Next.js 13 has landed in a somewhat confusing way. Many remarkable things have been added; however, a good part is still Beta. Nevertheless, the Beta features give us important signals on how the future of Next.js will be shaped, so there are good reasons to keep a close eye on them, even if you’re going to wait to adopt them.

This article is part of a series of experiences about the Beta features. Let’s play with Server Components today.

Making server components the default option is arguably the boldest change made in Next.js 13. The goal of server component is to reduce the size of JS shipped to the client by keeping component code only on the server side. I.e., the rendering happens and only happens on the server side, even if the loading of the component is triggered on the client side (via client-side routing). It’s quite a big paradigm shift.

It feels quite “reasearch-y” by then, so it was shocking when seeing that Next.js is already betting its future on it now. Time flies and the fantastic engineers from React must have done some really great work.

npx create-next-app@latest --experimental-app --ts --eslint next13-server-components

Let’s have some fun playing with the project.

Server Component

The first difference noticed is that a new app folder now sits along with old friend page. They’ll save the routing changes to another article, but what’s worth mentioning for now is that every component under the app folder is, by default, a server component, meaning that it’s rendered on the server side, and its code stays on the server side.

Let’s create our very first server component now:

// app/server/page.tsx

export default function Server() {
    console.log('Server page rendering: this should only be printed on the server');
    return (
        <div>
            <h1>Server Page</h1>
            <p>My secret key: {process.env.MY_SECRET_ENV}</p>
        </div>
    );
}

If you access the /server route, whether by a fresh browser load or client-side routing, you’ll only see the line of log printed in your server console but never in the browser console. The environment variable value is fetched from the server side as well.

Looking at network traffic in the browser, you’ll see the content of the Server component is loaded via a remote call which returns an octet stream of JSON data of the render result:

Pic courtesy: medium.com

{
    ...
    "childProp": {
        "current": [
            [
                "$",
                "div",
                null,
                {
                    "children": [
                        ["$", "h1", null, { "children": "Server Page" }],
                        [
                            "$",
                            "p",
                            null,
                            {
                                "children": ["My secret key: ", "abc123"]
                            }
                        ]
                    ]
                }
            ]
        ]
    }
}

Rendering a server component is literally an API call to get serialized virtual DOM and then materialize it in the browser.

The most important thing to remember is that server components are for rendering non-interactive content, so there are no event handlers, no React hooks, and no browser-only APIs.

The most significant benefit is you can freely access any backend resource and secrets in server components. It’s safer (data don’t leak) and faster (code doesn’t leak).

Client Component

To make a client component, you’ll need to mark it so explicitly with use client:

// app/client/page.tsx

'use client';

import { useEffect } from 'react';

export default function Client() {
    console.log(
        'Client page rendering: this should only be printed on the server during ssr, and client when routing'
    );

    useEffect(() => {
        console.log('Client component rendered');
    });

    return (
        <div>
            <h1>Client Page</h1>
            {/* Uncommenting this will result in an error complaining about inconsistent
            rendering between client and server, which is very true */}
            {/* <p>My secret env: {process.env.MY_SECRET_ENV}</p> */}
        </div>
    );
}

As you may already anticipate, this gives you a similar behavior to the previous Next.js versions.

When the page is first loaded, it’s rendered by SSR, so you should see the first log in the server console; during client-side routing, both log messages will appear in the browser console.

Mix and Match

One of the biggest differences between Server Component and SSR is that SSR is at page level, while Server Component, as its name says, is at component level. This means you can mix and match server and client components in a render tree as you wish.

// A server page containing client component and nested server component

// app/mixmatch/page.tsx
import Client from './client';
import NestedServer from './nested-server';

export default function MixMatchPage() {
    console.log('MixMatchPage rendering');
    return (
        <div>
            <h1>Server Page</h1>
            <div className="box">
                <Client message="A message from server">
                    <NestedServer />
                </Client>
            </div>
        </div>
    );
}

// app/mixmatch/client.tsx
'use client';

import { useEffect } from 'react';

export default function Client({
    message,
    children,
}: {
    message: string;
    children: React.ReactNode;
}) {
    console.log('Client component rendering');

    return (
        <div>
            <h2>Client Child</h2>
            <p>Message from parent: {message}</p>
            <div className="box-red">{children}</div>
        </div>
    );
}

// app/mixmatch/nested-server.tsx

export default function NestedServer() {
    console.log('Nested server component rendering');
    return (
        <div>
            <h3>Nested Server</h3>
            <p>Nested server content</p>
        </div>
    );
}

Pic courtesy: medium.com

In a mixed scenario like this, server and client components get rendered independently, and the results are assembled by React runtime. Props passed from server components to client ones are serialized across the network (and need to be serializable).

Server Components Can Degenerate

One caution you need to take is that if a server component is directly imported into a client one, it silently degenerates into a client component.

Let’s revise the previous example slightly to observe it:

// app/degenerate/page.tsx

import Client from './client';

export default function DegeneratePage() {
    console.log('Degenerated page rendering');
    return (
        <div>
            <h1>Degenerated Page</h1>
            <div className="box-blue">
                <Client message="A message from server" />
            </div>
        </div>
    );
}

// app/degenerate/client.tsx

'use client';

import NestedServer from './nested-server';

export default function Client({ message }: { message: string }) {
    console.log('Client component rendering');

    return (
        <div>
            <h2>Client Child</h2>
            <p>Message from parent: {message}</p>
            <div className="box-blue">
                <NestedServer />
            </div>
        </div>
    );
}

// app/degenerated/nested-server.tsx

export default function NestedServer() {
    console.log('Nested server component rendering');
    return (
        <div>
            <h3>Degenerated Server</h3>
            <p>Degenerated server content</p>
        </div>
    );
}

If you check out the log, you’ll see NestedServer has “degenerated” and is now rendered by the browser.

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Content Source:

1. medium.com