Tutorial to implement a node.js server with express, express-validator, and polyglot.js to easily adjust the translations to be shown to the end user in any language that you want, fulfilling i18n requirements while decoupling your code.

Recently, I was working on a project that has an i18n requirement. I needed the API to validate incoming user data, and depending on that data, return the specific success or error messages in the user’s provided language.

Regarding the actual translations, I wanted to easily provide the backend with the messages in both languages (Spanish and English to begin with), and I wanted to be able to eventually support more languages, being able to hire a translator if needed, and having him modify them “on the go” without requiring help from a developer.

So I started researching how to fulfill those requirements, and I ran into some hiccups along the way, thus, I thought it’d be nice to create a tutorial with my proposed (and implemented) solution.

Let’s code!

This tutorial uses ES6, Node.js and Express, creating a server that will be responding the calls. I’ve included a working solution with basic testing, you can go ahead and check that out in this repository, or work through the code step-by-step with me!


We’ll be using some battle tested libraries to speed up our development:

  • express, to create/manage the server
  • express-locale, to get the user’s locale
  • body-parser, to get the user’s input
  • express-validator, to validate the user’s input
  • node-polyglot, by Airbnb, to help us manage languages
  • object.fromentries, to convert an Array into an Object

And since we’ll be using ES6, we’ll also be needing babel!

  • @babel/cli
  • @babel/core
  • @babel/preset-env

So let’s get to the console and create the project

mkdir i18n-validation
cd i18n-validation
npm init

For this use case, we’ll leave all the defaults that npm gives us, except for the default entry which I changed to server.js

Now, lets install our main dependencies

npm i express express-locale body-parser express-validator node-polyglot object.fromentries

Now, let’s install our development dependencies

npm i @babel/cli @babel/core @babel/preset-env --save-dev

Now, all we need to do is add another file:

touch .babelrc

And inside, we’ll write:

    "presets": [

If you’re going to source control your project, don ’t forget to add a .gitignore with node_modules in it, to avoid committing them.

Remember that we’ll be using ES6, and we need to do some extra steps to be able to do so, so let’s go ahead and change our scripts in the package.json:

"main": "server.js",
  "scripts": {
    "clean": "rm -rf dist && mkdir dist",
    "transpile": "babel -d ./dist ./src",
    "build": "npm run clean && npm run transpile",
    "start": "npm run build && node ./dist/server.js",
    "test": "echo \"Error: no test specified\" && exit 1"

We’re using several scripts and daisy-chaining some of them.

  • clean to create the dist folder that will hold the ES5 transpiled code.
  • transpile to transpile the code from ES6 to ES5.
  • build runs clean and afterwards, transpile.
  • start runs build and afterwards, the server.js file with node from the new ES5 transpiled dist folder.
  • test is empty and won’t be covered in the tutorial, but the repository includes basic tests.

Finally, let’s create the src folder and inside, the server.js file:

mkdir src
cd src
touch server.js

Let’s now get express going by modifying the server.js

// Import dependencies
// =============================================================
import express from 'express'

// Setup the express router
// =============================================================
const router = express()

// Set the port to be used
const port = process.env.PORT || 8080

// Start the server!
// =============================================================
router.listen(port, () => {
    console.log(`App running on port ${port}`)

By now, we can run:

npm start

If all’s good, the console should be telling us that we’re running on port 8080. And with that, we’ll have a server… that does nothing!

Now, we need to actually get it going.
So we need to add more dependencies:

// Import dependencies
// =============================================================
import express from 'express'
import createLocaleMiddleware from 'express-locale'
import bodyParser from 'body-parser'

And we need to set them up on the server

// Setup the express router
// =============================================================
const router = express()

// Set the port to be used
const port = process.env.PORT || 8080

// Add data parsing to express
router.use(bodyParser.urlencoded({ extended: true }))

// Get the user's locale, and set a default in case there's none
    "priority": ["accept-language", "default"],
    "default": "en_US"

With these changes, now we’re checking on the user’s locale and parsing the data they’re sending. However, we need to add polyglot to express.

For that, we’ll first be creating our .js file where the translations will be living

mkdir i18n
cd i18n
touch i18n.js
cd ..

Let’s open this new file, where we’ll have two constants, an array that will show what languages are available

export const availableLangs = ['es', 'en']

And an object that will contain the actual translations

export const messages = {
    en: {
        // Error messages
        'emailRequiredField': "'email' is a required field.",
        'emailIsEmail': "This is not a valid email address.",
        'passwordRequiredField': "'password' is a required field.",

        // Success messages
        'loginSuccessful': "You've successfully logged in.",
        'emailSent': "Your password recovery email was sent."
    es: {
        // Mensajes de error
        'emailRequiredField': "'email' es un campo requerido.",
        'emailIsEmail': "Este no es un email válido.",
        'passwordRequiredField': "'password' es un campo requerido.",

        // Mensajes de éxito
        'loginSuccessful': "Has iniciado sesión exitosamente.",
        'emailSent': "Tu correo de recuperación de contraseña ha sido enviado."

With our messages ready, we’ll go ahead and create a middleware for express, that will import polyglot and these translations, to include them in the actual express request.

mkdir utilities
cd utilities
touch startPolyglot.js
cd ..

Open this new file, where we’ll import both polyglot and the translations

import Polyglot from 'node-polyglot'
import { messages } from '../i18n/i18n'

And we’ll create a function that will be used on every request as an Express’ middleware. It will get the user’s locale (which we got in the server.js), create an instance of Polyglot, and load it with the proper messages depending on the user’s language

exports.startPolyglot = (req, res, next) => {
    // Get the locale from express-locale
    const locale = req.locale.language

    // Start Polyglot and add it to the req
    req.polyglot = new Polyglot()

    // Decide which phrases for polyglot will be used
    if (locale == 'es') {
    } else {


If you remember, our server.js uses the createLocaleMiddleware to set the current locale, which lives on req.locale.language.

So we get that value, and for our use case, check if it is es for Spanish or en for English (our default in case it’s neither), and load the proper messages for the language, which are added to the Express’ ‘req’ object through polyglot’s extend function.

Adding Polyglot to Express

Now, we need to add this middleware to Express on the server.js, both by importing it, and adding it AFTER we create the locale middleware, as polyglot uses it.

import { startPolyglot } from './utilities/startPolyglot'

Add this line at the very top, with the rest of the imports.

// Start polyglot and set the language in the req with the phrases to be used

Add this lines after the createLocaleMiddleware function is called.

There, now our server is ready to send error or success messages in either Spanish or English, however, where will these messages originate?


So Express needs to know what to do with the different type of calls on the different routes. For that, we’ll start listening for calls in our server by first creating a routes folder and file.

mkdir routes
cd routes
touch auth.routes.js
cd ..

Let’s open this file and add the following code:

// Routes =============================================================
module.exports = router => {

    // POST route to mock a log endpoint"/api/login")

    // POST route to mock a forgotten password endpoint"/api/forgot-password")

What this code will do, is export a function that will take the Express instance as a parameter to create the actual routes we’ll be using in our test API. For now, it’s missing parameters, since it’s only adding the first one, which tells express the route to listen to. After that parameter, we may add as many Express’ middlewares as we need. We will be adding middleware to do the input data validation, the error processing in case there’s any, and finally, if all’s good, respond with a success message if there were no errors with the validation.

Now, let’s go ahead and add it to the server.js right before we start it

// Routes
// =============================================================

This line should be added right before we start our server with router.listen

So now our API is listening for POST requests on localhost:8080/api/login and localhost:8080/api/forgot-password, but we still have no functionality, let’s get there.

Validating user’s input

So it’s time to validate data, for that, we’ll be using express-validator, which is a handy middleware that lets us validate data as it comes from the req object, setting specific error messages for each of the parameters that we’re expecting.

mkdir validator
cd validator
touch auth.validator.js
cd ..

Now, open auth.validator.js and we’ll first import the check function from express-validator.

import { check } from 'express-validator/check'

Next, we’ll create a function that will be exported, which we’ll be using as middleware in our auth.routes.js. This function receives a String, which we define based on the use case of that route, inside we’ll use the check function that was just imported, to validate the data we’re receiving. We’ll be using a switch for that, so we can reuse the same validator both for the login, and the forgot-password.

Here’s the code:

exports.validator = functionName => {
    switch (functionName) {
        case 'login': {
            return [
        case 'forgotPassword': {
            return [

We won’t go deep into the details of how the check function works, but it basically adds another object inside of the req which will store the errors (if there’s any).

What’s important to note though, is the fact that instead of setting normal error messages, we’re using the variables that we created on our i18n file!

Why? Because we want to use those keys from our i18n.js in whatever language the user chooses, so we need to check the object for all the possible error messages, and check our translated errors object, and swap the error string with the actual error message that we wrote in the user’s language… but not yet.

For now, we’ll be adding this validator into our route file by going to auth.routes.js and importing it:

import { validator } from '../validator/auth.validator'

This line should go at the very top, before our module.exports

Now, we’ll use it on our actual routes:

// POST route to mock a login endpoint"/api/login", validator('login'))

// POST route to mock a forgotten password endpoint"/api/forgot-password", validator('forgotPassword'))

So now our server listens to post requests on those two routes, and validates the incoming payload. Now we need to make sure to transform those strings.

Translating the errors

For this, we’ll create another Express middleware which will check all the errors (if any) and convert them into strings in the user’s language.

cd utilities
touch processErrors.js
cd ..

Go ahead and open this new file, where we’ll import another function from express-validator and the npm package object.fromentries.

import { validationResult } from 'express-validator/check'
import fromEntries from 'object.fromentries'

Now, we need to create the function that will do the translation:

const translateMessages = (errObj, req) => {
    // Convert the errObj to an Array
    const errArr = Object.entries(errObj)

    // For each array(err), compare the error msg with the polyglot phrases, and replace it.
    errArr.forEach(err => {
        Object.keys(req.polyglot.phrases).forEach(phrase => {
            if (phrase == err[1].msg) {
                err[1].msg = req.polyglot.t(phrase)

    // Return a function that converts the Array to an Object
    return fromEntries(errArr)

In this code, we’re receiving both the error Object created with express-validator (which we’ll extract from the req object with the validationResult function in a bit), and the Express’ req object.

We’re creating an Array from the errObj, and then, for each entry, we’re taking the string we set as the error variable, and comparing it with the keys from the translation messages, changing the string in the errArr (each “err[1].msg”) to the actual phrase in polyglot in the desired language (each “phrase”).

Finally, we use the imported fromEntries function, to convert the Array back into an Object and return it.

Now, in that same file, we’ll export a middleware function that will be using this translateMessages function to process the errors (if any).

exports.procErr = (req, res, next) => {

    // Verifies if there were validation errors added to the request
    const validationErrors = validationResult(req)

    // If there were errors in the validation
    if (!validationErrors.isEmpty()) {
        // Return the result of the function below
        return res.status(400).send(translateMessages(validationErrors.mapped(), req))
    } else {
        // If no errors, go!

In this code we receive the regular req, res, next from Express, and we first verify if there were any errors using express-validator’s validationResult. Then, we check if there are errors, and if there are any, we return them with Express’ response. Check that return closely, as you can see, we send the results of the translateMessages function that is receiving the validationErrors, and the req object. We also have an else, that when there are no validation errors, calls next() to continue to the next Express middleware.

Sending the errors

So we’re able to manage the errors by converting them from the string to their translated version, and packaging it in an object, ready to be sent back to the user if needed.

Now, we just need to use that file!
Let’s go back to our auth.routes.js file and make use of this new function by importing it:

import { procErr } from '../utilities/processErrors'

As I mentioned earlier, we built it as an Express Middleware, so we can just add it inside of our chain of events.

And then using it in the actual routes:

// Routes =============================================================
module.exports = router => {

    // POST route to mock a login  endpoint"/api/login", validator('login'), procErr)

    // POST route to mock a forgotten password endpoint"/api/forgot-password", validator('forgotPassword'), procErr)


Moving past errors

So now, our code is ready to handle errors in both languages, but what about success messages?

We already have those in the i18n.js file, but we’re not using them. Lets write the final piece of code:

mkdir controller
cd controller
touch auth.controller.js
cd ..

Open this new file, where we’ll create a couple of exports to handle the final steps of the login and forgot-password processes. If express didn’t return an error on the last step, theoretically, there are no errors on the user’s data, so we’ll go ahead and send success messages here.

Of course, on a real world application we’d go to the database and check the user’s data and confirm it is actually correct and not just valid, but that’s beyond the scope of this tutorial.

So let’s write some code on the auth.controller.js.

exports.login = (req, res) => {
    // If no validation errors, get the req.body objects that were validated and are needed
    const { email, password } = req.body

    // Here, we would make use of that data, validating it against our database, creating a JWT token, etc...
    // Since all the validations passed, we send the loginSuccessful message, which would normally include a JWT or some other form of authorization
    return res.status(200).send({ auth: true, message: req.polyglot.t('loginSuccessful'), token: null })

exports.forgotPassword = (req, res) => {
    // If no validation errors, get the req.body objects that were validated and are needed
    const { email } = req.body
    // Here, we would make use of that data, validating it against our database, creating a JWT token, etc...
    // Since all the validations passed, we send the emailSent message
    return res.status(200).send({ auth: true, message: req.polyglot.t('emailSent') })

As you can see, both functions are exported to be used in the routes file, and both deconstruct the req.body to get the values we need to use.

I should emphasize that in both cases, further validation would be done in the controller, such as going to the database and checking if the user’s actually exist and are authorized to either login (and their password is correct) or if they’re not banned and are authorized to request a new password.

We’re assuming all of those things happened already, and just sending the response using Express’ res which includes the message with: req.polyglot.t('key'). This will take the value assigned to that key in the user’s selected language, and return that message.

Now, we need to go back to our routes to add this two functions there. The final version of auth.routes.js should now look something like this:

import { validator } from '../validator/auth.validator'
import { procErr } from '../utilities/processErrors'
import { login,
    forgotPassword } from '../controller/auth.controller'

// Routes =============================================================
module.exports = router => {

    // POST route to mock a log endpoint"/api/login", validator('login'), procErr, login)

    // POST route to mock a forgotten password endpoint"/api/forgot-password", validator('forgotPassword'), procErr, forgotPassword)

As you can see, we’re importing both login and forgotPassword, and adding them in the post as the final parameter. These last functions respond with the success messages when everything is ok!


Let’s check that our API is working as expected.

Go ahead and run npm run start. This will build our transpile our code and start the server. If we followed all steps, we should see: App running on port 8080 in our console.

Now open up Postman.

  • Set the Method to POST
  • Set the Request URL to localhost:8080/api/login
  • Set the Headers key to Accept-Language and the value to es_MX
  • Set the Body to { “email”: "" }

And click on Send. If all went well, you should see this response:

    "password": {
        "location": "body",
        "param": "password",
        "msg": "'password' es un campo requerido."

You can play around with the Request URL or the Headers or the Body to see the different responses from the API in either language. You can play around with the Request URL trying both routes or the Headers setting either en_US or es_MX or another option, also, try and modify the Body to see the different responses from the API.

So that’s it! Now, hopefully you have a clear understanding of how to set up an Express API that responds properly whether your headers are set to es_MX or en_US. Both for error and success messages.

If you have any questions, please create an issue on the repository, or send me a tweet. I’m more than glad to help.

Read you soon!