feat: Add localeCookie option for middleware

docs/pages/docs/routing/middleware.mdx

@@ -28,7 +28,7 @@ export const config = {
 };
 ```
 
-## Locale detection
+## Locale detection [#locale-detection-overview]
 
 The locale is negotiated based on your [`localePrefix`](/docs/routing#locale-prefix) and [`domains`](/docs/routing#domains) setting. Once a locale is detected, it will be remembered for future requests by being stored in the `NEXT_LOCALE` cookie.
 
@@ -122,6 +122,44 @@ In this case, only the locale prefix and a potentially [matching domain](#domain
 
 Note that by setting this option, the middleware will no longer return a `set-cookie` response header, which can be beneficial for CDN caching (see e.g. [the Cloudflare Cache rules for `set-cookie`](https://developers.cloudflare.com/cache/concepts/cache-behavior/#interaction-of-set-cookie-response-header-with-cache)).
 
+### Locale cookie [#locale-cookie]
+
+By default, the middleware will set a cookie called `NEXT_LOCALE` that contains the most recently detected locale. This is used to remember the user's locale preference for future requests (see [locale detection](#locale-detection-overview)).
+
+By default, the cookie will be configured with the following attributes:
+
+1. [**`maxAge`**](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#max-agenumber): This value is set to 1 year so that the preference of the user is kept as long as possible.
+2. [**`sameSite`**](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#samesitesamesite-value): This value is set to `lax` so that the cookie can be set when coming from an external site.
+3. [**`path`**](https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie#pathpath-value): This value is not set by default, but will use the value of your [`basePath`](#base-path) if configured.
+
+If you have more specific requirements, you can adjust these settings accordingly:
+
+```tsx filename="middleware.ts"
+import createMiddleware from 'next-intl/middleware';
+import {routing} from './i18n/routing';
+
+export default createMiddleware(routing, {
+  // These options will be merged with the defaults
+  localeCookie: {
+    // Expire in one day
+    maxAge: 60 * 60 * 24
+  }
+});
+```
+
+… or turn the cookie off entirely:
+
+```tsx filename="middleware.ts"
+import createMiddleware from 'next-intl/middleware';
+import {routing} from './i18n/routing';
+
+export default createMiddleware(routing, {
+  localeCookie: false
+});
+```
+
+Note that the cookie is only set once per detected locale and is not updated on every request.
+
 ### Alternate links [#alternate-links]
 
 The middleware automatically sets [the `link` header](https://developers.google.com/search/docs/specialty/international/localized-versions#http) to inform search engines that your content is available in different languages. Note that this automatically integrates with your routing strategy and will generate the correct links based on your configuration.

examples/example-app-router-playground/src/middleware.ts

@@ -1,7 +1,12 @@
 import createMiddleware from 'next-intl/middleware';
 import {routing} from './i18n/routing';
 
-export default createMiddleware(routing);
+export default createMiddleware(routing, {
+  localeCookie: {
+    // 200 days
+    maxAge: 200 * 24 * 60 * 60
+  }
+});
 
 export const config = {
   matcher: [

examples/example-app-router-playground/tests/main.spec.ts

@@ -682,6 +682,11 @@ it('can use `getPahname` to define a canonical link', async ({page}) => {
   await expect(getCanonicalPathname()).resolves.toBe('/de/neuigkeiten/3');
 });
 
+it('can define custom cookie options', async ({request}) => {
+  const response = await request.get('/');
+  expect(response.headers()['set-cookie']).toContain('Max-Age=17280000');
+});
+
 it('can use `t.has` in a Server Component', async ({page}) => {
   await page.goto('/');
   await expect(page.getByTestId('HasTitle')).toHaveText('true');

packages/next-intl/.size-limit.ts

@@ -60,7 +60,7 @@ const config: SizeLimitConfig = [
   {
     name: 'import createMiddleware from \'next-intl/middleware\'',
     path: 'dist/production/middleware.js',
-    limit: '9.63 KB'
+    limit: '9.675 KB'
   },
   {
     name: 'import * from \'next-intl/routing\'',
@@ -71,7 +71,7 @@ const config: SizeLimitConfig = [
     name: 'import * from \'next-intl\' (react-client, ESM)',
     path: 'dist/esm/index.react-client.js',
     import: '*',
-    limit: '14.265 kB'
+    limit: '14.245 kB'
   },
   {
     name: 'import {NextIntlProvider} from \'next-intl\' (react-client, ESM)',

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

@@ -1,8 +1,38 @@
+import {NextResponse} from 'next/server';
+
+type ResponseCookieOptions = Pick<
+  NonNullable<Parameters<typeof NextResponse.prototype.cookies.set>['2']>,
+  | 'maxAge'
+  | 'domain'
+  | 'expires'
+  | 'partitioned'
+  | 'path'
+  | 'priority'
+  | 'sameSite'
+  | 'secure'
+  // Not:
+  // - 'httpOnly' (the client side needs to read the cookie)
+  // - 'name' (the client side needs to know this as well)
+  // - 'value' (only the middleware knows this)
+>;
+
 export type MiddlewareOptions = {
-  /** Sets the `Link` response header to notify search engines about content in other languages (defaults to `true`). See https://developers.google.com/search/docs/specialty/international/localized-versions#http */
+  /**
+   * Sets the `Link` response header to notify search engines about content in other languages (defaults to `true`). See https://developers.google.com/search/docs/specialty/international/localized-versions#http
+   * @see https://next-intl-docs.vercel.app/docs/routing/middleware#alternate-links
+   **/
   alternateLinks?: boolean;
 
-  /** By setting this to `false`, the cookie as well as the `accept-language` header will no longer be used for locale detection. */
+  /**
+   * Can be used to disable the locale cookie or to customize it.
+   * @see https://next-intl-docs.vercel.app/docs/routing/middleware#locale-cookie
+   */
+  localeCookie?: boolean | ResponseCookieOptions;
+
+  /**
+   * By setting this to `false`, the cookie as well as the `accept-language` header will no longer be used for locale detection.
+   * @see https://next-intl-docs.vercel.app/docs/routing/middleware#locale-detection
+   **/
   localeDetection?: boolean;
 };
 

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

@@ -294,6 +294,24 @@ describe('prefix-based routing', () => {
       });
     });
 
+    it('can turn off the cookie', () => {
+      const response = createMiddleware(routing, {localeCookie: false})(
+        createMockRequest('/')
+      );
+      expect(response.cookies.get('NEXT_LOCALE')).toBeUndefined();
+    });
+
+    it('restricts which options of the cookie can be customized', () => {
+      createMiddleware(routing, {
+        localeCookie: {
+          // @ts-expect-error
+          httpOnly: true,
+          name: 'custom',
+          value: 'custom'
+        }
+      });
+    });
+
     it('retains request headers for the default locale', () => {
       middleware(
         createMockRequest('/', 'en', 'http://localhost:3000', undefined, {

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

@@ -49,7 +49,8 @@ export default function createMiddleware<
   const resolvedOptions = {
     alternateLinks: options?.alternateLinks ?? routing.alternateLinks ?? true,
     localeDetection:
-      options?.localeDetection ?? routing?.localeDetection ?? true
+      options?.localeDetection ?? routing?.localeDetection ?? true,
+    localeCookie: options?.localeCookie ?? routing?.localeCookie ?? true
   };
 
   return function middleware(request: NextRequest) {
@@ -311,8 +312,8 @@ export default function createMiddleware<
       }
     }
 
-    if (resolvedOptions.localeDetection) {
-      syncCookie(request, response, locale);
+    if (resolvedOptions.localeDetection && resolvedOptions.localeCookie) {
+      syncCookie(request, response, locale, resolvedOptions.localeCookie);
     }
 
     if (

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

@@ -4,19 +4,23 @@ import {
   COOKIE_MAX_AGE,
   COOKIE_SAME_SITE
 } from '../shared/constants';
+import {MiddlewareOptions} from './config';
 
 export default function syncCookie(
   request: NextRequest,
   response: NextResponse,
-  locale: string
+  locale: string,
+  localeCookie: MiddlewareOptions['localeCookie']
 ) {
   const hasOutdatedCookie =
     request.cookies.get(COOKIE_LOCALE_NAME)?.value !== locale;
+
   if (hasOutdatedCookie) {
     response.cookies.set(COOKIE_LOCALE_NAME, locale, {
       path: request.nextUrl.basePath || undefined,
       sameSite: COOKIE_SAME_SITE,
-      maxAge: COOKIE_MAX_AGE
+      maxAge: COOKIE_MAX_AGE,
+      ...(typeof localeCookie === 'object' && localeCookie)
     });
   }
 }