This page catalogs all the special exports One supports from route files.
_layout routes are special and do not support these exports unless specified.This export is needed for "ssg" type pages that use dynamic path segments (for example, [id].tsx). At build-time, One will expand the id segment into multiple pages, using the values you return from generateStaticParams.
The entire function and export generateStaticParams will be removed at build-time and everything it depends on will be tree-shaken out of your client-side bundle.
You'll need to return an array of objects, where the object keys match the name inside the brackets. As an example, we'll use nested dynamic segments:
app/blog/[month]/[year]/[slug]+ssg.tsx
export async function generateStaticParams() {
  const blogPosts = await getAllBlogPosts()
  return blogPosts.map(post => {
    return {
      month: post.month,
      year: post.year,
      slug: post.slug,
    }
  })
}
If this function returned this:
[
  {
    month: 10,
    year: 2025,
    slug: 'some-slug'
  }
]
Then you'd generate one page at /blog/10/2025/some-slug.
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