Function getFallbackRedirect

middleware/archived-enterprise-versions.js:253–301

(req)

const fallbackRedirectLookups = new Map()

function getFallbackRedirect(req) {
  // The file `lib/redirects/static/archived-frontmatter-valid-urls.json` which
  // we depend on here, is structured like this:
  //
  //  {
  //   "/enterprise/2.13/foo/bar": [
  //     "/enterprise/2.13/other/old/thing",
  //     "/enterprise/2.13/more/redirectable/url",
  //     "/enterprise/2.13/etc/etc"
  //   ],
  //   ...
  //
  // The keys are valid URLs that it can redirect to. I.e. these are
  // URLs that we definitely know are valid and will be found
  // in https://github.com/github/help-docs-archived-enterprise-versions
  // The array values are possible URLs we deem acceptable redirect
  // sources.
  // But to avoid an unnecessary, O(n), loop every time, we turn this
  // structure around to become:
  //
  //   {
  //     "/enterprise/2.13/other/old/thing": "/enterprise/2.13/foo/bar",
  //     "/enterprise/2.13/more/redirectable/url": "/enterprise/2.13/foo/bar",
  //     "/enterprise/2.13/etc/etc": "/enterprise/2.13/foo/bar",
  //     ...
  //
  // Now potential lookups are fast.
  if (!fallbackRedirectLookups.size) {
    for (const [destination, sources] of Object.entries(archivedFrontmatterValidURLS())) {
      for (const source of sources) {
        fallbackRedirectLookups.set(source, destination)
      }
    }
  }

  // But before we proceed, remember that the
  // file lib/redirects/static/archived-frontmatter-valid-urls.json never
  // contains a language prefix.
  // E.g. only `/enterprise/2.13/foo/bar` but the requested URL can be
  // `/en/enterprise/2.13/foo/bar`, `/pt/enterprise/2.13/foo/bar`,
  // or just `/enterprise/2.13/foo/bar`.
  // Whatever it is, pop the language prefix, operate, and put it back
  // again. In the end, it always has to have a language prefix.
  const [language, withoutLanguage] = splitPathByLanguage(req.path)
  const fallback = fallbackRedirectLookups.get(withoutLanguage)
  if (fallback) {
    return `/${language}${fallback}`
  }
}