I follow the demo code provided by i18next

const i18next = require("i18next");
const i18m = require("koa-i18next-middleware");
 
// add custom detector.
i18m.LanguageDetector.addDetector({
    name: "mySessionDetector",
 
    lookup(ctx, options) {
        let found;
        if (options.lookupSession && ctx && ctx.sessions) {
            found = ctx.sessions[options.lookupMySession];
        }
        return found;
    },
 
    cacheUserLanguage(ctx, lng, options = {}) {
        if (options.lookupMySession && ctx && ctx.session) {
            ctx.session[options.lookupMySession] = lng;
        }
    }
});
 
i18next.use(i18m.LanguageDetector).init({
    fallbackLng: "en",
    preload: ["en", "es"],
    resources: {
        en: {
            translation: {
                "key": "hello world"
            }
        },
        es: {
            translation: {
                "key": "es hello world es"
            }
        }
    },
    detection: {
        order: ["querystring", "path", /*"cookie", "header",*/ "session", "mySessionDetector"],
 
        lookupQuerystring: "lng",
 
        lookupParam: "lng", // for route like: "path1/:lng/result"
        lookupFromPathIndex: 0,
 
        // currently using ctx.cookies
        lookupCookie: "i18next",
        // cookieExpirationDate: new Date(), // default: +1 year
        // cookieDomain: "", // default: current domain.
 
        // currently using ctx.session
        lookupSession: "lng",
 
        // other options
        lookupMySession: "lang",
 
        // cache user language
        caches: ["cookie", "mySessionDetector"]
    }
}, (err, t) => {
    // initialized and ready to go!
    const hw = i18next.t("key"); // hw = "hello world"
    console.log(hw);
});
 
app.use(i18m.getHandler(i18next, {
        locals: "locals",
        ignoreRoutes: ["/no-lng-route"],
    })
);
 

console prompt error

what to do with this!