t.ts

import { I18nTypes } from './I18nTypes';
const lang = 'en';

interface Langs {
  [key: string]: string | string[];
}

/*tslint:disable:no-var-requires*/
const langs: Langs = require(`./langs/${lang}.json`);

export const i18n = (key: I18nTypes, index: number = 0, ...replacements: string[]): string => {
  let value = langs[key];
  value = Array.isArray(value) ? value[index] : value;

  if (replacements.length) {
    let i = 0;
    const replaceFn = () => replacements[i++];
    value = value.replace(/{(.*?)}/gim, replaceFn);
  }

  // empty string could be provided value as well
  if (value === undefined && (process.env.NODE_ENV === 'development' || process.env.NODE_ENV === 'test')) {
    console.warn(`Could not find translation by provided key: ${key}`);
  }

  return value;
};