React Server Components

This separate environment is the “server” in React Server Components. Server Components can run once at build time on your CI server, or they can be run for each request using a web server.

• Server Components without a Server
• Server Components with a Server
• Adding interactivity to Server Components
• Async components with Server Components

Server Components without a Server

Server components can run at build time to read from the filesystem or fetch static content, so a web server is not required. For example, you may want to read static data from a content management system.

Without Server Components, it’s common to fetch static data on the client with an Effect:

// bundle.js
import marked from 'marked'; // 35.9K (11.2K gzipped)
import sanitizeHtml from 'sanitize-html'; // 206K (63.3K gzipped)

function Page({page}) {
  const [content, setContent] = useState('');
  // NOTE: loads *after* first page render.
  useEffect(() => {
    fetch(`/api/content/${page}`).then((data) => {
      setContent(data.content);
    });
  }, [page]);
  
  return <div>{sanitizeHtml(marked(content))}</div>;
}

// api.js
app.get(`/api/content/:page`, async (req, res) => {
  const page = req.params.page;
  const content = await file.readFile(`${page}.md`);
  res.send({content});
});

This pattern means users need to download and parse an additional 75K (gzipped) of libraries, and wait for a second request to fetch the data after the page loads, just to render static content that will not change for the lifetime of the page.

With Server Components, you can render these components once at build time:

import marked from 'marked'; // Not included in bundle
import sanitizeHtml from 'sanitize-html'; // Not included in bundle

async function Page({page}) {
  // NOTE: loads *during* render, when the app is built.
  const content = await file.readFile(`${page}.md`);
  
  return <div>{sanitizeHtml(marked(content))}</div>;
}

The rendered output can then be server-side rendered (SSR) to HTML and uploaded to a CDN. When the app loads, the client will not see the original Page component, or the expensive libraries for rendering the markdown. The client will only see the rendered output:

<div><!-- html for markdown --></div>

This means the content is visible during first page load, and the bundle does not include the expensive libraries needed to render the static content.

async function Page({page}) {
  //...
}

Server Components with a Server

Server Components can also run on a web server during a request for a page, letting you access your data layer without having to build an API. They are rendered before your application is bundled, and can pass data and JSX as props to Client Components.

Without Server Components, it’s common to fetch dynamic data on the client in an Effect:

// bundle.js
function Note({id}) {
  const [note, setNote] = useState('');
  // NOTE: loads *after* first render.
  useEffect(() => {
    fetch(`/api/notes/${id}`).then(data => {
      setNote(data.note);
    });
  }, [id]);
  
  return (
    <div>
      <Author id={note.authorId} />
      <p>{note}</p>
    </div>
  );
}

function Author({id}) {
  const [author, setAuthor] = useState('');
  // NOTE: loads *after* Note renders.
  // Causing an expensive client-server waterfall.
  useEffect(() => {
    fetch(`/api/authors/${id}`).then(data => {
      setAuthor(data.author);
    });
  }, [id]);

  return <span>By: {author.name}</span>;
}

// api
import db from './database';

app.get(`/api/notes/:id`, async (req, res) => {
  const note = await db.notes.get(id);
  res.send({note});
});

app.get(`/api/authors/:id`, async (req, res) => {
  const author = await db.authors.get(id)




    
;
  res.send({author});
});

With Server Components, you can read the data and render it in the component:

import db from './database';

async function Note({id}) {
  // NOTE: loads *during* render.
  const note = await db.notes.get(id);
  return (
    <div>
      <Author id={note.authorId} />
      <p>{note}</p>
    </div>
  );
}

async function Author({id}) {
  // NOTE: loads *after* Note,
  // but is fast if data is co-located.
  const author = await db.authors.get(id);
  return <span>By: {author.name}</span>;
}

The bundler then combines the data, rendered Server Components and dynamic Client Components into a bundle. Optionally, that bundle can then be server-side rendered (SSR) to create the initial HTML for the page. When the page loads, the browser does not see the original Note and Author components; only the rendered output is sent to the client:

<div>
  <span>By: The React Team</span>
  <p>React 19 is...</p>
</div>

Server Components can be made dynamic by re-fetching them from a server, where they can access the data and render again. This new application architecture combines the simple “request/response” mental model of server-centric Multi-Page Apps with the seamless interactivity of client-centric Single-Page Apps, giving you the best of both worlds.

Adding interactivity to Server Components

Server Components are not sent to the browser, so they cannot use interactive APIs like useState . To add interactivity to Server Components, you can compose them with Client Component using the "use client" directive.

In the following example, the Notes Server Component imports an Expandable Client Component that uses state to toggle its expanded state:

// Server Component
import Expandable from './Expandable';

async function Notes() {
  const notes = await db.notes.getAll();
  return (
    <div>
      {notes.map(note => (
        <Expandable key={note.id}>
          <p note={note} />
        </Expandable>
      ))}
    </div>
  )
}

// Client Component
"use client"

export default function Expandable({children}) {
  const [expanded, setExpanded] = useState(false);
  return (
    <div>
      <button
        onClick={() => setExpanded(!expanded)}
      >
        Toggle
      </button>
      {expanded && children}
    </div>
  )
}

This works by first rendering Notes as a Server Component, and then instructing the bundler to create a bundle for the Client Component Expandable . In the browser, the Client Components will see output of the Server Components passed as props:

<head>
  <!-- the bundle for Client Components -->
  <script src="bundle.js" />
</head>
<body>
  <div>
    <Expandable key={1}>
      <p>this is the first note</p>
    </Expandable>
    <Expandable key={2}>
      <p>this is the second note</p>
    </Expandable>
    <!--...-->
  </div> 
</body>

Async components with Server Components