React 19.x Development

This skill provides guidance on React 19.x features that may not be in LLM training data. Focus is on new APIs, patterns, and migration from older React patterns.

Quick API Reference

New Hooks (React 19)

Hook	Purpose	Import
useActionState	Form state + pending + error handling	react
useFormStatus	Read parent form's pending state	react-dom
useOptimistic	Optimistic UI updates	react
use	Read promises/context conditionally	react

New Hooks (React 19.2)

Hook	Purpose	Import
useEffectEvent	Stable event handlers in effects	react

New Components (React 19.2)

Component	Purpose
<Activity>	Control visibility/priority of subtrees

Decision Tree: Which API to Use?

Handling form submission?
├── Need pending state in child component? → useFormStatus
├── Need optimistic updates? → useOptimistic
├── Need form state + error handling? → useActionState
└── Simple async action? → useTransition with async

Reading async data?
├── In render, with Suspense? → use(promise)
└── In effect/event handler? → Regular await

Reading context conditionally?
└── After early return? → use(Context) instead of useContext

Effect needs reactive value but shouldn't re-run?
└── → useEffectEvent

Conditional rendering with state preservation?
└── → <Activity mode="visible|hidden">

Core Patterns

1. Form Actions (replaces manual submit handlers)

// OLD: Manual state management
function OldForm() {
  const [isPending, setIsPending] = useState(false);
  const [error, setError] = useState(null);

  const handleSubmit = async (e) => {
    e.preventDefault();
    setIsPending(true);
    const result = await submitData(new FormData(e.target));
    setIsPending(false);
    if (result.error) setError(result.error);
  };

  return <form onSubmit={handleSubmit}>...</form>;
}

// NEW: Form Actions with useActionState
function NewForm() {
  const [error, submitAction, isPending] = useActionState(
    async (prevState, formData) => {
      const result = await submitData(formData);
      if (result.error) return result.error;
      redirect('/success');
      return null;
    },
    null
  );

  return (
    <form action={submitAction}>
      <input name="field" />
      <button disabled={isPending}>Submit</button>
      {error && <p>{error}</p>}
    </form>
  );
}

2. ref as Prop (no more forwardRef)

// OLD: Required forwardRef wrapper
const OldInput = forwardRef(function OldInput({ label }, ref) {
  return <input ref={ref} aria-label={label} />;
});

// NEW: ref is just a prop
function NewInput({ label, ref }) {
  return <input ref={ref} aria-label={label} />;
}

// Both used the same way
<NewInput ref={inputRef} label="Name" />

3. Context as Provider

// OLD
<ThemeContext.Provider value={theme}>
  {children}
</ThemeContext.Provider>

// NEW
<ThemeContext value={theme}>
  {children}
</ThemeContext>

4. use() API for Conditional Context

// useContext cannot be called after early return
function Component({ show }) {
  if (!show) return null;
  // const theme = useContext(ThemeContext); // ERROR!
  const theme = use(ThemeContext); // OK!
  return <div style={{ color: theme.color }}>...</div>;
}

5. Activity Component (React 19.2)

// OLD: Unmounts and loses state
{isVisible && <ExpensiveComponent />}

// NEW: Preserves state, defers updates when hidden
<Activity mode={isVisible ? 'visible' : 'hidden'}>
  <ExpensiveComponent />
</Activity>

6. useEffectEvent (React 19.2)

// Problem: theme change causes reconnect
function ChatRoom({ roomId, theme }) {
  useEffect(() => {
    const conn = connect(roomId);
    conn.on('connected', () => showNotification(theme));
    return () => conn.disconnect();
  }, [roomId, theme]); // theme shouldn't trigger reconnect!
}

// Solution: useEffectEvent
function ChatRoom({ roomId, theme }) {
  const onConnected = useEffectEvent(() => {
    showNotification(theme); // Always reads latest theme
  });

  useEffect(() => {
    const conn = connect(roomId);
    conn.on('connected', onConnected);
    return () => conn.disconnect();
  }, [roomId]); // Only roomId triggers reconnect
}

Common Pitfalls

useActionState

• First param receives (previousState, formData) - don't forget previousState
• Returns [state, action, isPending] - order matters
• Renamed from useFormState in React DOM

useFormStatus

• Must be used in a component that's a child of <form>
• Cannot be used in the same component that renders the form
• Returns { pending, data, method, action }

useOptimistic

• Automatically reverts on error - no manual rollback needed
• Use with startTransition or form actions

use()

• Only for promises created outside render (from cache, loader, etc.)
• Never create promises in render and pass to use()
• Works with Suspense boundaries

ref Cleanup Functions

// NEW: Cleanup function syntax
<input
  ref={(node) => {
    // Setup
    node.focus();
    return () => {
      // Cleanup when ref changes or unmounts
    };
  }}
/>

// BREAKING: Implicit returns now fail TypeScript
<div ref={current => (instance = current)} />     // BAD
<div ref={current => { instance = current; }} />  // GOOD

useEffectEvent

• Do NOT add to dependency array (it's stable)
• Only use for "event" logic, not to silence linter

References

For detailed documentation on specific topics:

• Actions & Forms: useActionState, useFormStatus, useOptimistic, form actions
• use() API: Reading promises and context with use()
• Server Components: RSC, Server Actions, cacheSignal
• Rendering APIs: prerender, Activity, Partial Pre-rendering
• Syntax Improvements: ref as prop, Context provider, metadata, stylesheets, scripts