more efficient language redirectPatterns middleware (#23412)

* more efficient language redirectPatterns middleware

Part of #1322

* feedbacked

middleware/redirects/language-code-redirects.js

@@ -1,18 +1,37 @@
 import languages from '../../lib/languages.js'
 
+const redirectPatterns = Object.values(languages)
+  .map((language) => language.redirectPatterns || [])
+  .flat()
+// If the enabled languages have `.redirectPatterns`, combine them
+// into one which we only need to use to determine if we should bother
+// doing the redirect at all.
+const combinedRedirectPatternRegex =
+  redirectPatterns.length > 0
+    ? new RegExp(redirectPatterns.map((rex) => rex.source).join('|'))
+    : null
+
 // This middleware handles redirects for mistyped language codes
 //
 // Examples:
 // /jp*    -> /ja*
 // /zh-TW* -> /cn*
 export default function languageCodeRedirects(req, res, next) {
-  for (const code in languages) {
-    const language = languages[code]
-    const redirectPatterns = language.redirectPatterns || []
-    for (const i in redirectPatterns) {
-      const redirectPattern = redirectPatterns[i]
-      if (redirectPattern.test(req.path)) {
-        return res.redirect(301, req.path.replace(redirectPattern, `/${language.code}`))
+  // Only in the unlikely event that the `req.path` starts with one of these
+  // prefixes do we bother looking up what the redirect should be.
+  if (
+    !req.path.startsWith('/_next/static') &&
+    combinedRedirectPatternRegex &&
+    combinedRedirectPatternRegex.test(req.path)
+  ) {
+    // This loop is almost never ever used to it doesn't have to be
+    // particularly smart or fast.
+    for (const language of Object.values(languages)) {
+      const redirectPatterns = language.redirectPatterns || []
+      for (const redirectPattern of redirectPatterns) {
+        if (redirectPattern.test(req.path)) {
+          return res.redirect(301, req.path.replace(redirectPattern, `/${language.code}`))
+        }
       }
     }
   }