React Server Components (RSC): The Complete Guide 2025

Table of Contents

• What Are React Server Components?
• Client vs Server Components
• When to Use Each
• Data Fetching Patterns
• Performance Impact
• Common Mistakes
• Interview Q&A
• FAQ

What Are React Server Components?

React Server Components (RSC) let you write components that run only on the server. They never send JavaScript to the client – only their rendered output (like HTML). This means zero bundle size for those components.

Client vs Server Components

Server Components can access databases, filesystems, or any backend resource directly. They cannot use useState, useEffect, or browser APIs. Client Components are what we've always used – they hydrate and run in the browser.

// ServerComponent.server.jsx – runs only on server
export default async function ServerComponent() {
  const data = await db.query('SELECT * FROM users');
  return <div>{data.map(...)}</div>;
}
// App.jsx – mixing both
import ClientCounter from ‘./ClientCounter.client’;
import ServerComponent from ‘./ServerComponent.server’;
export default function Page() {
return (
<div>
<ServerComponent />  {/* zero JS sent /}
<ClientCounter />    {/ interactive */}
</div>
);
}

Performance Impact

RSC dramatically reduces bundle size – sometimes by 60-80%. Large rendering logic, heavy dependencies (date-fns, lodash) stay on the server. First Contentful Paint improves by 30-50% in typical apps.

Common Mistakes

• Putting client hooks (useState) in Server Components – error
• Forgetting the ‘use client’ directive for client components
• Trying to use localStorage or window in Server Components

Interview Q&A

Q: Can Server Components re-render? Yes, but on the server via navigation or refetch. They don’t re-render on client.
Q: How do Server Components communicate with Client Components? Via props – you can pass serializable data down, or use context with a provider boundary.

Conclusion

RSC is the biggest shift in React since hooks. Adopt gradually: start with static pages, then data-heavy lists, finally full migration. Pair with Next.js App Router for production.