Server vs Client Component Boundaries

Role: Choose between Server and Client Components effectively and manage boundaries between them.

When to Activate

• User mentions Server Components, Client Components, or 'use client'
• Architecting component hierarchy
• Accessing server-only or client-only APIs
• Working with React Server Components frameworks (Next.js, Remix)
• Errors about hooks or browser APIs in Server Components

Component Comparison

Feature	Server Component	Client Component
Directive	None (default)	'use client'
Hooks; Event handlers; Browser APIs	❌	✅
Async/await (top-level)	✅	⚠️ Limited
Database/server APIs	✅ Direct	❌ Use Server Actions
Import Server Components	✅	❌ Pass as children
Bundle impact	📦 Zero	📦 Sent to client
Bundle reduction	20%-90%	—

Key Decision: Needs interactivity/hooks/browser APIs → Client Component; static/server-only data → Server Component

Quick Checklist

Choose Client Component if: needs hooks, event handlers, browser APIs, or state management.

Choose Server Component if: purely presentational, fetches server data, accesses databases/server APIs.

Implementing Components

Step 1: Add 'use client' at file top (Client Components only)

'use client';

import { useState } from 'react';

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

Step 2: Compose Server + Client

Server Components import Client Components; pass Server Components as children to avoid circular imports:

// Server Component: Can fetch data, import CC
import { Counter } from './Counter';

async function Page() {
  const data = await db.getData();
  return (
    <div>
      <h1>{data.title}</h1>
      <Counter />
    </div>
  );
}

// ❌ WRONG: Client Component importing Server Component
('use client');
import ServerComponent from './ServerComponent'; // ERROR

// ✅ RIGHT: Pass Server Component as children
<ClientWrapper>
  <ServerComponent />
</ClientWrapper>;

Common Patterns

Interactivity at leaf nodes: Server Component fetches data, passes to Client Component for interaction

async function ProductPage({ id }) {
  const product = await db.products.find(id);
  return (
    <>
      <ProductDetails product={product} />
      <AddToCartButton productId={id} />
    </>
  );
}

Server data in Client Component: Pass as props (serialize data)

async function ServerComponent() {
  const data = await fetchData();
  return <ClientComponent data={data} />;
}

Server logic from Client: Use Server Actions

async function ServerComponent() {
  async function serverAction() {
    'use server';
    await db.update(...);
  }
  return <ClientForm action

={serverAction} />;
}

Example: Product Page with Cart

// Server Component
import { AddToCart } from './AddToCart';
import { Reviews } from './Reviews';

async function ProductPage({ productId }) {
  const product = await db.products.find(productId);
  const reviews = await db.reviews.findByProduct(productId);

  return (
    <main>
      <img src={product.image} alt={product.name} />
      <section>
        <h1>{product.name}</h1>
        <p>{product.description}</p>
        <p>${product.price}</p>
        <AddToCart productId={productId} />
      </section>
      <Reviews reviews={reviews} />
    </main>
  );
}

// Client Component
('use client');
import { useState } from 'react';

export function AddToCart({ productId }) {
  const [adding, setAdding] = useState(false);

  async function handleAdd() {
    setAdding(true);
    await fetch('/api/cart', {
      method: 'POST',
      body: JSON.stringify({ productId }),
    });
    setAdding(false);
  }

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

// Server Component with real-time Client
async function Dashboard() {
  const stats = await db.stats.getLatest();
  return (
    <>
      <DashboardStats stats={stats} />
      <LiveMetrics />
    </>
  );
}

// Client Component with WebSocket
('use client');
import { useEffect, useState } from 'react';

export function LiveMetrics() {
  const [metrics, setMetrics] = useState(null);

  useEffect(() => {
    const ws = new WebSocket('/api/metrics');
    ws.onmessage = (event) => setMetrics(JSON.parse(event.data));
    return () => ws.close();
  }, []);

  return metrics ? <div>Active Users: {metrics.activeUsers}</div> : <div>Connecting...</div>;
}

// Server Component with Server Action
async function ContactPage() {
  async function submitContact(formData) {
    'use server';
    await db.contacts.create({
      email: formData.get('email'),
      message: formData.get('message'),
    });
  }

  return (
    <form action={submitContact}>
      <input name="email" type="email" />
      <textarea name="message" />
      <button type="submit">Send</button>
    </form>
  );
}

Requirements

MUST: 'use client' at file top for Client Components; before any imports; serialize props (Server → Client); use Server Actions for server-side logic from Client

SHOULD: Keep most components as Server Components (smaller bundle); place 'use client' at leaf nodes (smallest boundary); use Server Components for data fetching; use Client Components only for interactivity

NEVER: Import Server Components into Client Components; use hooks in Server Components; access browser APIs in Server Components; pass non-serializable props (functions, classes, symbols); om

it 'use client' directive

Validation Checklist

1. Component Types: Client Components have 'use client' at top; Server Components have no directive; no hooks/event handlers in Server Components
2. Data Flow: Server → Client props are serializable; Client → Server uses Server Actions; no Server Components imported in Client
3. Functionality: Server Components fetch data correctly; Client Components handle interaction; no hydration mismatches; no runtime errors about hooks/browser APIs
4. Bundle: Only necessary components are Client Components; most stay on server; JavaScript minimized

References

• Server Components: research/react-19-comprehensive.md (lines 71-82)
• Server Actions: forms/skills/server-actions/SKILL.md
• Component Composition: component-composition/SKILL.md