Disallow localeDetection with localePrefix: "as-needed"

[apfelbox]

Hi,

using localeDetection: true in combination with localePrefix: "as-needed" should preferably be disallowed.

First: it isn't really useful, as on / you probably don't want locale detection, as there already is a valid locale at this address. And if the matched locale of your browser doesn't match the default locale, this page would not be accessible (as you will always be redirected away).

But also, I have a URL structure like this:

/[locale]/[[...url]]/page.tsx

with localeDetection enabled, you will get weird issues, as locale will just be the first URL segment, no matter whether this is an actual URL.

With it disabled, the [locale] param will be injected correctly.

I don't really know the internals, but maybe this is due to the fact, that we basically only have fully dynamic ([[...url]]) routes?

Thanks for your great library and all the awesome work! 🙌

[amannn]

Thank you for the feedback @apfelbox! 🙌

on / you probably don't want locale detection, as there already is a valid locale at this address.

I guess it depends a bit on what you consider "valid" here. Valid in terms of we can assume a locale to render for the request—for sure—but it might not be the best-matching one, or yet one the user understands at all. It seems to me like at least being able to support both use cases is reasonable.

you will get weird issues, as locale will just be the first URL segment, no matter whether this is an actual URL.

I would probably have to check in detail. Do you have details for specific cases where you found issues? The middleware runs independently of how you've defined pages in /app, so I think there shouldn't be a difference. Or are you using localized pathnames and those interfere?

There's quite an extensive test suite at middleware.test.tsx, if you'd like to contribute some failing cases I'm happy to have a look!

[apfelbox]

Hi @amannn, thank you for your response!

I have the following case: my German version is located at /, the English version is at /en/. My personal browser is English, so the locale redirect redirects me to /en/. If I enable the localeDetection the German homepage is not accessible for me anymore: every time I access it (by opening /), the locale detection kicks in and redirects me to /en/. This might be possible to fix by manually overwriting the locale cookie used by next-intl (to influence the locale detection) when using the "language selector" in my UI, but I am not sure if there is a proper way to set that cookie or if it's only ever just set internally in next-intl?

About the issues:
I have the structure /[locale]/[[...url]] and I got this results:

• /karriere -> {locale: "karriere", url: []} ❌
• /en/karriere -> {locale: "en" url: ["karriere"]} ✅

when I tweaked the issues and deactivated the locale detection I got the correct result:

• /karriere -> {locale: "de", url: ["karriere"]} ✅
• /en/karriere -> {locale: "en" url: ["karriere"]} ✅

not sure about how this occurred, but I will try to recreate it with your testsuite.

[amannn]

This might be possible to fix by manually overwriting the locale cookie used by next-intl (to influence the locale detection) when using the "language selector" in my UI, but I am not sure if there is a proper way to set that cookie or if it's only ever just set internally in next-intl?

This is automatically done for you when using the navigation APIs from next-intl for changing the locale (example).

The trick is that /de is always accessible and will take care of overwriting the cookie for you and then redirecting to /. If you hit that route with the cookie set, then you'll stay there. This is also mentioned in the localePrefix: 'as-needed' docs.

I will try to recreate it with your testsuite.

That's really nice of you, thank you @apfelbox!