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How to return a 404 Not Found page for dynamic routes in Next.js

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For a Next.js blog you typically have a dynamic route posts/[slug].js which fetches the blog posts from a headless CMS.

If the blog post is not found in the CMS, because it has e.g. been deleted or the slug has changed you can return a 404 status code and page by returning { notFound: true }.

export async function getServerSideProps(context) {
  const res = await fetch(`https://...`);
  const data = await res.json();

  if (!data) {
    return {
      notFound: true,
    };
  }

  return {
    props: {}, // will be passed to the page component as props
  };
}

This works both for getStaticProps and getServerSideProps.

For getStaticProps the page will return a 404 even if there was a successfully generated page before. This is meant to support use-cases like user generated content getting removed by its author.