Next.js 13: complete guide to Server Components and the App Directory
Tags: nextjs, react, webdev, javascript
Published: 2023-06-03 19:12:07

*This article was originally published on [makerkit.dev](https://makerkit.dev)*

Hi and welcome! In this tutorial, we'll learn how to use Next.js 13 with server components and the app directory.

## What does Next.js 13 bring to the table?

Next.js 13 is a major release of the most popular React framework: in fact, it ships with a new routing system, also called App Router. In many ways, this new routing system is a complete rewrite of the previous one, and **fundamentally changes the way we will write Next.js applications**.

The new `app` directory brings many improvements to the existing `pages` directory, and it's the new default way to write Next.js apps, after a period of experimental releases.

### Server Components (RSC)
The biggest change in the new App Router is the introduction of Server Components, **a new type of React components that run on the server and return compiled JSX that is sent to the client**. Server Components are useful for rendering the skeleton of a page, fetching data from the server in parallel, and passing it to the "client components".

Server Components are the default component type in the new `app` directory.

### Layouts
Powered by Server Components, layouts are foundational components that wrap pages. Next.js Layouts are not only useful for displaying a common UI across pages, but also to reuse logic and data fetching across pages.

Layouts also solve the waterfall problem, which is a common issue with the current Next.js routing system. In fact, with the new App Router, **we can fetch data in parallel**, and pass it to the page component: **this is a substantial performance improvement** over the current routing system.

### Server Actions

Still in alpha, Server Actions are a new way to **execute functions on the server**, instead of wiring them through an API handler. Server actions are useful for executing server-side logic, such as sending emails, or updating a database. Server actions are useful for submitting forms, execute server-side logic, and redirect the user to a new page.

Additionally, server actions offer us the ability to **revalidate data fetched from Server Components**, eliminating the need for complex client-side state management as a result of data mutation (such as updating a Redux store).

### Enhanced Router

Using conventional filenames, we can add various types of components in the `app` directory. This is a big improvement over the current routing system, which requires us to use a specific directory structure to define pages, API handlers, and so on.

What does this mean? From now on, we can create various types of components in the `app` directory using a specific filename convention specific to Next.js:
- pages are defined as `page.tsx`
- layouts are defined as `layout.tsx`
- errors are defined as `error.tsx`
- loading states are defined as `loading.tsx`

## What are Server Components?

Server Components are a new type of React components that run on the server and return compiled JSX that is sent to the client. Next.js, with its new app directory released in Next.js 13, fully embraced Server Components by making them the default type components.

This is a big shift from traditional React components that run both on the server and on the client. In fact, as we have specified, React Server components do not execute on the client.

As such, there are some constraints to using Server Components that we need to keep in mind:
- Server components cannot use browser-only APIs
- Server components cannot use React hooks
- Server components cannot use Context

---

So, what are they for?

React Server Components are useful for rendering the skeleton of a page, while leaving the interactive bits to the so-called "client components".

Despite their name, "client components" (which, IMHO, is unfortunate) are also server-rendered, and they run on both the server and the client.

React Server Components can be useful because they allows us to:
- render pages faster
- reduce the amount of JavaScript that needs to be sent to the client
- improve the routing performance of server-rendered pages

In short, we use Server Components to fetch data from the server and render the skeleton of a page: then, we can pass the data to the "client components".

### Server Components vs Client Components

As we have seen, Server Components are useful for rendering the skeleton of a page, while Client Components are the components as we know them today.

[This comparison in the Next.js docs](https://beta.nextjs.org/docs/rendering/server-and-client-components#when-to-use-server-vs-client-components) is a good way to understand the difference between the two.

#### Defining Server Components

Server components do not need a notation to be defined as such: server components are the default components when rendered in the app directory.

We cannot use React hooks, Context, or browser-only APIs in Server Components. However, we can use Server Components only APIs, such as `headers`, `cookies`, etc.

Server components **can import client components**.

There is no need to specify a notation to define a Server Component: in fact, Server Components **are the default component type** in the new `app` directory.

Assuming the component `ServerComponent` is not a child of a Client Component, **it will be rendered on the server and sent to the client as compiled JSX**:

```tsx
export default function ServerComponent() {
  return <div>Server Component</div>;
}
```

#### Defining Client components

On the contrary, in the Next.js app directory, we need to specifically define **client components**.

We can do this by specifying the `use client` pragma at the top of the file:

```tsx
'use client';

export default function ClientComponent() {
  return <div>Client Component</div>;
}
```

When we use client components, we can use React hooks, Context, and browser-only APIs. However, we cannot use some Server Components only APIs, such as `headers`, `cookies`, etc.

NB: Client components **cannot import server components**, but you can pass a Server Component as a child or prop of a Client Component.

## App directory

The new "app" directory released in Next.js 13 is 
an experimental
 a new way to build Next.js apps. It coexists with the `pages` directory, and we can use it to incrementally migrate an existing project to the new directory structure.

This new directory structure is not just a new way to write apps, it's a whole new routing system underneath, much more powerful than the current one.

### File Structure

What does the new Next.js 13 file structure look like? Let's take a look at the example app we'll be using in this tutorial.

Below is an example of a Next.js 13 app with the new `app` directory:

```bash
- app
  - layout.tsx
  - (site)
    - page.tsx
    - layout.tsx
  - app
    - dashboard
      - page.tsx
    - layout.tsx
```

As you can see, the name of the file reflects the type of component it is. For example, `layout.tsx` is a layout component, while `page.tsx` is a page component, and so on.

Don't worry, we will go through all the different types of components in the next sections.

#### Colocation

One important side-effect of the new `app` directory is that it allows us to colocate our files. Since filenames are conventional, we can define any file in the `app` directory without these becoming pages components.

For example, we could place our components for a specific page right in the folder where it's defined:

```bash
- app
  - (site)
    - components
      - Dashboard.tsx
    - hooks
      - use-fetch-data-hook.ts
    - page.tsx
```

Why is `(site)` in parentheses? By using parenthesis, we make the directory `site` "pathless", which means we can create new layouts, loading files, and pages in the `site` directory, without adding a new path segment to the routing.

All pages under `(site)` will be accessed from the root path `/`: for example, the page `app/(site)/page.tsx` will be accessible at `/`.

### Layouts

Layouts are one of the biggest new functionality made possible by the new App Router.

Layouts are foundational components that wrap pages: this is not only useful for displaying a common UI across pages, but also to reuse data-fetching and logic.

Next.js **needs one root layout component**:

```tsx
export const metadata = {
  title: 'Next.js Tutorial',
  description: 'A Next.js tutorial using the App Router',
};

async function RootLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  return (
    <html lang={'en'}>
      <body>{children}</body>
    </html>
  );
}

export default RootLayout;
```

Layouts are defined using the convention `layout.tsx` in the `app` directory: Next.js will automatically wrap all pages within the folder where the layout is defined.

For example, if we have a layout defined in `app/(site)/layout.tsx`, Next.js will wrap all pages in the `app/(site)` directory with this layout:

```tsx
export default async function SiteLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  return (
    <div>
      <main>
        {children}
      </main>
    </div>
  );
}
```

As a result - all the pages in the `app/(site)` directory will be wrapped with the `SiteLayout` component.

### Loading data in Layout Components

Layout components can be extremely useful also in case you need to load some data needed in all pages of a directory: for example, we could load the user's profile in the layout component, and pass it to the page components.

To fetch data in Layout Components in Next.js, we can use the new `use` hook, an experimental hook in React that uses `Suspense` to fetch data on the server.

```tsx
import { use } from "react";

export default function SiteLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  const data = use(getData());

  return (
    <div>
      <header>
        { data.user ? <ProfileDropown /> : null }
      </header>

      <main>
        {children}
      </main>
    </div>
  );
}

function getData() {
  return fetch('/api/data').then(res => res.json());
}
```

In the example above:

1. we fetch the data in the layout component using the `use` hook
2. we conditionally render the `ProfileDropdown` component based on the data.user property

NB: we used the `use` hook to fetch the data in a (seemingly) synchronous way. This is because the `use` hook uses `Suspense` under the hood, which allows us to write asynchronous code in a synchronous way.

#### Using Async/Await in Server Components

An alternative way would be to make the component an `async` component, and use `async/await` to fetch the data from `getData`:

```tsx
export default async function SiteLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  const data = await getData()

  return (
    <div>
      <header>
        { data.user ? <ProfileDropown /> : null }
      </header>

      <main>
        {children}
      </main>
    </div>
  );
}

function getData() {
  return fetch('/api/data').then(res => res.json());
}
```

#### Reading Cookies and Headers

If you're using a Server Component, you can read cookies and headers using from the `next/headers` package.

NB: At the time of writing, we can only use these functions to read their values, but not to set or delete them.

```tsx
import { cookies } from 'next/headers';

export function Layout(
  { children }: { children: React.ReactNode },
) {
  const lang = cookies.get('lang');

  return (
    <html lang={lang}>
      <body>
        {children}
      </body>
    </html>
  );
}
```

If you feel like something is missing, don't worry, it's not just you. In fact, unlike `getServerSideProps`, we do not have access to the `request` object. This is why Next.js is exposing these utilities to read data from the request.

#### Redirecting from Layouts

In layouts, we can also redirect users to a different page.

For example, if we want to redirect users to the login page if they are not authenticated, we can do it in the layout component:

```tsx
import { use } from 'react';
import { redirect } from 'next/navigation';

function AuthLayout(
  props: React.PropsWithChildren,
) {
  const session = use(getSession());

  if (session) {
    return redirect('/dashboard');
  }

  return (
    <div className={'auth'}>
      {props.children}
    </div>
  );
}


function getSession() {
  return fetch('/api/session').then(res => res.json());
}
```
Now, we can use the `loadSession` function in the layout component:

```tsx
import { use } from 'react';

function AuthLayout(
  props: React.PropsWithChildren,
) {
  const response = use(loadSession());
  const data = response.data;

  // do something with data

  return (
    <div className={'auth'}>
      {props.children}
    </div>
  );
}
```

#### Using the redirect side-effect in Next.js

The new Next.js function `redirect` will throw an error: in fact, its return type is `never`. If you catch the error, you need to be careful and ensure to follow the redirect thrown by the error.

To do that, we can use some utilities exported by the Next.js package:

```tsx
import { use } from 'react';

import {
  isRedirectError,
  getURLFromRedirectError,
} from 'next/dist/client/components/redirect';

import { redirect } from "next/navigation";

async function loadData() {
  try {
    const data = await getData();

    if (!data) {
      return redirect('/login');
    }

    const user = data.user;

    console.log(`User ${user.name} logged in`);

    return user;
  } catch (e) {
    if (isRedirectError(e)) {
      return redirect(getURLFromRedirectError(e));
    }

    throw e;
  }
}

function Layout(
  props: React.PropsWithChildren,
) {
  const data = use(loadData());

  // do something with data

  return (
    <div>
      {props.children}
    </div>
  );
}
```

### Pages

To define pages in the new app directory, we use the special convention `page.tsx`. That means, if we want to define a page in the `app` directory, we need to name the file `page.tsx`.

For example, if we want to define the home page of your website, we can place the page in the `app/(site)` directory and name it `page.tsx`:

```tsx
function SitePage() {
  return <div>Site Page</div>;
}

export default SitePage;
```

#### Page Metadata and SEO

To specify the metadata of a page, we can export the constant `metadata` property in the `page.tsx` file:

```tsx
export const metadata = {
  title: 'Site Page',
  description: 'This is the site page',
};
```

If you need to access dynamic data, you can use the `generateMetadata` function:

```tsx
export async function generateMetadata(
  { params, searchParams }
) {
  return { title: '...' };
}
```

Check out the [Next.js documentation for the full list of supported metadata properties](https://beta.nextjs.org/docs/api-reference/metadata).

#### Generating Static Pages

To generate a list of static pages to be used with dynamic parameters, we can use the `generateStaticParams` function:

```tsx
export async function generateStaticParams() {
  const posts = await getPosts();

  return posts.map((post) => ({
    slug: post.slug,
  }));
}
```

Check out the full documentation for [generating static paths](https://beta.nextjs.org/docs/api-reference/generate-static-params).

### Loading Indicators

When navigation between pages, we may want to display a loading indicator. To do this, we can use the `loading.tsx` file which we can define in every directory:

```tsx 
export default function Loading() {
  return <div>Loading...</div>;
}
```

Here you can add any component you want to display while the page is loading, such as a top bar loader, or a loading spinner, or both.

### Error Handling

At the moment, you can define a "not found" page using the convention `not-found.tsx`:

```tsx
export default function NotFound() {
  return (
    <>
      <h2>Not Found</h2>
      <p>Could not find requested resource</p>
    </>
  );
}
```

This file will only be displayed if used in conjunction with the `notFound` function. This is why it's still recommended to use custom 400 and 500 pages using the old `pages` directory.

#### Custom 404 and 500 pages

At the time of writing, we need to stick with the regular `pages` directory to define custom 404 and 500 pages. This is because Next.js does not support custom 404 and 500 pages in the `app` directory.

### Fonts

We can use the package `next/font` to load fonts in our application.

To do so, we need to define a client component, and import it in the root layout `app/layout.tsx` file:

```tsx
'use client';

import { Inter } from 'next/font/google';
import { useServerInsertedHTML } from 'next/navigation';

const heading = Inter({
  subsets: ['latin'],
  variable: '--font-family-heading',
  fallback: ['--font-family-sans'],
  weight: ['400', '500'],
  display: 'swap',
});

export default function Fonts() {
  useServerInsertedHTML(() => {
    return (
      <style
        dangerouslySetInnerHTML={{
          __html: `
          :root {
            --font-family-sans: '-apple-system', 'BlinkMacSystemFont',
              ${sans.style.fontFamily}, 'system-ui', 'Segoe UI', 'Roboto',
              'Ubuntu', 'sans-serif';

            --font-family-heading: ${heading.style.fontFamily};
          }
        `,
        }}
      />
    );
  });

  return null;
}
```

After that, we can import the `Fonts` component in the root layout:

```tsx
import Fonts from '~/components/Fonts';

export default async function RootLayout({
  children,
}: {
  children: React.ReactNode;
}) {
  return (
    <html>
      <Fonts />

      <body>{children}</body>
    </html>
  );
}
```

### API Routes

The new app directory also supports API routes. The convention to define an API route is to create a file named `route.tsx` in the `app` directory.

API routes now use the standard `Request` object rather than the `express`-like `req` and `res` objects.

When we define an API route, we can export the handler for the methods we want to support. For example, if we want to support the `GET` and `POST` methods, we can export the `GET` and `POST` functions:

```tsx
import { NextResponse } from 'next/server';

export async function GET() {
  return NextResponse.json({ hello: 'world' });
}

export async function POST(
  request: Request
) {
  const body = await request.json();
  const data = await getData(body);

  return NextResponse.json(data);
}
```

If we want to manipulate the response, for example by setting cookies, we can use the `NextResponse` object:

```tsx
export async function POST(
  request: Request
) {
  const organizationId = getOrganizationId();
  const response = NextResponse.json({ organizationId });

  response.cookies.set('organizationId', organizationId, {
    path: '/',
    httpOnly: true,
    sameSite: 'lax',
  });

  return response;
}
```

In API routes, just like in Server Components, we can also redirect users using the `redirect` function imported from `next/navigation`:

```tsx
import { redirect } from 'next/navigation';

export async function GET(
  request: Request
) {
  return redirect('/login');
}
```

#### Handling Webhooks

Handling webhooks is a common use case for API routes, and getting the raw body request is now much simpler. In fact, we can get the raw body request by using the `request.text()` method:

```tsx

export async function POST(
  request: Request
) {
  const rawBody = await request.text();

  // handle webhook here
}
```

## Server Actions

Server Actions are a new concept introduced in Next.js 13. They are a way to define server-side actions that can be called from the client. All you need to do is to define a function and use the `use server` keyword at the top:

For example, the below is a valid server action:

```tsx
async function myActionFunction() {
  'use server';

  // do something
}
```

If you are defining a server action from a client component, this needs to be exported from a separate file, and imported in the client component. The file needs the keyword `use server` at the top:

```tsx
 'use server';

async function myActionFunction() {
  // do something
}
```

To call the server action from the client, you have multiple ways

1. Defining the action as the `action` property of a `form` component
2. Calling the action from a `button` component using the `formAction` property
3. Calling the action using the `useTransition` hook (if it mutates data)
4. Simply calling the action like a normal function (if it does not mutate data)

If you want to know more about Server Actions, I will soon post a whole article dedicated to them soon.

## Conclusion

In this article, we learned how to use the new App Router in Next.js 13.

While Next.js App Router is now out of beta, some of the patterns and conventions we learned in this article are new and may be still not production-ready, and may change in the future.

However, they are already very useful, and we can already start using them in our projects.