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React 19 Development Patterns

Overview

React 19 patterns for Next.js App Router, Server Actions, optimistic UI, and concurrent features. See Quick Reference for API summary and Examples for copy-paste patterns.

When to Use

Building React 19 applications with Next.js App Router
Implementing optimistic UI with useOptimistic or useTransition
Creating Server Actions with form validation
Migrating from class components to hooks
Optimizing concurrent rendering with React Compiler
Managing complex state with useReducer or custom hooks
Wrapping async operations in Suspense boundaries

Quick Reference

Pattern	Hook / API	Use Case
Local state	`useState`	Simple component state
Complex state	`useReducer`	Multi-action state machines
Side effects	`useEffect`	Subscriptions, data fetching
Shared state	`useContext` / `createContext`	Cross-component data
DOM access	`useRef`	Focus, measurements, timers
Performance	`useMemo` / `useCallback`	Expensive computations
Non-urgent updates	`useTransition`	Search/filter on large lists
Defer expensive UI	`useDeferredValue`	Stale-while-updating
Read resources	`use()` (React 19)	Promises and context in render
Optimistic UI	`useOptimistic` (React 19)	Instant feedback on mutations
Form status	`useFormStatus` (React 19)	Pending state in child components
Form state	`useActionState` (React 19)	Server action results
Auto-memoization	React Compiler	Eliminates manual memo/callback

Instructions

Identify Component Type: Determine if Server Component or Client Component is needed
Select Hooks: Use appropriate hooks for state management and side effects
Type Props: Define TypeScript interfaces for all component props
Handle Async: Wrap data-fetching components in Suspense boundaries
Optimize: Use React Compiler or manual memoization for expensive renders
Handle Errors: Add ErrorBoundary for graceful error handling
Validate Server Actions: Define Zod/schema validation, then test:

Submit invalid inputs → verify rejection
Submit valid inputs → verify success

Examples

Server Component with Client Interaction

tsx

// Server Component (default) — async, fetches data
async function ProductPage({ id }: { id: string }) {
  const product = await db.product.findUnique({ where: { id } });

  return (
    <div>
      <h1>{product.name}</h1>
      <AddToCartButton productId={product.id} />
    </div>
  );
}

// Client Component — handles interactivity
'use client';
function AddToCartButton({ productId }: { productId: string }) {
  const [isPending, startTransition] = useTransition();

  const handleAdd = () => {
    startTransition(async () => {
      await addToCart(productId);
    });
  };

  return (
    <button onClick={handleAdd} disabled={isPending}>
      {isPending ? 'Adding...' : 'Add to Cart'}
    </button>
  );
}

useOptimistic for Instant Feedback

tsx

'use client';
import { useOptimistic } from 'react';

function TodoList({ todos, addTodo }: { todos: Todo[]; addTodo: (t: Todo) => Promise<void> }) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (state, newTodo: Todo) => [...state, { ...newTodo, pending: true }]
  );

  const handleSubmit = async (formData: FormData) => {
    const newTodo = { id: Date.now(), text: formData.get('text') as string };
    addOptimisticTodo(newTodo);  // Immediate UI update
    await addTodo(newTodo);      // Actual backend call
  };

  return (
    <form action={handleSubmit}>
      {optimisticTodos.map(todo => (
        <div key={todo.id} style={{ opacity: todo.pending ? 0.5 : 1 }}>
          {todo.text}
        </div>
      ))}
      <input type="text" name="text" />
      <button type="submit">Add</button>
    </form>
  );
}

Server Action with Form

tsx

// app/actions.ts
'use server';
import { z } from 'zod';
import { revalidatePath } from 'next/cache';

const schema = z.object({
  title: z.string().min(5),
  content: z.string().min(10),
});

export async function createPost(prevState: any, formData: FormData) {
  const parsed = schema.safeParse({
    title: formData.get('title'),
    content: formData.get('content'),
  });

  if (!parsed.success) {
    return { errors: parsed.error.flatten().fieldErrors };
  }

  await db.post.create({ data: parsed.data });
  revalidatePath('/posts');
  return { success: true };
}

// app/blog/new/page.tsx
'use client';
import { useActionState } from 'react';
import { createPost } from '../actions';

export default function NewPostPage() {
  const [state, formAction, pending] = useActionState(createPost, {});

  return (
    <form action={formAction}>
      <input name="title" placeholder="Title" />
      {state.errors?.title && <span>{state.errors.title[0]}</span>}
      <textarea name="content" placeholder="Content" />
      <button type="submit" disabled={pending}>
        {pending ? 'Publishing...' : 'Publish'}
      </button>
    </form>
  );
}

Custom Hook

tsx

export function useOnlineStatus() {
  const [isOnline, setIsOnline] = useState(true);

  useEffect(() => {
    function handleOnline() { setIsOnline(true); }
    function handleOffline() { setIsOnline(false); }

    window.addEventListener('online', handleOnline);
    window.addEventListener('offline', handleOffline);

    return () => {
      window.removeEventListener('online', handleOnline);
      window.removeEventListener('offline', handleOffline);
    };
  }, []);

  return isOnline;
}

useTransition for Non-Urgent Updates

tsx

function SearchableList({ items }: { items: Item[] }) {
  const [query, setQuery] = useState('');
  const [isPending, startTransition] = useTransition();
  const [filteredItems, setFilteredItems] = useState(items);

  const handleChange = (e: React.ChangeEvent<HTMLInputElement>) => {
    setQuery(e.target.value);
    startTransition(() => {
      setFilteredItems(items.filter(i => i.name.toLowerCase().includes(e.target.value.toLowerCase())));
    });
  };

  return (
    <div>
      <input value={query} onChange={handleChange} />
      {isPending && <span>Filtering...</span>}
      <ul>{filteredItems.map(i => <li key={i.id}>{i.name}</li>)}</ul>
    </div>
  );
}

Best Practices

Server vs Client Decision

Start with Server Component (no directive needed)
Add 'use client' only for: hooks, browser APIs, event handlers

State Management

Keep state minimal — compute derived values during render, not in effects
Use useReducer for state with multiple related actions
Lift state up to the nearest common ancestor

Effects

Use effects only for external system synchronization
Always specify correct dependency arrays
Return cleanup functions for subscriptions and timers
Never mutate state directly — always create new references

Performance

With React Compiler: avoid manual useMemo, useCallback, memo
Without React Compiler: use useMemo for expensive computations, useCallback for stable callbacks
Use useTransition for low-priority state updates
Use stable IDs as list keys, not array indices

React 19 Specifics

Wrap use(promise) components in Suspense boundaries
Use useActionState for form-server action integration
Validate Server Action inputs — they are public endpoints
Pass serializable data from Server to Client Components

Constraints and Warnings

Server Components: Cannot use hooks, event handlers, or browser APIs
use() Hook: Can only be called during render, not in callbacks or effects
Server Actions: Must include 'use server' directive; always validate inputs
State Mutations: Never mutate state directly — always create new references
Effect Dependencies: Include all dependencies in useEffect dependency arrays
Memory Leaks: Always clean up subscriptions and event listeners in useEffect return

References

Consult these files for detailed patterns:

[references/hooks-patterns.md](references/hooks-patterns.md) — useState, useEffect, useRef, useReducer, custom hooks, common pitfalls
[references/component-patterns.md](references/component-patterns.md) — Props, composition, lifting state, context, compound components, error boundaries
[references/react19-features.md](references/react19-features.md) — use(), useOptimistic, useFormStatus, useActionState, Server Actions, Server Components, migration guide
[references/performance-patterns.md](references/performance-patterns.md) — React Compiler setup, useMemo, useCallback, useTransition, useDeferredValue, lazy loading
[references/typescript-patterns.md](references/typescript-patterns.md) — Typed props, generic components, event handlers, discriminated unions, context typing
[references/learn.md](references/learn.md) — Progressive learning guide from basics to advanced React 19
[references/reference.md](references/reference.md) — Complete API reference for all React hooks and component APIs