unexpected params in getStaticProps

Bug report

Describe the bug

I’m getting unexpected or undocumented results for getStaticProps with a dynamic route parameter. The params slug is an array of path parts (looks like path.split('/')). It occurs for me when the page is in a dynamic folder and named index.js and not with a spread filename ie [...slug].js. Here’s an example:

Problem case:

// at pages/abc/[slug]/index.js
export async function getStaticProps({ params }) {
  console.log(params); // --- @build: { slug: 'a' } @runtime: { slug: ['abc', 'a'] }
}

What I expected:

// at pages/abc/[slug]/index.js
export async function getStaticProps({ params }) {
  console.log(params); // { slug: 'some-slug' }
}

I get these errors at runtime after deploying to now serverless. Builds are successful and the page appears to work but found the error in the now function logs.

[Aleksion]

For anyone else stumbling on this, I wrote this ugly fix that seems to catch the edge cases I’ve hit so far:

const cleanNextParams = (queryParams: string[]) => {
  let params = [...queryParams]

  // Sort out next params
  if (params.includes("_next")) {
    const indexOfNext = params.findIndex((e) => e === "_next")
    params.splice(indexOfNext, 3)
  }

  // remove .json from params
  params = params.map((p) => p.replace(".json", ""))

  return params
}

export const getStaticParam = (queryParam: string | string[], path: string, param: string) => {
  if (typeof queryParam === "string") {
    return queryParam
  }

  const indexOfParam = path.split("/").findIndex((el) => el === param)

  if (indexOfParam < 0) {
    return null
  }

  const cleanedParams = cleanNextParams(queryParam)

  return cleanedParams[indexOfParam]
}

// pages/blog/[article].ts
export const getStaticProps = ({params}) => {
   const path = "/blog/[article]"
   const articleId = getStaticParam(params.article, path, "[article]")
}

[timneutkens]

Ideally in this iteration: https://github.com/vercel/next.js/milestone/46