React 19 Patterns - Comprehensive Guide

When to Use This Skill

Use this skill when:

• Writing React components (Server or Client)
• Using React hooks (standard or new React 19 hooks)
• Implementing forms with Server Actions
• Working with Suspense and streaming
• Managing state and transitions
• Optimistic UI updates
• Migrating from React 18 to React 19

What This Skill Covers

Core Patterns

• Server vs Client Components - Complete decision tree
• All React Hooks - Complete reference with TypeScript
• Suspense Patterns - Boundaries, streaming, error handling
• Server Components - Data fetching, caching, composition
• Client Components - Interactivity, state, effects
• Transitions - useTransition, startTransition, isPending
• Streaming - Progressive rendering patterns
• Migration Guide - React 18 → React 19

New in React 19

• use() - For async data in components
• useOptimistic() - For optimistic UI updates
• useFormStatus() - For form submission state
• useActionState() - For Server Action state
• Enhanced useTransition() - Better performance
• Improved error boundaries
• Better hydration

Quick Reference

Server Component Pattern

// ✅ Default - async data fetching
export default async function ProjectsPage() {
  const projects = await db.project.findMany()
  return <ProjectList projects={projects} />
}

Client Component Pattern

// ✅ Use 'use client' for interactivity
'use client'

import { useState } from 'react'

export function InteractiveComponent() {
  const [count, setCount] = useState(0)
  return <button onClick={() => setCount(count + 1)}>{count}</button>
}

New React 19 Hook Pattern

'use client'

import { useOptimistic } from 'react'

export function TodoList({ todos }: Props) {
  const [optimisticTodos, addOptimisticTodo] = useOptimistic(
    todos,
    (state, newTodo: string) => [...state, { id: 'temp', text: newTodo, pending: true }]
  )

  return (
    <form action={async (formData) => {
      addOptimisticTodo(formData.get('todo'))
      await createTodo(formData)
    }}>
      <input name="todo" />
      <button type="submit">Add</button>
    </form>
  )
}

File Structure

This skill is organized into detailed guides:

1. server-vs-client.md - Decision tree for component type
2. hooks-complete.md - All React hooks with TypeScript
3. suspense-patterns.md - Suspense boundaries and streaming
4. server-components-complete.md - Server Component patterns
5. client-components-complete.md - Client Component patterns
6. transitions.md - useTransition and concurrent features
7. streaming-patterns.md - Progressive rendering
8. migration-guide.md - React 18 → 19 migration
9. validate-react.py - Validation tool for React rules

Decision Flow

START: Creating new component
│
├─ Does it need interactivity (onClick, onChange)?
│  ├─ YES → Read client-components-complete.md
│  └─ NO → Continue
│
├─ Does it need React hooks (useState, useEffect)?
│  ├─ YES → Read client-components-complete.md + hooks-complete.md
│  └─ NO → Continue
│
├─ Does it fetch data?
│  ├─ YES → Read server-components-complete.md
│  └─ NO → Continue
│
└─ Default → Server Component (read server-components-complete.md)

Need specific hook help?
└─ Read hooks-complete.md (complete reference)

Need Suspense/streaming?
└─ Read suspense-patterns.md + streaming-patterns.md

Need optimistic UI?
└─ Read hooks-complete.md (useOptimistic section)

Need form handling?
└─ Read hooks-complete.md (useFormStatus, useActionState)

Migrating from React 18?
└─ Read migration-guide.md

Common Mistakes Prevented

❌ Async Client Component

'use client'
export default async function Bad() {} // ERROR!

✅ Use Server Component or useEffect

// Option 1: Server Component
export default async function Good() {} // ✅

// Option 2: Client with useEffect
'use client'
export default function Good() {
  useEffect(() => {
    fetchData()
  }, [])
}

❌ Hooks in Conditions

if (condition) {
  useState(0) // ERROR: Rules of Hooks violation
}

✅ Hooks at Top Level

const [value, setValue] = useState(0)
if (condition) {
  // Use the hook result here
}

❌ Browser APIs in Server Component

export default function Bad() {
  const data = localStorage.getItem('key') // ERROR!
  return <div>{data}</div>
}

✅ Use Client Component

'use client'
export default function Good() {
  const [data, setData] = useState(() =>
    localStorage.getItem('key')
  )
  return <div>{data}</div>
}

Validation

Use validate-react.py to check your React code:

# Validate single file
python .claude/skills/react-19-patterns/validate-react.py src/components/Button.tsx

# Validate directory
python .claude/skills/react-19-patterns/validate-react.py src/components/

# Auto-fix (where possible)
python .claude/skills/react-19-patterns/validate-react.py --fix src/components/

Checks for:

• Rules of Hooks violations
• Server/Client component mistakes
• Missing 'use client' directives
• Invalid async Client Components
• Browser API usage in Server Components
• Non-serializable props to Client Components

Best Practices

1. Default to Server Components

   • Better performance (no JS to client)
   • Direct data access
   • SEO friendly

2. Use Client Components Sparingly

   • Only when interactivity needed
   • Keep them small
   • Minimize bundle size

3. Compose Server + Client

   • Fetch data in Server Components
   • Pass as props to Client Components
   • Best of both worlds

4. Use New React 19 Hooks

   • use() for async data
   • useOptimistic() for instant feedback
   • useFormStatus() for form states
   • useActionState() for Server Actions

5. Leverage Suspense

   • Stream data progressively
   • Better perceived performance
   • Parallel data loading

Resources

• React 19 Docs: https://react.dev/
• Server Components: https://react.dev/reference/rsc/server-components
• React 19 Changelog: https://react.dev/blog/2024/12/05/react-19
• Hooks API: https://react.dev/reference/react/hooks
• Server Actions: https://react.dev/reference/rsc/server-actions

Quick Links

• Server vs Client Decision Tree
• All Hooks Reference
• Suspense Patterns
• Server Components Guide
• Client Components Guide
• Transitions Guide
• Streaming Patterns
• Migration Guide
• Validation Tool

Last Updated: 2025-11-23 React Version: 19.2.0 Next.js Version: 15.5