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Middleware

Middleware

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The middleware is only needed when you're using i18n routing.

The middleware handles redirects and rewrites based on the detected user locale.

middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // A list of all locales that are supported
  locales: ['en', 'de'],
 
  // Used when no locale matches
  defaultLocale: 'en'
});
 
export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

In addition to handling i18n routing, the middleware sets the link header to inform search engines that your content is available in different languages (see alternate links).

Strategies

There are two strategies for detecting the locale:

  1. Prefix-based routing (default)
  2. Domain-based routing

Once a locale is detected, it will be saved in the NEXT_LOCALE cookie.

Strategy 1: Prefix-based routing (default)

Since your pages are nested within a [locale] folder, all routes are by default prefixed with one of your supported locales (e.g. /en/about).

Locale detection

The locale is detected based on these priorities:

  1. A locale prefix is present in the pathname (e.g. /en/about)
  2. A cookie is present that contains a previously detected locale
  3. A locale can be matched based on the accept-language header (opens in a new tab)
  4. As a last resort, the defaultLocale is used

To change the locale, users can visit a prefixed route. This will take precedence over a previously matched locale that is saved in a cookie or the accept-language header and will update the previous cookie value.

Example workflow:

  1. A user requests / and based on the accept-language header, the en locale is matched.
  2. The en locale is saved in a cookie and the user is redirected to /en.
  3. The app renders <Link locale="de" href="/">Switch to German</Link> to allow the user to change the locale to de.
  4. When the user clicks on the link, a request to /de is initiated.
  5. The middleware will update the cookie value to de.
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You can optionally remove the locale prefix in pathnames by changing the localePrefix setting.

Which algorithm is used to match the accept-language header against the available locales?

To determine the best-matching locale based on the available options from your app, the middleware uses the "best fit" algorithm of @formatjs/intl-localematcher (opens in a new tab). This algorithm is expected to provide better results than the more conservative "lookup" algorithm that is specified in RFC 4647 (opens in a new tab).

To illustrate this with an example, let's consider your app supports these locales:

  1. en-US
  2. de-DE

The "lookup" algorithm works by progressively removing subtags from the user's accept-language header until a match is found. This means that if the user's browser sends the accept-language header en-GB, the "lookup" algorithm will not find a match, resulting in the default locale being used.

In contrast, the "best fit" algorithm compares the distance between the user's accept-language header and the available locales, while taking into consideration regional information. Due to this, the "best fit" algorithm is able to match en-US as the best-matching locale in this case.

Strategy 2: Domain-based routing

If you want to serve your localized content based on different domains, you can provide a list of mappings between domains and locales to the middleware.

Examples:

  • us.example.com/en
  • ca.example.com/en
  • ca.example.com/fr
middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // All locales across all domains
  locales: ['en', 'fr'],
 
  // Used when no domain matches (e.g. on localhost)
  defaultLocale: 'en',
 
  domains: [
    {
      domain: 'us.example.com',
      defaultLocale: 'en',
      // Optionally restrict the locales available on this domain
      locales: ['en']
    },
    {
      domain: 'ca.example.com',
      defaultLocale: 'en'
      // If there are no `locales` specified on a domain,
      // all available locales will be supported here
    }
  ]
});
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You can optionally remove the locale prefix in pathnames by changing the localePrefix setting.

Locale detection

To match the request against the available domains, the host is read from the x-forwarded-host header, with a fallback to host.

The locale is detected based on these priorities:

  1. A locale prefix is present in the pathname (e.g. ca.example.com/fr)
  2. A locale is stored in a cookie and is supported on the domain
  3. A locale that the domain supports is matched based on the accept-language header (opens in a new tab)
  4. As a fallback, the defaultLocale of the domain is used

Since the middleware is aware of all your domains, if a domain receives a request for a locale that is not supported (e.g. en.example.com/fr), it will redirect to an alternative domain that does support the locale.

Example workflow:

  1. The user requests us.example.com and based on the defaultLocale of this domain, the en locale is matched.
  2. The app renders <Link locale="fr" href="/">Switch to French</Link> to allow the user to change the locale to fr.
  3. When the link is clicked, a request to us.example.com/fr is initiated.
  4. The middleware recognizes that the user wants to switch to another domain and responds with a redirect to ca.example.com/fr.
How is the best matching domain for a given locale detected?

The bestmatching domain is detected based on these priorities:

  1. Stay on the current domain if the locale is supported here
  2. Use an alternative domain where the locale is configured as the defaultLocale
  3. Use an alternative domain where the available locales are restricted and the locale is supported
  4. Use an alternative domain that supports all locales

Further configuration

Locale prefix

By default, the pathnames of your app will be available under a prefix that matches your directory structure (e.g. src/app/[locale]/about/page.tsx โ†’ /en/about). You can however customize the routing to optionally remove the prefix.

Note that if you're using the navigation APIs from next-intl, you want to make sure your localePrefix setting matches your middleware configuration.

Always use a locale prefix (default)

By default, pathnames always start with the locale (e.g. /en/about).

middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localePrefix: 'always' // This is the default
});
How can I redirect unprefixed pathnames?

If you want to redirect unprefixed pathnames like /about to a prefixed alternative like /en/about, you can adjust your middleware matcher to match unprefixed pathnames too.

Don't use a locale prefix for the default locale

If you don't want to include a locale prefix for the default locale, but only for non-default locales, you can configure the middleware accordingly:

middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localePrefix: 'as-needed'
});

In this case, requests where the locale prefix matches the default locale will be redirected (e.g. /en/about to /about). This will affect both prefix-based as well as domain-based routing.

Note that:

  1. If you use this strategy, you should make sure that your matcher detects unprefixed pathnames.
  2. If you use the Link component, the initial render will point to the prefixed version but will be patched immediately on the client once the component detects that the default locale has rendered. The prefixed version is still valid, but SEO tools might report a hint that the link points to a redirect.

Never use a locale prefix

๐Ÿ’ก

If you'd like to provide a locale to next-intl, e.g. based on user settings, you can consider setting up next-intl without i18n routing. This way, you don't need to use the middleware in the first place.

In case you're using the middleware, but you don't want your pathnames to be prefixed with a locale, you can configure the middleware to never show a locale prefix in the URL.

This can be useful in the following cases:

  1. You're using domain-based routing and you support only a single locale per domain
  2. You're using a cookie to determine the locale but would like to enable static rendering
middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localePrefix: 'never'
});

In this case, requests for all locales will be rewritten to have the locale only 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.

Note that:

  1. If you use this strategy, you should make sure that your matcher detects unprefixed pathnames.
  2. If you don't use domain-based routing, the cookie is now the source of truth for determining the locale in the middleware. Make sure that your hosting solution reliably returns the set-cookie header from the middleware (e.g. Vercel and Cloudflare are known to potentially strip this header (opens in a new tab) for cacheable requests).
  3. Alternate links are disabled in this mode since there might not be distinct URLs per locale.

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.

middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  localeDetection: false
});

In this case, only the locale prefix and a potentially matching domain are used to determine the locale.

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 (opens in a new tab)).

Alternate links

The middleware automatically sets the link header (opens in a new tab) 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.

If you prefer to include these links yourself, e.g. because you're using a CMS to manage localized slugs of your pages, you can opt-out of this behavior.

middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  // ... other config
 
  alternateLinks: false // Defaults to `true`
});

If you decide to manage alternate links yourself, a good option can be to include them in a sitemap.

Localizing pathnames

Many apps choose to localize pathnames, especially when search engine optimization is relevant, e.g.:

  • /en/about
  • /de/ueber-uns

Since you want to define these routes only once internally, you can use the next-intl middleware to rewrite (opens in a new tab) such incoming requests to shared pathnames.

middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  defaultLocale: 'en',
  locales: ['en', 'de'],
 
  // The `pathnames` object holds pairs of internal and
  // external paths. Based on the locale, the external
  // paths are rewritten to the shared, internal ones.
  pathnames: {
    // If all locales use the same pathname, a single
    // external path can be used for all locales.
    '/': '/',
    '/blog': '/blog',
 
    // If locales use different paths, you can
    // specify each external path per locale.
    '/about': {
      en: '/about',
      de: '/ueber-uns'
    },
 
    // Dynamic params are supported via square brackets
    '/news/[articleSlug]-[articleId]': {
      en: '/news/[articleSlug]-[articleId]',
      de: '/neuigkeiten/[articleSlug]-[articleId]'
    },
 
    // Static pathnames that overlap with dynamic segments
    // will be prioritized over the dynamic segment
    '/news/just-in': {
      en: '/news/just-in',
      de: '/neuigkeiten/aktuell'
    },
 
    // Also (optional) catch-all segments are supported
    '/categories/[...slug]': {
      en: '/categories/[...slug]',
      de: '/kategorien/[...slug]'
    }
  }
});
๐Ÿ’ก

If you have pathname localization set up in the middleware, you likely want to use the localized navigation APIs in your components.

How can I revalidate localized pathnames?

Depending on if a route is generated statically (at build time) or dynamically (at runtime), revalidatePath (opens in a new tab) needs to be called either for the localized or the internal pathname.

Consider this example:

app
โ””โ”€โ”€ [locale]
    โ””โ”€โ”€ news
        โ””โ”€โ”€ [slug]

โ€ฆ with this middleware configuration:

middleware.ts
import createMiddleware from 'next-intl/middleware';
 
export default createMiddleware({
  defaultLocale: 'en',
  locales: ['en', 'fr'],
  pathnames: {
    '/news/[slug]': {
      en: '/news/[slug]',
      fr: '/infos/[slug]'
    }
  }
});

Depending on whether some-article was included in generateStaticParams (opens in a new tab) or not, you can revalidate the route like this:

// Statically generated at build time
revalidatePath('/fr/news/some-article');
 
// Dynamically generated at runtime:
revalidatePath('/fr/infos/some-article');

When in doubt, you can revalidate both paths to be on the safe side.

See also: vercel/next.js#59825 (opens in a new tab)

How can I localize dynamic segments?

If you have a route like /news/[articleSlug]-[articleId], you may want to localize the articleSlug part in the pathname like this:

/en/news/launch-of-new-product-94812
/de/neuigkeiten/produktneuheit-94812

In this case, the localized slug can either be provided by the backend or generated in the frontend by slugifying the localized article title.

A good practice is to include the ID in the URL, allowing you to retrieve the article based on this information from the backend. The ID can be further used to implement self-healing URLs (opens in a new tab), where a redirect is added if the articleSlug doesn't match.

If you localize the values for dynamic segments, you might want to turn off alternate links and provide your own implementation that considers localized values for dynamic segments.

Matcher config

The middleware is intended to only run on pages, not on arbitrary files that you serve independently of the user locale (e.g. /favicon.ico).

Because of this, the following config is generally recommended:

middleware.ts
export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

This enables:

  1. A redirect at / to a suitable locale
  2. Internationalization of all pathnames starting with a locale (e.g. /en/about)
Can I avoid hardcoding the locales in the matcher config?

A Next.js matcher (opens in a new tab) needs to be statically analyzable, therefore you can't use variables to generate this value dynamically. However, in case you're self-hosting Next.js via a Node.js server, you can implement the matcher dynamically instead:

middleware.ts
import {NextRequest} from 'next/server';
import createIntlMiddleware from 'next-intl/middleware';
 
// Can be imported from a shared module
const locales = ['en', 'de'];
 
const intlMiddleware = createIntlMiddleware({
  locales
  // ...
});
 
export default function middleware(request: NextRequest) {
  const {pathname} = request.nextUrl;
 
  const shouldHandle =
    pathname === '/' ||
    new RegExp(`^/(${locales.join('|')})(/.*)?$`).test(
      request.nextUrl.pathname
    );
  if (!shouldHandle) return;
 
  return intlMiddleware(request);
}

Pathnames without a locale prefix

There are two use cases where you might want to match pathnames without a locale prefix:

  1. You're using a config for localePrefix other than always
  2. You want to implement redirects that add a locale for unprefixed pathnames (e.g. /about โ†’ /en/about)

For these cases, the middleware should run on requests for pathnames without a locale prefix as well.

A popular strategy is to match all routes that don't start with certain segments (e.g. /_next) and also none that include a dot (.) since these typically indicate static files. However, if you have some routes where a dot is expected (e.g. /users/jane.doe), you should explicitly provide a matcher for these.

middleware.ts
export const config = {
  // Matcher entries are linked with a logical "or", therefore
  // if one of them matches, the middleware will be invoked.
  matcher: [
    // Match all pathnames except for
    // - โ€ฆ if they start with `/api`, `/_next` or `/_vercel`
    // - โ€ฆ the ones containing a dot (e.g. `favicon.ico`)
    '/((?!api|_next|_vercel|.*\\..*).*)',
    // However, match all pathnames within `/users`, optionally with a locale prefix
    '/([\\w-]+)?/users/(.+)'
  ]
};

Note that some third-party providers like Vercel Analytics (opens in a new tab) and umami (opens in a new tab) typically use internal endpoints that are then rewritten to an external URL (e.g. /_vercel/insights/view). Make sure to exclude such requests from your middleware matcher so they aren't accidentally rewritten.

Base path

The next-intl middleware as well as the navigation APIs will automatically pick up a basePath (opens in a new tab) that you might have configured in your next.config.js.

Note however that you should make sure that your middleware matcher handles the root of your base path:

middleware.ts
// ...
 
export const config = {
  // The `matcher` is relative to the `basePath`
  matcher: [
    // This entry handles the root of the base
    // path and should always be included
    '/'
 
    // ... other matcher config
  ]
};

Composing other middlewares

By calling createMiddleware, you'll receive a function of the following type:

middleware(request: NextRequest): NextResponse

If you need to incorporate additional behavior, you can either modify the request before the next-intl middleware receives it, or modify the response that is returned.

middleware.ts
import createIntlMiddleware from 'next-intl/middleware';
import {NextRequest} from 'next/server';
 
export default async function middleware(request: NextRequest) {
  // Step 1: Use the incoming request (example)
  const defaultLocale = request.headers.get('x-your-custom-locale') || 'en';
 
  // Step 2: Create and call the next-intl middleware (example)
  const handleI18nRouting = createIntlMiddleware({
    locales: ['en', 'de'],
    defaultLocale
  });
  const response = handleI18nRouting(request);
 
  // Step 3: Alter the response (example)
  response.headers.set('x-your-custom-locale', defaultLocale);
 
  return response;
}
 
export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

Example: Additional rewrites

If you need to handle rewrites apart from the ones provided by next-intl, you can adjust the pathname of the request before invoking the next-intl middleware (based on "A/B Testing with Cookies" by Vercel (opens in a new tab)).

This example rewrites requests for /[locale]/profile to /[locale]/profile/new if a special cookie is set.

middleware.ts
import createIntlMiddleware from 'next-intl/middleware';
import {NextRequest} from 'next/server';
 
export default async function middleware(request: NextRequest) {
  const [, locale, ...segments] = request.nextUrl.pathname.split('/');
 
  if (locale != null && segments.join('/') === 'profile') {
    const usesNewProfile =
      (request.cookies.get('NEW_PROFILE')?.value || 'false') === 'true';
 
    if (usesNewProfile) {
      request.nextUrl.pathname = `/${locale}/profile/new`;
    }
  }
 
  const handleI18nRouting = createIntlMiddleware({
    locales: ['en', 'de'],
    defaultLocale: 'en'
  });
  const response = handleI18nRouting(request);
  return response;
}
 
export const config = {
  matcher: ['/', '/(de|en)/:path*']
};

Note that if you use a localePrefix other than always, you need to adapt the handling appropriately to handle unprefixed pathnames too.

Example: Integrating with Clerk

@clerk/nextjs (opens in a new tab) provides a middleware that can be combined (opens in a new tab) with other middlewares like the one provided by next-intl. By combining them, the middleware from @clerk/next will first ensure protected routes are handled appropriately. Subsequently, the middleware from next-intl will run, potentially redirecting or rewriting incoming requests.

middleware.ts
import {clerkMiddleware, createRouteMatcher} from '@clerk/nextjs/server';
import createMiddleware from 'next-intl/middleware';
 
const intlMiddleware = createMiddleware({
  locales: ['en', 'de'],
  defaultLocale: 'en'
});
 
const isProtectedRoute = createRouteMatcher(['/:locale/dashboard(.*)']);
 
export default clerkMiddleware((auth, req) => {
  if (isProtectedRoute(req)) auth().protect();
 
  return intlMiddleware(req);
});
 
export const config = {
  // Match only internationalized pathnames
  matcher: ['/', '/(de|en)/:path*']
};

Example: Integrating with Supabase Authentication

In order to use Supabase Authentication with next-intl, you need to combine the Supabase middleware with the one from next-intl.

You can do so by following the setup guide from Supabase (opens in a new tab) and adapting the middleware as follows:

middleware.ts
import {type NextRequest} from 'next/server';
import {createServerClient, type CookieOptions} from '@supabase/ssr';
import createIntlMiddleware from 'next-intl/middleware';
 
const handleI18nRouting = createIntlMiddleware({
  locales: ['en', 'de'],
  defaultLocale: 'en'
});
 
export async function middleware(request: NextRequest) {
  const response = handleI18nRouting(request);
 
  const supabase = createServerClient(
    process.env.NEXT_PUBLIC_SUPABASE_URL!,
    process.env.NEXT_PUBLIC_SUPABASE_ANON_KEY!,
    {
      cookies: {
        get(name: string) {
          return request.cookies.get(name)?.value;
        },
        set(name: string, value: string, options: CookieOptions) {
          request.cookies.set({name, value, ...options});
          response.cookies.set({name, value, ...options});
        },
        remove(name: string, options: CookieOptions) {
          request.cookies.set({name, value: '', ...options});
          response.cookies.set({name, value: '', ...options});
        }
      }
    }
  );
 
  await supabase.auth.getUser();
  return response;
}
 
export const config = {
  matcher: ['/', '/(de|en)/:path*']
};

Note that cookies need to be set simultaneously for the request and the response.

Example: Integrating with Auth.js (aka NextAuth.js)

The Next.js middleware of Auth.js (opens in a new tab) requires an integration with their control flow to be compatible with other middlewares. The success callback (opens in a new tab) can be used to run the next-intl middleware on authorized pages. However, public pages need to be treated separately.

For pathnames specified in the pages object (opens in a new tab) (e.g. signIn), Auth.js will skip the entire middleware and not run the success callback. Therefore, we have to detect these pages before running the Auth.js middleware and only run the next-intl middleware in this case.

middleware.ts
import {withAuth} from 'next-auth/middleware';
import createIntlMiddleware from 'next-intl/middleware';
import {NextRequest} from 'next/server';
 
const locales = ['en', 'de'];
const publicPages = ['/', '/login'];
 
const intlMiddleware = createIntlMiddleware({
  locales,
  localePrefix: 'as-needed',
  defaultLocale: 'en'
});
 
const authMiddleware = withAuth(
  // Note that this callback is only invoked if
  // the `authorized` callback has returned `true`
  // and not for pages listed in `pages`.
  function onSuccess(req) {
    return intlMiddleware(req);
  },
  {
    callbacks: {
      authorized: ({token}) => token != null
    },
    pages: {
      signIn: '/login'
    }
  }
);
 
export default function middleware(req: NextRequest) {
  const publicPathnameRegex = RegExp(
    `^(/(${locales.join('|')}))?(${publicPages
      .flatMap((p) => (p === '/' ? ['', '/'] : p))
      .join('|')})/?$`,
    'i'
  );
  const isPublicPage = publicPathnameRegex.test(req.nextUrl.pathname);
 
  if (isPublicPage) {
    return intlMiddleware(req);
  } else {
    return (authMiddleware as any)(req);
  }
}
 
export const config = {
  matcher: ['/((?!api|_next|.*\\..*).*)']
};
๐Ÿ’ก

Usage without middleware (static export)

If you're using the static export feature from Next.js (opens in a new tab) (output: 'export'), the middleware will not run. You can use prefix-based routing nontheless to internationalize your app, but a few tradeoffs apply.

Static export limitations:

  1. There's no default locale that can be used without a prefix (same as localePrefix: 'always')
  2. The locale can't be negotiated at runtime (same as localeDetection: false)
  3. You can't use pathname localization
  4. This requires static rendering
  5. You need to add a redirect for the root of the app
app/page.tsx
import {redirect} from 'next/navigation';
 
// Redirect the user to the default locale when `/` is requested
export default function RootPage() {
  redirect('/en');
}

Note that other limitations as documented by Next.js (opens in a new tab) will apply too.

Troubleshooting

"Unable to find next-intl locale because the middleware didn't run on this request."

This can happen either because:

  1. You're using a setup with i18n routing but the middleware is not set up.
  2. You're using a setup without i18n routing but are reading the locale param passed to the function within getRequestConfig.
  3. The middleware is set up in the wrong file (e.g. you're using the src folder, but middleware.ts was added in the root folder).
  4. The middleware matcher didn't match a request, but you're using APIs from next-intl in a component.
  5. You're attempting to implement static rendering via force-static (opens in a new tab).

To recover from this error, please make sure that:

  1. You're consistently using a setup with or without i18n routing (i.e. with or without the routing APIs).
  2. If you're not using i18n routing:
    1. You don't read the locale param in getRequestConfig but instead return it.
  3. If you're using i18n routing:
    1. You're using APIs from next-intl (including the navigation APIs) exclusively within the [locale] segment.
    2. Your middleware matcher matches all routes of your application, including dynamic segments with potentially unexpected characters like dots (e.g. /users/jane.doe).
    3. If you're using localePrefix: 'as-needed', the locale segment effectively acts like a catch-all for all unknown routes. You should make sure that the locale is validated before it's used by any APIs from next-intl.
    4. To implement static rendering, make sure to provide a static locale to next-intl instead of using force-static.

Note that next-intl will invoke the notFound() function to abort the render if the locale can't be found. You should consider adding a not-found page due to this.

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