Exploring Next.js 13 Server Components | by Yiming Cao | Nov, 2022

It feels quite “reasearch-y” by then, so I was shocked 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, so I created a shiny new Next.js 13 project to play with it.

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

Let’s have some fun playing with the project. You can find the full project code here.

The first difference noticed is that a new app folder now sits along with our old friend page. I’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:

{
...
"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).

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.

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>
);
}

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).

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 MixMatchPage() {
console.log('MixMatchPage rendering');
return (
<div>
<h1>MixMatch Server 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>
);
}

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

Next.js is trying everything it can to move things to the server side, exactly how people did web development two decades ago. So now we’re completing a full circle, but with greatly improved development experiences and end-user experiences.

For the end users, it’s a clear win since computing on the server side is faster and more reliable. The result will be a much more rapid first content paint.

For developers, the paradigm shift will be mentally challenging, mixed with confusion, bugs, and anti-patterns. It will be a hell of a journey.

Thanks for reading.

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I'm the creator of ZenStack, a toolkit for building 
secure  CRUD services with Next.js + TypeScript. 
Our goal is to let you save time writing boilerplate code 
and focus on building what matters - the user experience.