Internationalization (i18n) is one essential part of any application. This can be achieved easily in Nodejs with the the module i18n. There is another module called i18n-2, which is based on i18n and designed specifically to work out-of-the-box with Express.js. In this post, I will focus mostly on i18n-2.

First, you need to install i18n-2 using npm

$ npm install --save i18n-2

After that, add these config line into your app.js file. Remember to add it after you have loaded the cookieParser.

app.use(express.cookieParser('your secret here')); // put the config after this line

i18n.expressBind(app, {
  // setup some locales - other locales default to vi silently
  locales: ['vi', 'en'],
  // set the default locale
  defaultLocale: 'vi',
  // set the cookie name
  cookieName: 'locale'

// set up the middleware
app.use(function(req, res, next) {

The i18n object will now reside within the request object of each request. The above config also allows the locale to be set from query string or from cookie. For example, the will automatically set the locale to en. To use the i18n object, simply use the __ function

function handlerFunc1(req, res){
  res.render('index', { title: req.i18n.__("hello") });

Or if you want to use it in your view, simply use __ again

  <%= __("hello") %>

i18n-2 will then look up the key hello in the locale files (by default located in locales/en.js and locales/vi.js). If the keys or the files is not exist yet, it will then create those files and keys automatically for you so you don’t have to worry about the errors. You can then open those files and edit the values already there or add your new one. The file syntax is just normal JSON syntax.

Note: you cannot use // for comments.

  "hello": "Hello",
  "title": "title",

To change the language, you can set it directly using setLocale(locale) function. Beside that, you can set the cookie locale value for the browser to remember the current language for the next access.

function handlerFunc(req, res){
  // you can set it directly like this

  // or set it via the cookie
  res.cookie('locale', 'vi');

  // redirect back