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Here is some essential information you should know about getStaticProps.

Fetch External API or Query Database

In lib/posts.js, we’ve implemented getSortedPostsData which fetches data from the file system. But you can fetch the data from other sources, like an external API endpoint, and it’ll work just fine:

export async function getSortedPostsData() {
  // Instead of the file system,
  // fetch post data from an external API endpoint
  const res = await fetch('..')
  return res.json()
}

Note: Next.js polyfills fetch() on both the client and server. You don't need to import it.

You can also query the database directly:

import someDatabaseSDK from 'someDatabaseSDK'

const databaseClient = someDatabaseSDK.createClient(...)

export async function getSortedPostsData() {
  // Instead of the file system,
  // fetch post data from a database
  return databaseClient.query('SELECT posts...')
}

This is possible because getStaticProps runs only on the server-side. It will never 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.

Development vs. Production

• In development (npm run dev or yarn dev), getStaticProps runs on every request.
• In production, getStaticProps runs at build time.

Because it’s meant to be run at build time, you won’t be able to use data that’s only available during request time, such as query parameters or HTTP headers.

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.

What If I Need to Fetch Data at Request Time?

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 try Server-side Rendering or skip pre-rendering. Let’s talk about these strategies before we move on to the next lesson.