feat: Add localePrefix: 'never' option for middleware (#388 by @bor…

…is-arkenaar)

fixes #366

---------

Co-authored-by: Jan Amann <[email protected]>

docs/pages/docs/routing/middleware.mdx

@@ -138,14 +138,37 @@ import createMiddleware from 'next-intl/middleware';
 export default createMiddleware({
   // ... other config
 
-  localePrefix: 'always' // Defaults to 'as-needed'
+  localePrefix: 'always'
 });
 ```
 
 In this case, requests without a prefix will be redirected accordingly (e.g. `/about` to `/en/about`).
 
 Note that this will affect both prefix-based as well as domain-based routing.
 
+### Never use a locale prefix
+
+For applications behind an authentication layer, where there is no need for SEO, it is possible to have the locale never show up in the URL.
+
+```tsx filename="middleware.ts" {6}
+import createMiddleware from 'next-intl/middleware';
+
+export default createMiddleware({
+  // ... other config
+
+  localePrefix: 'never'
+});
+```
+
+In this case all requests for all locales will be rewritten to have the locale
+prefixed internally. You still need to place all your pages inside a
+`[locale]` folder for the routes to be able to receive the `locale` param.
+
+<Callout>
+  Note that [alternate links](#disable-alternate-links) are disabled in this
+  mode since there are no distinct URLs per language.
+</Callout>
+
 ### Disable automatic locale detection
 
 If you want to rely entirely on the URL to resolve the locale, you can disable locale detection based on the `accept-language` header and a potentially existing cookie value from a previous visit.
@@ -156,11 +179,11 @@ import createMiddleware from 'next-intl/middleware';
 export default createMiddleware({
   // ... other config
 
-  localeDetection: false // Defaults to `true`
+  localeDetection: false
 });
 ```
 
-Note that in this case other detection mechanisms will remain in place regardless (e.g. based on a locale prefix in the pathname or a matched domain).
+In this case, only the locale prefix and a potentially [matching domain](#domain-based-routing) are used to determine the locale.
 
 ### Disable alternate links
 
@@ -322,8 +345,9 @@ export default function RootPage() {
 
 <Callout type="warning">
   Note that this is currently limited to the usage of `next-intl` in [Client
-  Components](/docs/getting-started/app-router-client-components) (no [Server
-  Components](/docs/getting-started/app-router-server-components)).
+  Components](/docs/getting-started/app-router-client-components) ([Server
+  Components](/docs/getting-started/app-router-server-components) are not
+  supported).
 </Callout>
 
 You can explore a working demo by building [the Next.js 13 example](/examples/app-router) after enabling the static export:

packages/next-intl/src/middleware/NextIntlMiddlewareConfig.tsx

@@ -1,4 +1,4 @@
-type LocalePrefix = 'as-needed' | 'always';
+type LocalePrefix = 'as-needed' | 'always' | 'never';
 
 type RoutingBaseConfig = {
   /** A list of all locales that are supported. */

packages/next-intl/src/middleware/middleware.tsx

@@ -114,8 +114,9 @@ export default function createMiddleware(config: MiddlewareConfig) {
       }
 
       if (
-        hasMatchedDefaultLocale &&
-        configWithDefaults.localePrefix === 'as-needed'
+        configWithDefaults.localePrefix === 'never' ||
+        (hasMatchedDefaultLocale &&
+          configWithDefaults.localePrefix === 'as-needed')
       ) {
         response = rewrite(pathWithSearch);
       } else {
@@ -140,7 +141,9 @@ export default function createMiddleware(config: MiddlewareConfig) {
       if (hasLocalePrefix) {
         const basePath = pathWithSearch.replace(`/${pathLocale}`, '') || '/';
 
-        if (pathLocale === locale) {
+        if (configWithDefaults.localePrefix === 'never') {
+          response = redirect(basePath);
+        } else if (pathLocale === locale) {
           if (
             hasMatchedDefaultLocale &&
             configWithDefaults.localePrefix === 'as-needed'
@@ -168,9 +171,10 @@ export default function createMiddleware(config: MiddlewareConfig) {
         }
       } else {
         if (
-          hasMatchedDefaultLocale &&
-          (configWithDefaults.localePrefix === 'as-needed' ||
-            configWithDefaults.domains)
+          configWithDefaults.localePrefix === 'never' ||
+          (hasMatchedDefaultLocale &&
+            (configWithDefaults.localePrefix === 'as-needed' ||
+              configWithDefaults.domains))
         ) {
           response = rewrite(`/${locale}${pathWithSearch}`);
         } else {
@@ -186,6 +190,7 @@ export default function createMiddleware(config: MiddlewareConfig) {
     }
 
     if (
+      configWithDefaults.localePrefix !== 'never' &&
       configWithDefaults.alternateLinks &&
       configWithDefaults.locales.length > 1
     ) {