fix: detect language error on /_next/data/ input (#22419)

Author: mikesurowiec

middleware/detect-language.js

@@ -36,7 +36,7 @@ function getUserLanguage(browserLanguages) {
 
 // determine language code from a path. Default to en if no valid match
 export function getLanguageCodeFromPath(path) {
-  const maybeLanguage = path.split('/')[path.startsWith('/_next/data/') ? 4 : 1].slice(0, 2)
+  const maybeLanguage = (path.split('/')[path.startsWith('/_next/data/') ? 4 : 1] || '').slice(0, 2)
   return languageCodes.includes(maybeLanguage) ? maybeLanguage : 'en'
 }
 

tests/unit/detect-language.js

@@ -6,6 +6,10 @@ describe('detect-language - getLanguageCodeFromPath', () => {
     expect(getLanguageCodeFromPath('/_next/data/development/ja/articles/foo')).toBe('ja')
   })
 
+  test('should not error on /_next/data/ input', () => {
+    expect(getLanguageCodeFromPath('/_next/data/')).toBe('en')
+  })
+
   test('should return for paths with an extension', () => {
     expect(getLanguageCodeFromPath('/ja.json')).toBe('ja')
     expect(getLanguageCodeFromPath('/_next/data/development/ja.json')).toBe('ja')