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Next.js Pages

In Next.js, a page is a React Component exported from a .js, .jsx, .ts, or .tsx file in the pages directory. Each page is associated with a route based on its file name.

Example: If you create pages/about.js that exports a React component like below, it will be accessible at /about.

function About() {
  return <div>About</div>
}

export default About

Pages with Dynamic Routes

Next.js supports pages with dynamic routes. For example, if you create a file called pages/posts/[id].js, then it will be accessible at posts/1, posts/2, etc.

To learn more about dynamic routing, check the Dynamic Routing documentation.

Pre-rendering

By default, Next.js pre-renders every page. This means that Next.js generates HTML for each page in advance, instead of having it all done by client-side JavaScript. Pre-rendering can result in better performance and SEO.

Each generated HTML is associated with minimal JavaScript code necessary for that page. When a page is loaded by the browser, its JavaScript code runs and makes the page fully interactive. (This process is called hydration.)

Two forms of Pre-rendering

Next.js has two forms of pre-rendering: Static Generation and Server-side Rendering. The difference is in when it generates the HTML for a page.

Static Generation (Recommended): The HTML is generated at build time and will be reused on each request.
Server-side Rendering: The HTML is generated on each request.

Importantly, Next.js lets you choose which pre-rendering form you'd like to use for each page. You can create a "hybrid" Next.js app by using Static Generation for most pages and using Server-side Rendering for others.

We recommend using Static Generation over Server-side Rendering for performance reasons. Statically generated pages can be cached by CDN with no extra configuration to boost performance. However, in some cases, Server-side Rendering might be the only option.

You can also use Client-side Rendering along with Static Generation or Server-side Rendering. That means some parts of a page can be rendered entirely by client side JavaScript. To learn more, take a look at the Data Fetching documentation.

Static Generation (Recommended)

If a page uses Static Generation, the page HTML is generated at build time. That means in production, the page HTML is generated when you run next build . This HTML will then be reused on each request. It can be cached by a CDN.

In Next.js, you can statically generate pages with or without data. Let's take a look at each case.

Static Generation without data

By default, Next.js pre-renders pages using Static Generation without fetching data. Here's an example:

function About() {
  return <div>About</div>
}

export default About

Note that this page does not need to fetch any external data to be pre-rendered. In cases like this, Next.js generates a single HTML file per page during build time.

Static Generation with data

Some pages require fetching external data for pre-rendering. There are two scenarios, and one or both might apply. In each case, you can use a special function Next.js provides:

Your page content depends on external data: Use getStaticProps.
Your page paths depend on external data: Use getStaticPaths (usually in addition to getStaticProps).

Scenario 1: Your page content depends on external data

Example: Your blog page might need to fetch the list of blog posts from a CMS (content management system).

// TODO: Need to fetch `posts` (by calling some API endpoint)
//       before this page can be pre-rendered.
function Blog({ posts }) {
  return (
    <ul>
      {posts.map((post) => (
        <li key="{post}">{post.title}</li>
      ))}
    </ul>
  )
}

export default Blog

To fetch this data on pre-render, Next.js allows you to export an async function called getStaticProps from the same file. This function gets called at build time and lets you pass fetched data to the page's props on pre-render.

function Blog({ posts }) {
  // Render posts...
}

// This function gets called at build time
export async function getStaticProps() {
  // Call an external API endpoint to get posts
  const res = await fetch('https://.../posts')
  const posts = await res.json()

  // By returning { props: { posts } }, the Blog component
  // will receive `posts` as a prop at build time
  return {
    props: {
      posts
    }
  }
}

export default Blog

To learn more about how getStaticProps works, check out the Data Fetching documentation.

Scenario 2: Your page paths depend on external data

Next.js allows you to create pages with dynamic routes. For example, you can create a file called pages/posts/[id].js to show a single blog post based on id. This will allow you to show a blog post with id: 1 when you access posts/1.

To learn more about dynamic routing, check the Dynamic Routing documentation.

However, which id you want to pre-render at build time might depend on external data.

Example: suppose that you've only added one blog post (with id: 1) to the database. In this case, you'd only want to pre-render posts/1 at build time.

Later, you might add the second post with id: 2. Then you'd want to pre-render posts/2 as well.

So your page paths that are pre-rendered depend on external data**.** To handle this, Next.js lets you export an async function called getStaticPaths from a dynamic page (pages/posts/[id].js in this case). This function gets called at build time and lets you specify which paths you want to pre-render.

// This function gets called at build time
export async function getStaticPaths() {
  // Call an external API endpoint to get posts
  const res = await fetch('https://.../posts')
  const posts = await res.json()

  // Get the paths we want to pre-render based on posts
  const paths = posts.map((post) => ({
    params: { id: post.id }
  }))

  // We'll pre-render only these paths at build time.
  // { fallback: false } means other routes should 404.
  return { paths, fallback: false }
}

Also in pages/posts/[id].js, you need to export getStaticProps so that you can fetch the data about the post with this id and use it to pre-render the page:

function Post({ post }) {
  // Render post...
}

export async function getStaticPaths() {
  // ...
}

// This also gets called at build time
export async function getStaticProps({ params }) {
  // params contains the post `id`.
  // If the route is like /posts/1, then params.id is 1
  const res = await fetch(`https://.../posts/${params.id}`)
  const post = await res.json()

  // Pass post data to the page via props
  return { props: { post } }
}

export default Post

To learn more about how getStaticPaths works, check out the Data Fetching documentation.

When should I use Static Generation?

We recommend using Static Generation (with and without data) whenever possible because your page can be built once and served by CDN, which makes it much faster than having a server render the page on every request.

You can use Static Generation for many types of pages, including:

Marketing pages
Blog posts
E-commerce product listings
Help and documentation

You should ask yourself: "Can I pre-render this page ahead of a user's request?" If the answer is yes, then you should choose Static Generation.

On the other hand, Static Generation is not a good idea if you cannot pre-render a page ahead of a user's request. Maybe your page shows frequently updated data, and the page content changes on every request.

In cases like this, you can do one of the following:

Use Static Generation with Client-side Rendering: You can skip pre-rendering some parts of a page and then use client-side JavaScript to populate them. To learn more about this approach, check out the Data Fetching documentation.
Use Server-Side Rendering: Next.js pre-renders a page on each request. It will be slower because the page cannot be cached by a CDN, but the pre-rendered page will always be up-to-date. We'll talk about this approach below.

Server-side Rendering

Also referred to as "SSR" or "Dynamic Rendering".

If a page uses Server-side Rendering, the page HTML is generated on each request.

To use Server-side Rendering for a page, you need to export an async function called getServerSideProps. This function will be called by the server on every request.

For example, suppose that your page needs to pre-render frequently updated data (fetched from an external API). You can write getServerSideProps which fetches this data and passes it to Page like below:

function Page({ data }) {
  // Render data...
}

// This gets called on every request
export async function getServerSideProps() {
  // Fetch data from external API
  const res = await fetch(`https://.../data`)
  const data = await res.json()

  // Pass data to the page via props
  return { props: { data } }
}

export default Page

As you can see, getServerSideProps is similar to getStaticProps, but the difference is that getServerSideProps is run on every request instead of on build time.

To learn more about how getServerSideProps works, check out our Data Fetching documentation

Summary

We've discussed two forms of pre-rendering for Next.js.

Static Generation (Recommended): The HTML is generated at build time and will be reused on each request. To make a page use Static Generation, either export the page component, or export getStaticProps (and getStaticPaths if necessary). It's great for pages that can be pre-rendered ahead of a user's request. You can also use it with Client-side Rendering to bring in additional data.
Server-side Rendering: The HTML is generated on each request. To make a page use Server-side Rendering, export getServerSideProps. Because Server-side Rendering results in slower performance than Static Generation, use this only if absolutely necessary.

2022-06-11