getStaticPaths API

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, Blitz 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 app/posts/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 Blitz will statically generate posts/1 and posts/2 at build time using the page component in app/posts/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 app/posts/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 Blitz 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]], Blitz 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 app/posts/pages/posts/[id].js. The list of blog posts will be fetched from your database or a CMS and returned by getStaticPaths . Then, for each page, it fetches the data for a single post using getStaticProps.

// app/posts/pages/posts/[id].js

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

// This function gets called during pre-rendering
export async function getStaticPaths() {
  // 1. Use a blitz query to get all posts
  // 2. Or call an external API endpoint to get posts
  const posts = /* ... */

  // 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 post = /* get one post */

  // 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, Blitz 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, Blitz 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, Blitz adds this path to the list of pre-rendered pages. Subsequent requests to the same path will serve the generated page, exactly 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:

// app/posts/pages/posts/[id].js
import {useRouter} from "blitz"

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 post = /* ... */

  // 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.

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, Blitz 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, Blitz 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'.

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 blitz:

import {GetStaticPaths} from "blitz"

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 (blitz dev), getStaticPaths will be called on every request.