docs/src/redirects/middleware/handle-redirects.ts

import type { NextFunction, Response } from 'express'

import patterns from '@/frame/lib/patterns.js'
import { pathLanguagePrefixed } from '@/languages/lib/languages'
import { deprecatedWithFunctionalRedirects } from '@/versions/lib/enterprise-server-releases.js'
import getRedirect from '../lib/get-redirect.js'
import { defaultCacheControl, languageCacheControl } from '@/frame/middleware/cache-control.js'
import { ExtendedRequest, URLSearchParamsTypes } from '@/types'

export default function handleRedirects(req: ExtendedRequest, res: Response, next: NextFunction) {
  if (!req.context) throw new Error('Request not contextualized')

  // Any double-slashes in the URL should be removed first
  // This must be done before checking if the path
  // is an asset (patterns.assetPaths)
  if (req.path.includes('//')) {
    return res.redirect(301, req.path.replace(/\/+/g, '/'))
  }

  // never redirect assets
  if (patterns.assetPaths.test(req.path)) return next()

  // All /api/ endpoints handle their own redirects
  // such as /api/pageinfo redirects to /api/pageinfo/v1
  if (req.path.startsWith('/api/')) return next()

  // blanket redirects for languageless homepage
  if (req.path === '/') {
    const language = getLanguage(req)
    languageCacheControl(res)
    // Forward query params to the new URL
    let queryParams = new URLSearchParams((req?.query as any) || '').toString()
    if (queryParams) {
      queryParams = `?${queryParams}`
    }
    return res.redirect(302, `/${language}${queryParams}`)
  }

  // begin redirect handling
  let redirect = req.path
  let queryParams = req.originalUrl.includes('?') ? req.originalUrl.split('?')[1] : null

  // Redirect `/some/uri?q=stuff` to `/en/search?query=stuff`
  // Redirect `/some/uri?query=stuff` to `/en/search?query=stuff`
  // Redirect `/fr/version@latest/some/uri?query=stuff`
  // to `/fr/version@latest/search?query=stuff`
  // The `q` param is deprecated, but we still need to support it in case
  // there are links out there that use it.
  const onSearch = req.path.endsWith('/search') || req.path.startsWith('/api/search')
  // We have legacy links that links to the GraphQL Explorer with
  // a `?query=...` in the URL. These should not redirect to the search page.
  const onGraphqlExplorer = req.path.includes('/graphql/overview/explorer')
  const hasQ = 'q' in req.query
  const hasQuery = 'query' in req.query
  if ((hasQ && !hasQuery) || (hasQuery && !onSearch && !onGraphqlExplorer)) {
    const language = getLanguage(req)
    const sp = new URLSearchParams(req.query as URLSearchParamsTypes)
    if (sp.has('q') && !sp.has('query')) {
      sp.set('query', sp.get('q')!)
      sp.delete('q')
    }

    let redirectTo = `/${language}`
    const { currentVersion } = req.context
    if (currentVersion !== 'free-pro-team@latest') {
      redirectTo += `/${currentVersion}`
      // The `req.context.currentVersion` is just the portion of the URL
      // pathname. It could be that the currentVersion is something
      // like `enterprise` which needs to be redirected to its new name.
      redirectTo = getRedirect(redirectTo, req.context) || redirectTo
    }

    redirectTo += `/search?${sp.toString()}`
    return res.redirect(301, redirectTo)
  }

  // have to do this now because searchPath replacement changes the path as well as the query params
  if (queryParams) {
    queryParams = '?' + queryParams
  }

  // remove query params temporarily so we can find the path in the redirects object
  let redirectWithoutQueryParams = removeQueryParams(redirect)

  const redirectTo = getRedirect(redirectWithoutQueryParams, req.context)

  redirectWithoutQueryParams = redirectTo || redirectWithoutQueryParams

  redirect = queryParams ? redirectWithoutQueryParams + queryParams : redirectWithoutQueryParams

  if (!redirectTo && !pathLanguagePrefixed(req.path)) {
    // No redirect necessary, but perhaps it's to a known page, and the URL
    // currently doesn't have a language prefix, then we need to add
    // the language prefix.
    // We can't always force on the language prefix because some URLs
    // aren't pages. They're other middleware endpoints such as
    // `/healthcheck` which should never redirect.
    // But for example, a `/authentication/connecting-to-github-with-ssh`
    // needs to become `/en/authentication/connecting-to-github-with-ssh`
    const possibleRedirectTo = `/en${req.path}`
    if (!req.context.pages) throw new Error('req.context.pages not yet set')
    if (possibleRedirectTo in req.context.pages || isDeprecatedVersion(req.path)) {
      const language = getLanguage(req)

      // Note, it's important to use `req.url` here and not `req.path`
      // because the full URL can contain query strings.
      // E.g. `/foo?json=breadcrumbs`
      redirect = `/${language}${req.url}`
    }
  }

  // do not redirect a path to itself
  if (redirect === req.originalUrl) {
    return next()
  }

  if (!req.context.pages) throw new Error('req.context.pages not yet set')

  // do not redirect if the redirected page can't be found
  if (
    !(req.context.pages[removeQueryParams(redirect)] || isDeprecatedVersion(req.path)) &&
    !redirect.includes('://')
  ) {
    // display error on the page in development, but not in production
    // include final full redirect path in the message
    if (process.env.NODE_ENV !== 'production' && req.context) {
      req.context.redirectNotFound = redirect
    }
    return next()
  }

  // do the redirect if the from-URL already had a language in it
  if (pathLanguagePrefixed(req.path) || redirect.includes('://')) {
    defaultCacheControl(res)
  } else {
    languageCacheControl(res)
  }

  const permanent = redirect.includes('://') || usePermanentRedirect(req)
  return res.redirect(permanent ? 301 : 302, redirect)
}

function getLanguage(req: ExtendedRequest, default_ = 'en') {
  // req.context.userLanguage, if it truthy, is always a valid supported
  // language. It's whatever was in the user's request in lib/languages.js
  return req.context!.userLanguage || default_
}

function usePermanentRedirect(req: ExtendedRequest) {
  // If the redirect was to essentially swap `enterprise-server@latest`
  // for `enterprise-server@3.x` then, we definitely don't want to
  // do a permanent redirect.
  // When this is the case, we don't want a permanent redirect because
  // it could overzealously cache in the users' browser which could
  // be bad when whatever "latest" means changes.
  if (req.path.includes('/enterprise-server@latest')) return false

  // If the redirect involved injecting a language prefix, then don't
  // permanently redirect because that could overly cache in users'
  // browsers if we some day want to make the language redirect
  // depend on a cookie or 'Accept-Language' header.
  if (pathLanguagePrefixed(req.path)) return true

  // The default is to *not* do a permanent redirect.
  return false
}

function removeQueryParams(redirect: string) {
  return new URL(redirect, 'https://docs.github.com').pathname
}

function isDeprecatedVersion(path: string) {
  // When we rewrote how redirects work, from a lookup model to a
  // functional model, the enterprise-server releases that got
  // deprecated since then fall between the cracks. Especially
  // for custom NextJS page-like pages like /admin/release-notes
  // These URLs don't come from any remaining .json lookup file
  // and they're not active pages either (e.g. req.context.pages)
  const split = path.split('/')
  for (const version of deprecatedWithFunctionalRedirects) {
    if (split.includes(`enterprise-server@${version}`)) {
      return true
    }
  }
  return false
}