react-i18next detect missing keys during test

i18nForTests.js

import i18n from 'i18next';
import { initReactI18next } from 'react-i18next';
import resources from '../locales/translations.json';

// we disable the translation texts for dev mode so that the
// tests would only break if an ID is missing but not
// if the translation has been changed by the translators.
const devTranslations = Object.keys(resources['en-US'].translation).reduce((acc, key) => {
  acc[key] = key;
  return acc;
}, {});

// you can use this for specific components with
// `<I18nextProvider i18n={i18nForTests}><Component...`
// but probably including it in setupTests is better ofr us.
i18n.use(initReactI18next).init({
  lng: 'dev',
  preload: ['dev'],
  resources: {
    dev: {
      translation: devTranslations
    }
  },
  // use `debug: true` to get more info about missing keys!
  parseMissingKeyHandler: (key) => {
    console.warn(`react-i18next key "${key}" not found.`);
    return key;
  }
});

export { i18n as i18nForTests };