version guard: If both React 18 and React 19 skills are loaded, follow the skill matching the project's react version in package.json.

---

Actions API

Actions are async functions used in transitions to handle pending states, errors, forms, and optimistic updates automatically.

useActionState

Manages form action state. Returns [state, action, isPending].

import { useActionState } from "react";

interface FormState {
  error?: string;
  success?: boolean;
}

async function updateName(
  prev: FormState,
  formData: FormData
): Promise<FormState> {
  const name = formData.get("name") as string;
  if (!name) return { error: "Name is required" };
  await saveToServer(name);
  return { success: true };
}

function NameForm() {
  const [state, submitAction, isPending] = useActionState(
    updateName,
    {}
  );

  return (
    <form action={submitAction}>
      <input type="text" name="name" />
      <button type="submit" disabled={isPending}>
        {isPending ? "Saving..." : "Save"}
      </button>
      {state.error && <p>{state.error}</p>}
    </form>
  );
}

Key points:

• Wraps an async function and returns the last result as state
• The wrapped action is passed to <form action={...}>
• isPending is true while the action is in flight
• Previously named useFormState — that name is deprecated

useFormStatus

Reads the status of a parent <form>. Import from react-dom.

import { useFormStatus } from "react-dom";

function SubmitButton() {
  const { pending } = useFormStatus();
  return (
    <button type="submit" disabled={pending}>
      {pending ? "Submitting..." : "Submit"}
    </button>
  );
}

Key points:

• Must be rendered inside a <form> — reads the nearest parent
• Returns { pending, data, method, action }
• Ideal for design system submit buttons that need loading state

useOptimistic

Shows an optimistic value while an async action is in flight.

import { useOptimistic } from "react";

function MessageList({
  messages,
  sendMessage,
}: {
  messages: Message[];
  sendMessage: (text: string) => Promise<void>;
}) {
  const [optimistic, addOptimistic] = useOptimistic(
    messages,
    (current, newText: string) => [
      ...current,
      { id: "temp", text: newText, sending: true },
    ]
  );

  const formAction = async (formData: FormData) => {
    const text = formData.get("text") as string;
    addOptimistic(text);
    await sendMessage(text);
  };

  return (
    <>
      {optimistic.map((m) => (
        <p key={m.id} style={{ opacity: m.sending ? 0.5 : 1 }}>
          {m.text}
        </p>
      ))}
      <form action={formAction}>
        <input name="text" />
        <button type="submit">Send</button>
      </form>
    </>
  );
}

Key points:

• Reverts to the real value automatically on success or error
• The reducer receives current state and the optimistic value
• Pair with useActionState for full form workflows

<form> with Actions

React 19 supports passing functions to action and formAction props. The form resets automatically on success for uncontrolled components.

<form action={async (formData) => {
  await submitData(formData);
}}>
  <input name="email" type="email" />
  <button type="submit">Subscribe</button>
</form>

---

use() Hook

Reads a resource (promise or context) during render. Unlike other hooks, use() can be called conditionally.

Reading a Promise

import { use, Suspense } from "react";

function UserProfile({
  userPromise,
}: {
  userPromise: Promise<User>;
}) {
  const user = use(userPromise);
  return <h2>{user.name}</h2>;
}

// Parent provides the promise and Suspense boundary
function Page({ userId }: { userId: string }) {
  const userPromise = fetchUser(userId);
  return (
    <Suspense fallback={<div>Loading...</div>}>
      <UserProfile userPromise={userPromise} />
    </Suspense>
  );
}

Reading Context Conditionally

import { use } from "react";

function Heading({ children }: { children?: React.ReactNode }) {
  if (!children) return null;

  // use() works after early returns — useContext() does not
  const theme = use(ThemeContext);
  return <h1 style={{ color: theme.color }}>{children}</h1>;
}

Key points:

• Suspends rendering until the promise resolves
• Do NOT create promises inside render — pass them from a parent or use a Suspense-compatible data fetching library
• For context, use() replaces useContext() when conditional reading is needed

---

React 19.2 Features

<Activity>

Controls UI visibility and state preservation. Two modes: visible and hidden.

import { Activity } from "react";

function TabPanel({
  isActive,
  children,
}: {
  isActive: boolean;
  children: React.ReactNode;
}) {
  return (
    <Activity mode={isActive ? "visible" : "hidden"}>
      {children}
    </Activity>
  );
}

Key points:

• hidden — hides children, unmounts effects, defers updates
• visible — shows children, mounts effects, processes updates
• Preserves component state across visibility toggles
• Use for tab panels, navigation pre-rendering, back-navigation state preservation
• Replaces conditional rendering ({show && <Page />}) when state preservation is needed

useEffectEvent()

Extracts non-reactive logic from effects so that "event" values do not cause the effect to re-run.

import { useEffect, useEffectEvent } from "react";

function ChatRoom({
  roomId,
  theme,
}: {
  roomId: string;
  theme: string;
}) {
  // theme changes should NOT reconnect the chat
  const onConnected = useEffectEvent(() => {
    showNotification("Connected!", theme);
  });

  useEffect(() => {
    const conn = createConnection(roomId);
    conn.on("connected", () => onConnected());
    conn.connect();
    return () => conn.disconnect();
  }, [roomId]);
  // onConnected is NOT a dependency
}

Key points:

• Always reads the latest props/state (like a DOM event handler)
• Must NOT appear in dependency arrays
• Must be declared in the same component or hook as the effect
• Requires eslint-plugin-react-hooks@latest (v6+) for correct linting
• Use when an effect needs to "fire an event" that reads reactive values without re-running the effect

cacheSignal

Allows aborting cached fetch calls when the cache lifetime ends. RSC-only — not applicable to client-side Vite projects.

import { cache, cacheSignal } from "react";

const dedupedFetch = cache(fetch);

async function ServerComponent() {
  await dedupedFetch(url, { signal: cacheSignal() });
}

Performance Tracks

React 19.2 adds custom tracks to Chrome DevTools performance profiles:

• Scheduler track — shows priority scheduling (blocking vs transition)
• Components track — shows mount/render/effect timing per component

Use the React DevTools Profiler alongside Performance Tracks for comprehensive performance analysis.

---

React 19 Quality-of-Life Improvements

Ref as Prop

Pass ref directly as a prop — forwardRef is no longer needed.

function TextInput({
  ref,
  ...props
}: {
  ref?: React.Ref<HTMLInputElement>;
} & React.InputHTMLAttributes<HTMLInputElement>) {
  return <input ref={ref} {...props} />;
}

// Usage
<TextInput ref={inputRef} placeholder="Type here" />

Context Without Provider

Render context directly as JSX instead of <Context.Provider>.

const ThemeContext = createContext("light");

function App({ children }: { children: React.ReactNode }) {
  return (
    <ThemeContext value="dark">
      {children}
    </ThemeContext>
  );
}

Ref Callbacks With Cleanup

Return a cleanup function from ref callbacks.

<div
  ref={(node) => {
    if (node) {
      const observer = new ResizeObserver(handleResize);
      observer.observe(node);
      return () => observer.disconnect();
    }
  }}
/>

Note: Do not use implicit returns in ref callbacks — TypeScript will reject them. Use explicit return.

Document Metadata in Components

Place <title>, <meta>, and <link> directly in components. React hoists them to <head> automatically.

function BlogPost({ post }: { post: Post }) {
  return (
    <article>
      <title>{post.title}</title>
      <meta name="author" content={post.author} />
      <h1>{post.title}</h1>
      <p>{post.content}</p>
    </article>
  );
}

useDeferredValue With Initial Value

Provide an initial value for the first render.

const deferredQuery = useDeferredValue(query, "");
// First render: ""
// Background re-render: actual query value

---

Concurrent Rendering Patterns

startTransition

Mark updates as non-urgent to keep the UI responsive.

import { startTransition } from "react";

function handleSearch(query: string) {
  // Urgent: update the input
  setInput(query);

  // Non-urgent: update the filtered results
  startTransition(() => {
    setResults(filterData(query));
  });
}

useDeferredValue

Defer a value to avoid blocking urgent updates.

import { useDeferredValue } from "react";

function SearchResults({ query }: { query: string }) {
  const deferredQuery = useDeferredValue(query);
  const isStale = query !== deferredQuery;

  return (
    <div style={{ opacity: isStale ? 0.5 : 1 }}>
      <Results query={deferredQuery} />
    </div>
  );
}

Suspense Boundaries

Wrap async components with <Suspense> for graceful loading.

<Suspense fallback={<Skeleton />}>
  <AsyncComponent />
</Suspense>

Key points:

• Nest multiple boundaries for granular loading states
• In React 19.2, SSR Suspense boundaries are batched for smoother reveals
• Pair with React.lazy() for code splitting at route boundaries

---

Server Components and Server Actions

Server Components (RSC)

• Run on the server, not bundled to the client
• Can async/await directly for data fetching
• No directive needed — components are server by default in RSC frameworks
• Mark client components with "use client" when they need interactivity

Server Actions

• Defined with "use server" directive
• Called from client components as regular async functions
• Framework handles the server request transparently

// actions.ts
"use server";

export async function saveUser(formData: FormData) {
  const name = formData.get("name") as string;
  await db.users.update({ name });
}

// UserForm.tsx
"use client";

import { saveUser } from "./actions";

export function UserForm() {
  return (
    <form action={saveUser}>
      <input name="name" />
      <button type="submit">Save</button>
    </form>
  );
}

Security Note

React Server Components had critical and follow-up advisories in December 2025 and January 2026. For impacted RSC packages, upgrade to fixed patch lines such as 19.0.4, 19.1.5, 19.2.4, or newer. Apps not using server/RSC paths are not affected.

---

TypeScript Patterns

Component Props

interface ButtonProps {
  variant: "primary" | "secondary";
  size?: "sm" | "md" | "lg";
  onClick?: () => void;
  children: React.ReactNode;
}

function Button({
  variant,
  size = "md",
  onClick,
  children,
}: ButtonProps) {
  return (
    <button
      className={`btn btn-${variant} btn-${size}`}
      onClick={onClick}
    >
      {children}
    </button>
  );
}

Discriminated Unions for Variants

type AlertProps =
  | { type: "success"; message: string }
  | { type: "error"; message: string; retry: () => void };

function Alert(props: AlertProps) {
  if (props.type === "error") {
    return (
      <div>
        <p>{props.message}</p>
        <button onClick={props.retry}>Retry</button>
      </div>
    );
  }
  return <div><p>{props.message}</p></div>;
}

Generic Components

interface ListProps<T> {
  items: T[];
  renderItem: (item: T) => React.ReactNode;
  keyExtractor: (item: T) => string;
}

function List<T>({
  items,
  renderItem,
  keyExtractor,
}: ListProps<T>) {
  return (
    <ul>
      {items.map((item) => (
        <li key={keyExtractor(item)}>{renderItem(item)}</li>
      ))}
    </ul>
  );
}

Hook Return Types

function useToggle(initial = false) {
  const [value, setValue] = useState(initial);
  const toggle = () => setValue((v) => !v);
  return [value, toggle] as const;
}
// Return type: readonly [boolean, () => void]

---

Decision Trees

State Management

Scenario	Solution
Local UI state (toggle, input)	useState
Form submission state	useActionState
Derived/computed value	Compute in render
Shared across siblings	Lift state up
Shared across tree	Context + use()
Complex state logic	useReducer
Global client state	Zustand / Redux Toolkit
Server state / caching	TanStack Query / SWR

Form Handling

Scenario	Solution
Simple client form	useActionState + <form action>
Loading indicator in form	useFormStatus in submit button
Optimistic UI during submit	useOptimistic
Server-side validation	Server Action + useActionState
Multi-step wizard	useReducer + startTransition

Effect Patterns

Scenario	Solution
Sync with external system	useEffect
Event in effect (no re-run)	useEffectEvent
Derived from props/state	Compute in render (no effect)
One-time setup	useEffect with [] deps
Subscription with cleanup	useSyncExternalStore

---

Common Mistakes to Flag

1. Creating promises in render — pass promises from parents or use data fetching libraries with use()
2. Manual memoization with React Compiler — useMemo and useCallback are rarely needed when the compiler is active
3. forwardRef in new code — use ref as a regular prop
4. Using legacy <Context.Provider> in new React 19 code — valid but verbose; prefer <Context value={...}>
5. useContext for conditional reads — use use() instead
6. useEffect for derived state — compute in render
7. eslint-plugin-react-hooks < v6 — upgrade to support useEffectEvent and React Compiler rules
8. Outdated RSC patch levels — use fixed patch lines (for example 19.0.4, 19.1.5, 19.2.4) or newer