Skip to content

Next.js

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 depth 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 fetch data on the client side.

getStaticProps (Static Generation)

Version History
VersionChanges
v12.0.0staticPageGenerationTimeout added.
v10.0.0locale, locales, defaultLocale, and notFound options added.
v9.5.0Stable Incremental Static Regeneration
v9.3.0getStaticProps introduced.

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:

getStaticProps should return an object with:

  • props - An optional object with the props that will be received by the page component. It should be a serializable object

  • revalidate - An optional amount in seconds after which a page re-generation can occur. Defaults to false. When revalidate is false it means that there is no revalidation, so the page will be cached as built until your next build. More on Incremental Static Regeneration

  • notFound - An optional boolean value to allow the page to return a 404 status and page. Below is an example of how it works:

    export async function getStaticProps(context) {
  const res = await fetch(`https://.../data`)
  const data = await res.json()

  if (!data) {
    return {
      notFound: true,
    }
  }

  return {
    props: { data }, // will be passed to the page component as props
  }
}

    Note: notFound is not needed for fallback: false mode as only paths returned from getStaticPaths will be pre-rendered.

    Note: With notFound: true the page will return a 404 even if there was a successfully generated page before. This is meant to support use-cases like user generated content getting removed by its author.

  • redirect - An optional redirect value to allow redirecting to internal and external resources. It should match the shape of { destination: string, permanent: boolean }. In some rare cases, you might need to assign a custom status code for older HTTP Clients to properly redirect. In these cases, you can use the statusCode property instead of the permanent property, but not both. Below is an example of how it works:

    export async function getStaticProps(context) {
  const res = await fetch(`https://...`)
  const data = await res.json()

  if (!data) {
    return {
      redirect: {
        destination: '/',
        permanent: false,
      },
    }
  }

  return {
    props: { data }, // will be passed to the page component as props
  }
}

    Note: Redirecting at build-time is currently not allowed and if the redirects are known at build-time they should be added in next.config.js.

Note: You can import modules in top-level scope for use in getStaticProps. Imports used in getStaticProps will not be bundled for the client-side.

This means you can write server-side code directly in getStaticProps. This includes reading from the filesystem or a database.

Note: You should not use fetch() to call an API route in getStaticProps. Instead, directly import the logic used inside your API route. You may need to slightly refactor your code for this approach.

Fetching from an external API is fine!

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 a 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) => {
  // ...
}

If you want to get inferred typings for your props, you can use InferGetStaticPropsType<typeof getStaticProps>, like this:

import { InferGetStaticPropsType } from 'next'

type Post = {
  author: string
  content: string
}

export const getStaticProps = async () => {
  const res = await fetch('https://.../posts')
  const posts: Post[] = await res.json()

  return {
    props: {
      posts,
    },
  }
}

function Blog({ posts }: InferGetStaticPropsType<typeof getStaticProps>) {
  // will resolve posts to type Post[]
}

export default Blog

Note: Next.js has a default static generation timeout of 60 seconds. If no new pages complete generating within the timeout, it will attempt generation three more times. If the fourth attempt fails, the build will fail. This timeout can be modified using the following configuration:

// next.config.js
module.exports = {
  // time in seconds of no pages generating during static
  // generation before timing out
  staticPageGenerationTimeout: 90,
}

Incremental Static Regeneration

Examples
Version History
VersionChanges
v9.5.0Base Path added.

Next.js allows you to create or update static pages after you’ve built your site. Incremental Static Regeneration (ISR) enables you to use static-generation on a per-page basis, without needing to rebuild the entire site. With ISR, you can retain the benefits of static while scaling to millions of pages.

Consider our previous getStaticProps example, but now with Incremental Static Regeneration enabled through the revalidate property:

function Blog({ posts }) {
  return (
    <ul>
      {posts.map((post) => (
        <li>{post.title}</li>
      ))}
    </ul>
  )
}

// This function gets called at build time on server-side.
// It may be called again, on a serverless function, if
// revalidation is enabled and a new request comes in
export async function getStaticProps() {
  const res = await fetch('https://.../posts')
  const posts = await res.json()

  return {
    props: {
      posts,
    },
    // Next.js will attempt to re-generate the page:
    // - When a request comes in
    // - At most once every 10 seconds
    revalidate: 10, // In seconds
  }
}

// This function gets called at build time on server-side.
// It may be called again, on a serverless function, if
// the path has not been generated.
export async function getStaticPaths() {
  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: blocking } will server-render pages
  // on-demand if the path doesn't exist.
  return { paths, fallback: 'blocking' }
}

export default Blog

When a request is made to a page that was pre-rendered at build time, it will initially show the cached page.

  • Any requests to the page after the initial request and before 10 seconds are also cached and instantaneous.
  • After the 10-second window, the next request will still show the cached (stale) page
  • Next.js triggers a regeneration of the page in the background.
  • Once the page has been successfully generated, Next.js will invalidate the cache and show the updated product page. If the background regeneration fails, the old page will stay unaltered.

When a request is made to a path that hasn’t been generated, Next.js will server-render the page on the first request. Future requests will serve the static file from the cache.

To learn how to persist the cache globally and handle rollbacks, learn more about Incremental Static Regeneration.

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 { promises as 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 = await fs.readdir(postsDirectory)

  const posts = filenames.map(async (filename) => {
    const filePath = path.join(postsDirectory, filename)
    const fileContents = await fs.readFile(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: await Promise.all(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.

You can use this tool to verify what Next.js eliminates from the client-side bundle.

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.

When using Incremental Static Generation getStaticProps will be executed out of band to generate the JSON needed for client-side navigation. You may see this in the form of multiple requests being made for the same page, however, this is intended and has no impact on end-user performance.

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)

Version History
VersionChanges
v9.5.0Stable Incremental Static Regeneration
v9.3.0getStaticPaths introduced.

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, false, or 'blocking' // 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.
  • If the page uses an optional catch-all route, supply null, [], undefined or false to render the root-most route. For example, if you supply slug: false for pages/[[...slug]], Next.js will statically generate the page /.

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

Examples

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

  • The paths returned from getStaticPaths will be rendered to HTML at build time by getStaticProps.
  • 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). Note: this "fallback" version will not be served for crawlers like Google and instead will render the path in blocking mode.
  • 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, like other pages pre-rendered at build time.

fallback: true is not supported when using next export.

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 },
    // Re-generate the post at most once per second
    // if a request comes in
    revalidate: 1,
  }
}

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.

fallback: true will not update generated pages, for that take a look at Incremental Static Regeneration.

fallback: 'blocking'

If fallback is 'blocking', new paths not returned by getStaticPaths will wait for the HTML to be generated, identical to SSR (hence why blocking), and then be cached for future requests so it only happens once per path.

getStaticProps will behave as follows:

  • The paths returned from getStaticPaths will be rendered to HTML at build time by getStaticProps.
  • The paths that have not been generated at build time will not result in a 404 page. Instead, Next.js will SSR on the first request and return the generated HTML.
  • When that’s done, the browser receives the HTML for the generated path. From the user’s perspective, it will transition from "the browser is requesting the page" to "the full page is loaded". There is no flash of loading/fallback state.
  • 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, like other pages pre-rendered at build time.

fallback: 'blocking' will not update generated pages by default. To update generated pages, use Incremental Static Regeneration in conjunction with fallback: 'blocking'.

fallback: 'blocking' is not supported when using next export.

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)

Version History
VersionChanges
v10.0.0locale, locales, defaultLocale, and notFound options added.
v9.3.0getServerSideProps introduced.

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:

getServerSideProps should return an object with:

  • props - An optional object with the props that will be received by the page component. It should be a serializable object or a Promise that resolves to a serializable object.

  • notFound - An optional boolean value to allow the page to return a 404 status and page. Below is an example of how it works:

    export async function getServerSideProps(context) {
  const res = await fetch(`https://...`)
  const data = await res.json()

  if (!data) {
    return {
      notFound: true,
    }
  }

  return {
    props: {}, // will be passed to the page component as props
  }
}

  • redirect - An optional redirect value to allow redirecting to internal and external resources. It should match the shape of { destination: string, permanent: boolean }. In some rare cases, you might need to assign a custom status code for older HTTP Clients to properly redirect. In these cases, you can use the statusCode property instead of the permanent property, but not both. Below is an example of how it works:

    export async function getServerSideProps(context) {
  const res = await fetch(`https://.../data`)
  const data = await res.json()

  if (!data) {
    return {
      redirect: {
        destination: '/',
        permanent: false,
      },
    }
  }

  return {
    props: {}, // will be passed to the page component as props
  }
}

Note: You can import modules in top-level scope for use in getServerSideProps. Imports used in getServerSideProps will not be bundled for the client-side.

This means you can write server-side code directly in getServerSideProps. This includes reading from the filesystem or a database.

Note: You should not use fetch() to call an API route in getServerSideProps. Instead, directly import the logic used inside your API route. You may need to slightly refactor your code for this approach.

Fetching from an external API is fine!

Provided req middleware in getServerSideProps

The req in the context passed to getServerSideProps provides built in middleware that parses the incoming request (req). That middleware is:

  • req.cookies - An object containing the cookies sent by the request. Defaults to {}

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) => {
  // ...
}

If you want to get inferred typings for your props, you can use InferGetServerSidePropsType<typeof getServerSideProps>, like this:

import { InferGetServerSidePropsType } from 'next'

type Data = { ... }

export const getServerSideProps = async () => {
  const res = await fetch('https://.../data')
  const data: Data = await res.json()

  return {
    props: {
      data,
    },
  }
}

function Page({ data }: InferGetServerSidePropsType<typeof getServerSideProps>) {
  // will resolve posts to type Data
}

export default Page

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.

You can use this tool to verify what Next.js eliminates from the client-side bundle.

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'

const fetcher = (url) => fetch(url).then((res) => res.json())

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

  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.
Pages（页面）内置 CSS 支持