@inlang/paraglide-js/dist/compiler/runtime/extract-locale-from-header.js

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import { isLocale } from "./is-locale.js";
/**
 * Extracts a locale from the accept-language header.
 *
 * Use the function on the server to extract the locale
 * from the accept-language header that is sent by the client.
 *
 * @example
 *   const locale = extractLocaleFromHeader(request);
 *
 * @type {(request: Request) => Locale}
 * @param {Request} request - The request object to extract the locale from.
 * @returns {string|undefined} The negotiated preferred language.
 */
export function extractLocaleFromHeader(request) {
    const acceptLanguageHeader = request.headers.get("accept-language");
    if (acceptLanguageHeader) {
        // Parse language preferences with their q-values and base language codes
        const languages = acceptLanguageHeader
            .split(",")
            .map((lang) => {
            const [tag, q = "1"] = lang.trim().split(";q=");
            // Get both the full tag and base language code
            const baseTag = tag?.split("-")[0]?.toLowerCase();
            return {
                fullTag: tag?.toLowerCase(),
                baseTag,
                q: Number(q),
            };
        })
            .sort((a, b) => b.q - a.q);
        for (const lang of languages) {
            if (isLocale(lang.fullTag)) {
                return lang.fullTag;
            }
            else if (isLocale(lang.baseTag)) {
                return lang.baseTag;
            }
        }
        return undefined;
    }
    return undefined;
}