middleware.ts

import { NextResponse } from "next/server";
import type { NextRequest } from "next/server";

import { i18n } from "./i18n-config";

import { match as matchLocale } from "@formatjs/intl-localematcher";
import Negotiator from "negotiator";

function getLocale(request: NextRequest): string | undefined {
   // Negotiator expects plain object so we need to transform headers
   const negotiatorHeaders: Record<string, string> = {};
   request.headers.forEach((value, key) => (negotiatorHeaders[key] = value));

   // @ts-ignore locales are readonly
   const locales: string[] = i18n.locales;

   // Use negotiator and intl-localematcher to get best locale
   let languages = new Negotiator({ headers: negotiatorHeaders }).languages(
      locales
   );

   const locale = matchLocale(languages, locales, i18n.defaultLocale);

   return locale;
}

const PUBLIC_FILE = /\.(.*)$/;

export function middleware(request: NextRequest) {
   const pathname = request.nextUrl.pathname;

   // skips adding the public files
   if (PUBLIC_FILE.test(pathname)) {
      return;
   }

   // Check if there is any supported locale in the pathname
   const pathnameIsMissingLocale = i18n.locales.every(
      (locale) =>
         !pathname.startsWith(`/${locale}/`) && pathname !== `/${locale}`
   );

   // Redirect if there is no locale
   if (pathnameIsMissingLocale) {
      const locale = getLocale(request);

      // e.g. incoming request is /products
      // The new URL is now /en-US/products
      return NextResponse.redirect(
         new URL(
            `/${locale}${pathname.startsWith("/") ? "" : "/"}${pathname}`,
            request.url
         )
      );
   }
}

export const config = {
   // Matcher ignoring `/_next/` and `/api/`
   matcher: ["/((?!api|_next/static|_next/image|favicon.ico).*)"],
};