How Next.js Works

What is Rendering?

There is an unavoidable unit of work to convert the code you write in React into the HTML representation of your UI. This process is called rendering.

Rendering can take place on the server or on the client. It can happen either ahead of time at build time, or on every request at runtime.

With Next.js, three types of rendering methods are available: Server-Side Rendering, Static Site Generation, and Client-Side Rendering.

Pre-Rendering

Server-Side Rendering and Static Site Generation are also referred to as Pre-Rendering because the fetching of external data and transformation of React components into HTML happens before the result is sent to the client.

Client-Side Rendering vs. Pre-Rendering

In a standard React application, the browser receives an empty HTML shell from the server along with the JavaScript instructions to construct the UI. This is called client-side rendering because the initial rendering work happens on the user's device.

Note: You can opt to use client-side rendering for specific components in your Next.js application by choosing to fetch data with React’s useEffect() or a data fetching hook such as useSWR.

In contrast, Next.js pre-renders every page by default. Pre-rendering means the HTML is generated in advance, on a server, instead of having it all done by JavaScript on the user's device.

In practice, this means that for a fully client-side rendered app, the user will see a blank page while the rendering work is being done. Compared to a pre-rendered app, where the user will see the constructed HTML:

Let’s discuss the two types of pre-rendering:

Server-Side Rendering

With server-side rendering, the HTML of the page is generated on a server for each request. The generated HTML, JSON data, and JavaScript instructions to make the page interactive are then sent to the client.

On the client, the HTML is used to show a fast non-interactive page, while React uses the JSON data and JavaScript instructions to make components interactive (for example, attaching event handlers to a button). This process is called hydration.

In Next.js, you can opt to server-side render pages by using getServerSideProps.

Note: React 18 and Next 12 introduce an alpha version of React server components. Server components are completely rendered on the server and do not require client-side JavaScript to render. In addition, server components allow developers to keep some logic on the server and only send the result of that logic to the client. This reduces the bundle size sent to the client and improves client-side rendering performance. Learn more about React server components here.

Static Site Generation

With Static Site Generation, the HTML is generated on the server, but unlike server-side rendering, there is no server at runtime. Instead, content is generated once, at build time, when the application is deployed, and the HTML is stored in a CDN and re-used for each request.

In Next.js, you can opt to statically generate pages by using getStaticProps.

Note: You can use Incremental Static Regeneration to create or update static pages after you’ve built your site. This means you do not have to rebuild your entire site if your data changes.

The beauty of Next.js is that you can choose the most appropriate rendering method for your use case on a page-by-page basis, whether that's Static Site Generation, Server-side Rendering, or Client-Side Rendering. To learn more about which rendering method is right for your specific use case, see the data fetching docs.

In the next section, we’ll discuss where your code can be stored or run after it’s deployed.

Quick Review: Can you have multiple rendering methods in a single Next.js application?