@inlang/paraglide-js/dist/services/lookup.js

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/**
 * Performs a lookup for the given language tag, among the available language tags,
 * according to the IETF BCP 47 spec.
 *
 * It **does not support Wildcards** at the moment.
 *
 * @see https://datatracker.ietf.org/doc/html/rfc4647#section-3.4
 */
export function lookup(locale, options) {
    const fallbackLanguages = [];
    const languageTagParts = locale.split("-").filter(Boolean);
    for (let i = languageTagParts.length; i > 0; i--) {
        //Skip the x separator
        if (languageTagParts[i - 1] === "x")
            continue;
        //Stringify the language tag parts
        const fallbackLanguageTag = languageTagParts.slice(0, i).join("-");
        if (!options.locales.includes(fallbackLanguageTag))
            continue;
        fallbackLanguages.push(fallbackLanguageTag);
    }
    return fallbackLanguages[0] ?? options.baseLocale;
}