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First, let’s set up the files:

• Create a file called [id].js inside the pages/posts directory.
• Also, remove first-post.js inside the pages/posts directory — we’ll no longer use this.

Then, add this to pages/posts/[id].js. We’ll fill in ... later.

import Layout from '../../components/layout'

export default function Post() {
  return <Layout>...</Layout>
}

Then, open lib/posts.js and add this function. This will return the list of file names (excluding .md) in the posts directory:

export function getAllPostIds() {
  const fileNames = fs.readdirSync(postsDirectory)

  // Returns an array that looks like this:
  // [
  //   {
  //     params: {
  //       id: 'ssg-ssr'
  //     }
  //   },
  //   {
  //     params: {
  //       id: 'pre-rendering'
  //     }
  //   }
  // ]
  return fileNames.map(fileName => {
    return {
      params: {
        id: fileName.replace(/\.md$/, '')
      }
    }
  })
}

Important: The returned list is not just an array of strings — it must be an array of objects that look like the comment above. Each object must have the params key and contain an object with the id key (because we’re using [id] in the file name). Otherwise, getStaticPaths will fail.

Finally, in pages/posts/[id].js, we’ll import this function:

import { getAllPostIds } from '../../lib/posts'

And create getStaticPaths which calls this function:

export async function getStaticPaths() {
  const paths = getAllPostIds()
  return {
    paths,
    fallback: false
  }
}

• The array of possible values for id must be the value of the paths key of the returned object. This is exactly what getAllPostIds() returns.
• Ignore fallback: false for now — we’ll explain that later.

We’re almost done — but we still need to implement getStaticProps. Let’s do that on the next page!