Why are both .js and .html files being output for getStaticProps?

[geelen]

So... weird behaviour I've encountered with NextJS's new static-first focus. For a project with the following structure:

> tree pages
pages
├── _app.js
├── _document.js
├── _error.js
├── about
│   └── index.js
├── blog
│   ├── [slug].js
│   └── index.js
...

I get this .next/serverless/pages structure.

> tree .next/serverless/pages
.next/serverless/pages
├── _error.js
├── about.html
├── blog
│   ├── [slug].js
│   ├── why-my-cat-is-best.html
│   ├── why-my-cat-is-best.json
│   ├── why-dogs-are-overrated.html
│   └── why-dogs-are-overrated.json
├── blog.html
├── blog.js     # <---
├── blog.json
...

Now, the way blog/[slug].js vs the prerendered posts make sense, since that's using getStaticPaths which I assume can be dynamic, but I'm a bit suprised to see blog.js there.

For reference, this is the getStaticProps logic I have in blog/index.js...

// blog/index.js
import { request } from "../../lib/datocms"

export default function BlogIndex(props) {
  const posts = this.props.posts.allBlogPosts

  return (
    //..
  )
}

export async function getStaticProps() {
  const blogPostList = `
    query blogPostList {
      allBlogPosts {
        title
        slug
      }
    }
  `

  const posts = await request({
    query: blogPostList
  })

  return {
    props: { posts }
  }
}

From my understanding, getStaticProps run only at build time, so this should mean that only .next/serverless/pages/blog.html is generated, not .next/serverless/pages/blog.js, yet both are there. Does anyone know why both are outputted?

Does every file that uses getStaticProps generate a .js file on the serverless build? And is that designed to allow the hosting platform to refresh the static snapshot? I thought you had to opt-in to that behaviour...

[ijjk]

The server JS file is always output for pages with getStaticProps since it is needed to prerender the page during the build, it is also used when leveraging preview mode. If you don't leverage API routes (required for enabling preview mode) and the pages are completely static (don't leverage fallback or revalidate support) with getStaticProps, lambdas won't be deployed for these pages on Vercel anymore though