TypeError: _this.services.languageUtils.isWhitelisted is not a function

const i18next = require("i18next");
const Backend = require("i18next-node-fs-backend");
const i18nextMiddleware = require("i18next-express-middleware");

i18next.use(Backend).init({
  backend: {
    loadPath: __dirname + "/locales/{{lng}}/{{ns}}.json",
  },
  debug: false,
  detection: {
    order: ["querystring", "cookie"],
    caches: ["cookie"],
  },
  preload: ["en", "ru"],
  saveMissing: true,
  fallbackLng: ["en"],
});

app.use(i18nextMiddleware.handle(i18next));

Your i18next version is >=v21.0.0 which removed some old deprecated api: https://www.i18next.com/misc/migration-guide#removed-deprecated

So either downgrade to <=v20 or update your other i18next dependencies, by replacing i18next-node-fs-backend with i18next-fs-backend and i18next-express-middleware with i18next-http-middleware

This package has been deprecated Author message:

replaced by i18next-http-middleware

const i18next = require('i18next');
const Backend = require('i18next-node-fs-backend');
const i18nextMiddleware = require('i18next-http-middleware');

i18next
  .use(Backend)
  .use(i18nextMiddleware.LanguageDetector)
  .init(
    {
      backend: {
        loadPath: path.resolve(__dirname, '../locales/{{lng}}/{{ns}}.json')
      },
      debug: false,
      detection: {
        order: ['querystring', 'cookie'],
        caches: ['cookie']
      },
      saveMissing: true,
      fallbackLng: 'en',
      preload: ['en', 'tr']
    },
    (err, t) => {
      if (err) return console.error(err);
      console.log('i18next is ready...');
      console.log(t('welcome'));
      console.log(t('welcome', { lng: 'tr' }));
    }
  );