React Server Components vs Client Components: The Decision Framework for Next.js 14

React Server Components (RSC) are the biggest mental model shift in Next.js 14. Most devs either avoid them entirely or use them incorrectly.

Here's the decision framework that makes it click.

The Core Distinction

Server Components run only on the server. They can:

• Fetch data directly (no useEffect, no loading states)
• Access databases, file systems, secrets
• Import heavy libraries without sending them to the browser
• NOT use hooks, event handlers, or browser APIs

Client Components run on the server (for SSR) AND in the browser. They can:

• Use useState, useEffect, useRef, and all hooks
• Handle user events (onClick, onChange)
• Access browser APIs (localStorage, window, navigator)
• NOT directly access server-side resources

The Decision Tree

Does this component need:
  - onClick, onChange, or other event handlers? → Client
  - useState or useReducer? → Client
  - useEffect? → Client
  - Browser APIs (window, localStorage)? → Client
  - Real-time updates? → Client

Does this component need:
  - Database queries? → Server
  - API keys or secrets? → Server
  - Large dependencies (markdown parsers, etc.)? → Server
  - Static or async data? → Server

Not sure? → Default to Server, add 'use client' only when needed

Default Is Server in App Router

In app/, all components are Server Components by default:

// app/dashboard/page.tsx -- Server Component (no directive needed)
import { db } from '@/lib/db'

export default async function DashboardPage() {
  // Direct DB access -- no API call needed
  const user = await db.user.findUnique({ where: { id: getCurrentUserId() } })
  const stats = await db.analytics.findMany({ where: { userId: user.id } })

  return (
    <div>
      <h1>Welcome {user.name}</h1>
      <StatsChart data={stats} /> {/* This can be a client component */}
    </div>
  )
}

Adding 'use client'

The directive marks a boundary. Everything in that file and its imports becomes client-side:

'use client'

// app/dashboard/stats-chart.tsx
import { useState } from 'react'
import { Chart } from 'recharts' // Heavy library -- sent to browser

export function StatsChart({ data }) {
  const [view, setView] = useState<'week' | 'month'>('week')

  return (
    <div>
      <button onClick={() => setView('week')}>Week</button>
      <button onClick={() => setView('month')}>Month</button>
      <Chart data={data} />
    </div>
  )
}

The parent (DashboardPage) remains a Server Component. It passes data down to the Client Component as props.

The Composition Pattern

Keep Server Components at the top, push Client Components to the leaves:

DashboardPage (Server)          -- fetches data, no JS sent
  Header (Server)               -- static, no JS sent
  UserStats (Server)            -- DB query, no JS sent
    StatsChart (Client)         -- interactive chart
  RecentActivity (Server)       -- DB query, no JS sent
    ActivityFilter (Client)     -- search/filter UI
  ProfileSettings (Client)      -- form with state

This minimizes the JavaScript sent to the browser while keeping interactivity where needed.

Passing Server Data to Client Components

// Server Component
async function ProductPage({ id }: { id: string }) {
  const product = await db.product.findUnique({ where: { id } })

  return (
    <div>
      <h1>{product.name}</h1>
      {/* Pass serializable data -- no class instances, functions, or Dates */}
      <AddToCartButton
        productId={product.id}
        price={product.price}
        inStock={product.inStock}
      />
    </div>
  )
}

// Client Component
'use client'
function AddToCartButton({ productId, price, inStock }) {
  const [adding, setAdding] = useState(false)

  async function handleAdd() {
    setAdding(true)
    await addToCart(productId)
    setAdding(false)
  }

  return (
    <button onClick={handleAdd} disabled={!inStock || adding}>
      {adding ? 'Adding...' : `Add to Cart -- $${price}`}
    </button>
  )
}

What Can't Cross the Boundary

Props from Server to Client must be serializable:

// OK -- primitives and plain objects
<ClientComponent name="Atlas" count={42} active={true} />

// OK -- arrays and nested objects
<ClientComponent items={[{ id: 1, name: 'foo' }]} />

// NOT OK -- functions
<ClientComponent onSave={async () => { /* server function */ }} />
// Use Server Actions instead

// NOT OK -- class instances
<ClientComponent user={prismaUser} />
// Serialize first: user={{ id: prismaUser.id, name: prismaUser.name }}

Server Actions for Mutations

Server Actions let Client Components call server-side functions:

// lib/actions.ts
'use server'

export async function updateProfile(formData: FormData) {
  const name = formData.get('name') as string
  await db.user.update({ where: { id: getSession().userId }, data: { name } })
  revalidatePath('/dashboard')
}

// components/profile-form.tsx
'use client'
import { updateProfile } from '@/lib/actions'

export function ProfileForm({ currentName }) {
  return (
    <form action={updateProfile}>
      <input name="name" defaultValue={currentName} />
      <button type="submit">Save</button>
    </form>
  )
}

AI SaaS Starter: RSC Pre-Configured

The AI SaaS Starter Kit ships with this pattern already implemented:

• Dashboard page (Server) fetching user data directly
• Interactive widgets (Client) receiving data as props
• Server Actions for all mutations
• Loading UI with Suspense boundaries

AI SaaS Starter Kit -- $99 one-time -- skip the RSC learning curve, start with a working architecture.

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