Using Next.js's Static Generation method

next.js

3 min read

In Next.js, there are two forms of pre-rendering, Static Generation and Server-side Rendering. We can apply these methods for each page individually.

The Static Generation method fetches the page’s data on build-time, and renders static HTML files. To apply the Static Generation method, we need to export a getStaticProps method from our page:

// pages/index.tsx

import type { GetStaticProps } from 'next'

import type { Lesson } from 'types/lesson'
import { fetchLessons } from 'utils/lessons'

type Props = {
  lessons: Lesson[]
}

const Home = ({ lessons }: Props) => {
  return (
    <ul>
      {lessons.map((lesson) => (...))}
    </ul>
  )
}

export const getStaticProps: GetStaticProps<Props> = async (context) => {
  const lessons = await fetchLessons()

  const props: Props = {
    lessons
  }

  return {
    props
  }
}

export default Home

For our Home page, Next.js will execute the fetchLessons method asynchroniously, and pass the lessons array as a prop in our Home component. If there is no data for the given query, we need to return notFound: true instead of the props. If the lessons data changes and we want to update the page, we can either rebuild our website, or use the Incremental Static Regeneration method.

In our getStaticProps method we can also obtain the context, which holds data like:

  • params: the route parameters for pages that use dynamic routes
  • preview: a boolean which is true if the page is in the Preview Mode, otherwise undefined
  • previewData: an object that holds the preview data set by setPreviewData
  • locale: the active locale, if you’ve enabled Internationalized Routing
  • locales: an array that contains all of the locales
  • defaultLocale: the default configured locale

Incremental Static Regeneration

The ISR method is an extension of the Static Generation method. We can enable ISR if we provide a revalidate property in our getStaticProps result:

export const getStaticProps: GetStaticProps<Props> = async (context) => {
  return {
    props: {...},
    revalidate: 60 * 60,
  }
}

The revalidate property will tell Next.js to “revalidate” our data maximum one time in the given timeframe (in our case is 1 hour, 60 seconds times 60).

Building pages with dynamic routes

Since the Static Generation happens on build-time, if we have a page that uses Dynamic Routes we also need to export the getStaticPaths method. The getStaticPaths method returns a list of paths that have to be rendered to HTML at build-time.

import type { GetStaticProps, GetStaticPaths } from 'next'

...

export const getStaticPaths: GetStaticPaths = async () => {
  const lessons = await fetchLessons()

  return {
    paths: lessons.map({ slug } => ({ params: { slug } })),
    fallback: false
  }
}

The paths key determines which paths will be pre-rendered. If for example we had 3 lessons, Next.js will pre-render the following URLs:

  • /lessons/getting-started
  • /lessons/create-pages
  • /lessons/create-dynamic-routes

For each lesson, Next.js will execute the getStaticProps method. That pre-renders every page and generates static HTML files for them.

The fallback key is a boolean key that we must include in our getStaticPaths result. If it’s set to false, then any paths that are not returned by the getStaticPaths method will result in a 404 page. If it’s set to true, Next.js will render a “fallback” page while it statically generates the HTML and JSON (this includes running the getStaticProps method). When it’s done, the page will receive the brand new data in its props and it will render its contents. At the end of the process, Next.js will add the new path to the list of pre-rendered pages.

If our page supports a fallback, we can use Next.js’s router to check if Next.js wants us to render a fallback page:

import { useRouter } from 'next/router'

const Home = () => {
  const router = useRouter()

  if (router.isFallback) {
    return <div>Loading...</div>
  }

  ...
}

If we don’t want to display a Loading page, we can set the fallback property in getStaticPaths to 'blocking'. This will make the browser wait for Next.js to pre-render the HTML.