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Next.js

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Data fetching

This document is for Next.js versions 9.3 and up. If you’re using older versions of Next.js, refer to our previous documentation.

Examples

In the Pages documentation, we’ve explained that Next.js has two forms of pre-rendering: Static Generation and Server-side Rendering. In this page, we’ll talk in depths about data fetching strategies for each case. We recommend you to read through the Pages documentation first if you haven’t done so.

We’ll talk about the three unique Next.js functions you can use to fetch data for pre-rendering:

In addition, we’ll talk briefly about how to do fetch data on the client side.

getStaticProps (Static Generation)

If you export an async function called getStaticProps from a page, Next.js will pre-render this page at build time using the props returned by getStaticProps.

export async function getStaticProps(context) {
  return {
    props: {}, // will be passed to the page component as props
  }
}

The context parameter is an object containing the following keys:

  • params contains the route parameters for pages using dynamic routes. For example, if the page name is [id].js , then params will look like { id: ... }. To learn more, take a look at the Dynamic Routing documentation. You should use this together with getStaticPaths, which we’ll explain later.
  • preview is true if the page is in the preview mode and false otherwise. See the Preview Mode documentation.
  • previewData contains the preview data set by setPreviewData. See the Preview Mode documentation.

Simple Example

Here’s an example which uses getStaticProps to fetch a list of blog posts from a CMS (content management system). This example is also in the Pages documentation.

// posts will be populated at build time by getStaticProps()
function Blog({ posts }) {
  return (
    <ul>
      {posts.map(post => (
        <li>{post.title}</li>
      ))}
    </ul>
  )
}

// This function gets called at build time on server-side.
// It won't be called on client-side, so you can even do
// direct database queries. See the "Technical details" section.
export async function getStaticProps() {
  // Call an external API endpoint to get posts.
  // You can use any data fetching library
  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

When should I use getStaticProps?

You should use getStaticProps if:

  • The data required to render the page is available at build time ahead of a user’s request.
  • The data comes from headless CMS.
  • The data can be publicly cached (not user-specific).
  • The page must be pre-rendered (for SEO) and be very fast — getStaticProps generates HTML and JSON files, both of which can be cached by a CDN for performance.

TypeScript: Use GetStaticProps

For TypeScript, you can use the GetStaticProps type from next:

import { GetStaticProps } from 'next'

export const getStaticProps: GetStaticProps = async context => {
  // ...
}

Reading files: Use process.cwd()

Files can be read directly from the filesystem in getStaticProps.

In order to do so you have to get the full path to a file.

Since Next.js compiles your code into a separate directory you can't use __dirname as the path it will return will be different from the pages directory.

Instead you can use process.cwd() which gives you the directory where Next.js is being executed.

import fs from 'fs'
import path from 'path'

// posts will be populated at build time by getStaticProps()
function Blog({ posts }) {
  return (
    <ul>
      {posts.map(post => (
        <li>
          <h3>{post.filename}</h3>
          <p>{post.content}</p>
        </li>
      ))}
    </ul>
  )
}

// This function gets called at build time on server-side.
// It won't be called on client-side, so you can even do
// direct database queries. See the "Technical details" section.
export async function getStaticProps() {
  const postsDirectory = path.join(process.cwd(), 'posts')
  const filenames = fs.readdirSync(postsDirectory)

  const posts = filenames.map(filename => {
    const filePath = path.join(postsDirectory, filename)
    const fileContents = fs.readFileSync(filePath, 'utf8')

    // Generally you would parse/transform the contents
    // For example you can transform markdown to HTML here

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

export default Blog

Technical details

Only runs at build time

Because getStaticProps runs at build time, it does not receive data that’s only available during request time, such as query parameters or HTTP headers as it generates static HTML.

Write server-side code directly

Note that getStaticProps runs only on the server-side. It will never be run on the client-side. It won’t even be included in the JS bundle for the browser. That means you can write code such as direct database queries without them being sent to browsers. You should not fetch an API route from getStaticProps — instead, you can write the server-side code directly in getStaticProps.

Statically Generates both HTML and JSON

When a page with getStaticProps is pre-rendered at build time, in addition to the page HTML file, Next.js generates a JSON file holding the result of running getStaticProps.

This JSON file will be used in client-side routing through next/link (documentation) or next/router (documentation). When you navigate to a page that’s pre-rendered using getStaticProps, Next.js fetches this JSON file (pre-computed at build time) and uses it as the props for the page component. This means that client-side page transitions will not call getStaticProps as only the exported JSON is used.

Only allowed in a page

getStaticProps can only be exported from a page. You can’t export it from non-page files.

One of the reasons for this restriction is that React needs to have all the required data before the page is rendered.

Also, you must use export async function getStaticProps() {} — it will not work if you add getStaticProps as a property of the page component.

Runs on every request in development

In development (next dev), getStaticProps will be called on every request.

Preview Mode

In some cases, you might want to temporarily bypass Static Generation and render the page at request time instead of build time. For example, you might be using a headless CMS and want to preview drafts before they're published.

This use case is supported by Next.js by the feature called Preview Mode. Learn more on the Preview Mode documentation.

getStaticPaths (Static Generation)

If a page has dynamic routes (documentation) and uses getStaticProps it needs to define a list of paths that have to be rendered to HTML at build time.

If you export an async function called getStaticPaths from a page that uses dynamic routes, Next.js will statically pre-render all the paths specified by getStaticPaths.

export async function getStaticPaths() {
  return {
    paths: [
      { params: { ... } } // See the "paths" section below
    ],
    fallback: true or false // See the "fallback" section below
  };
}

The paths key (required)

The paths key determines which paths will be pre-rendered. For example, suppose that you have a page that uses dynamic routes named pages/posts/[id].js. If you export getStaticPaths from this page and return the following for paths:

return {
  paths: [
    { params: { id: '1' } },
    { params: { id: '2' } }
  ],
  fallback: ...
}

Then Next.js will statically generate posts/1 and posts/2 at build time using the page component in pages/posts/[id].js.

Note that the value for each params must match the parameters used in the page name:

  • If the page name is pages/posts/[postId]/[commentId], then params should contain postId and commentId.
  • If the page name uses catch-all routes, for example pages/[...slug], then params should contain slug which is an array. For example, if this array is ['foo', 'bar'], then Next.js will statically generate the page at /foo/bar.

The fallback key (required)

The object returned by getStaticPaths must contain a boolean fallback key.

fallback: false

If fallback is false, then any paths not returned by getStaticPaths will result in a 404 page. You can do this if you have a small number of paths to pre-render - so they are all statically generated during build time. It’s also useful when the new pages are not added often. If you add more items to the data source and need to render the new pages, you’d need to run the build again.

Here’s an example which pre-renders one blog post per page called pages/posts/[id].js. The list of blog posts will be fetched from a CMS and returned by getStaticPaths . Then, for each page, it fetches the post data from a CMS using getStaticProps. This example is also in the Pages documentation.

// pages/posts/[id].js

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

// 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 }
}

// 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

fallback: true

If fallback is true, then the behavior of getStaticProps changes:

  • The paths returned from getStaticPaths will be rendered to HTML at build time.
  • The paths that have not been generated at build time will not result in a 404 page. Instead, Next.js will serve a “fallback” version of the page on the first request to such a path (see “Fallback pages” below for details).
  • In the background, Next.js will statically generate the requested path HTML and JSON. This includes running getStaticProps.
  • When that’s done, the browser receives the JSON for the generated path. This will be used to automatically render the page with the required props. From the user’s perspective, the page will be swapped from the fallback page to the full page.
  • At the same time, Next.js adds this path to the list of pre-rendered pages. Subsequent requests to the same path will serve the generated page, just like other pages pre-rendered at build time.

Fallback pages

In the “fallback” version of a page:

  • The page’s props will be empty.
  • Using the router, you can detect if the fallback is being rendered, router.isFallback will be true.

Here’s an example that uses isFallback:

// pages/posts/[id].js
import { useRouter } from 'next/router'

function Post({ post }) {
  const router = useRouter()

  // If the page is not yet generated, this will be displayed
  // initially until getStaticProps() finishes running
  if (router.isFallback) {
    return <div>Loading...</div>
  }

  // Render post...
}

// This function gets called at build time
export async function getStaticPaths() {
  return {
    // Only `/posts/1` and `/posts/2` are generated at build time
    paths: [{ params: { id: '1' } }, { params: { id: '2' } }],
    // Enable statically generating additional pages
    // For example: `/posts/3`
    fallback: true,
  }
}

// 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

When is fallback: true useful?

fallback: true is useful if your app has a very large number of static pages that depend on data (think: a very large e-commerce site). You want to pre-render all product pages, but then your builds would take forever.

Instead, you may statically generate a small subset of pages and use fallback: true for the rest. When someone requests a page that’s not generated yet, the user will see the page with a loading indicator. Shortly after, getStaticProps finishes and the page will be rendered with the requested data. From now on, everyone who requests the same page will get the statically pre-rendered page.

This ensures that users always have a fast experience while preserving fast builds and the benefits of Static Generation.

When should I use getStaticPaths?

You should use getStaticPaths if you’re statically pre-rendering pages that use dynamic routes.

TypeScript: Use GetStaticPaths

For TypeScript, you can use the GetStaticPaths type from next:

import { GetStaticPaths } from 'next'

export const getStaticPaths: GetStaticPaths = async () => {
  // ...
}

Technical details

Use together with getStaticProps

When you use getStaticProps on a page with dynamic route parameters, you must use getStaticPaths.

You cannot use getStaticPaths with getServerSideProps.

Only runs at build time on server-side

getStaticPaths only runs at build time on server-side.

Only allowed in a page

getStaticPaths can only be exported from a page. You can’t export it from non-page files.

Also, you must use export async function getStaticPaths() {} — it will not work if you add getStaticPaths as a property of the page component.

Runs on every request in development

In development (next dev), getStaticPaths will be called on every request.

getServerSideProps (Server-side Rendering)

If you export an async function called getServerSideProps from a page, Next.js will pre-render this page on each request using the data returned by getServerSideProps.

export async function getServerSideProps(context) {
  return {
    props: {}, // will be passed to the page component as props
  }
}

The context parameter is an object containing the following keys:

Simple example

Here’s an example which uses getServerSideProps to fetch data at request time and pre-renders it. This example is also in the Pages documentation.

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

When should I use getServerSideProps?

You should use getServerSideProps only if you need to pre-render a page whose data must be fetched at request time. Time to first byte (TTFB) will be slower than getStaticProps because the server must compute the result on every request, and the result cannot be cached by a CDN without extra configuration.

If you don’t need to pre-render the data, then you should consider fetching data on the client side. Click here to learn more.

TypeScript: Use GetServerSideProps

For TypeScript, you can use the GetServerSideProps type from next:

import { GetServerSideProps } from 'next'

export const getServerSideProps: GetServerSideProps = async context => {
  // ...
}

Technical details

Only runs on server-side

getServerSideProps only runs on server-side and never runs on the browser. If a page uses getServerSideProps , then:

  • When you request this page directly, getServerSideProps runs at the request time, and this page will be pre-rendered with the returned props.
  • When you request this page on client-side page transitions through next/link (documentation) or next/router (documentation), Next.js sends an API request to the server, which runs getServerSideProps. It’ll return JSON that contains the result of running getServerSideProps, and the JSON will be used to render the page. All this work will be handled automatically by Next.js, so you don’t need to do anything extra as long as you have getServerSideProps defined.

Only allowed in a page

getServerSideProps can only be exported from a page. You can’t export it from non-page files.

Also, you must use export async function getServerSideProps() {} — it will not work if you add getServerSideProps as a property of the page component.

Fetching data on the client side

If your page contains frequently updating data, and you don’t need to pre-render the data, you can fetch the data on the client side. An example of this is user-specific data. Here’s how it works:

  • First, immediately show the page without data. Parts of the page can be pre-rendered using Static Generation. You can show loading states for missing data.
  • Then, fetch the data on the client side and display it when ready.

This approach works well for user dashboard pages, for example. Because a dashboard is a private, user-specific page, SEO is not relevant and the page doesn’t need to be pre-rendered. The data is frequently updated, which requires request-time data fetching.

SWR

The team behind Next.js has created a React hook for data fetching called SWR. We highly recommend it if you’re fetching data on the client side. It handles caching, revalidation, focus tracking, refetching on interval, and more. And you can use it like so:

import useSWR from 'swr'

function Profile() {
  const { data, error } = useSWR('/api/user', fetch)

  if (error) return <div>failed to load</div>
  if (!data) return <div>loading...</div>
  return <div>hello {data.name}!</div>
}

Check out the SWR documentation to learn more.

Learn more

We recommend you to read the following sections next:

Preview Mode

Learn more about the preview mode in Next.js.

Routing

Learn more about routing in Next.js.

TypeScript

Add TypeScript to your pages.
PagesBuilt-in CSS Support