Getting Started internationalizing (i18n) with Intlayer and Express

express-intlayer is a powerful internationalization (i18n) middleware for Express applications, designed to make your backend services globally accessible by providing localized responses based on the client's preferences.

Why Internationalise Your Backend?

Internationalising your backend is essential for serving a global audience effectively. It allows your application to deliver content and messages in the preferred language of each user. This capability enhances user experience and broadens your application's reach by making it more accessible and relevant to people from different linguistic backgrounds.

Practical Use Cases

• Displaying Backend Errors in User's Language: When an error occurs, displaying messages in the user's native language improves understanding and reduces frustration. This is especially useful for dynamic error messages that might be shown in front-end components like toasts or modals.

• Retrieving Multilingual Content: For applications pulling content from a database, internationalisation ensures that you can serve this content in multiple languages. This is crucial for platforms like e-commerce sites or content management systems that need to display product descriptions, articles, and other content in the language preferred by the user.

• Sending Multilingual Emails: Whether it's transactional emails, marketing campaigns, or notifications, sending emails in the recipient’s language can significantly increase engagement and effectiveness.

• Multilingual Push Notifications: For mobile applications, sending push notifications in a user's preferred language can enhance interaction and retention. This personal touch can make notifications feel more relevant and actionable.

• Other Communications: Any form of communication from the backend, such as SMS messages, system alerts, or user interface updates, benefits from being in the user's language, ensuring clarity and enhancing the overall user experience. By internationalising the backend, your application not only respects cultural differences but also aligns better with global market needs, making it a key step in scaling your services worldwide.

Getting Started

Installation

To begin using express-intlayer, install the package using npm:

npm install intlayer express-intlayer

Setup

Configure the internationalisation settings by creating an intlayer.config.ts in your project root:

import { Locales, type IntlayerConfig } from "intlayer";const config: IntlayerConfig = {  internationalization: {    locales: [      Locales.ENGLISH,      Locales.FRENCH,      Locales.SPANISH_MEXICO,      Locales.SPANISH_SPAIN,    ],    defaultLocale: Locales.ENGLISH,  },};export default config;

Declare Your Content

Create and manage your content declarations to store translations:

import { t, type Dictionary } from "intlayer";const indexContent = {  key: "index",  content: {    exampleOfContent: t({      en-GB: "Example of returned content in English",      en: "Example of returned content in English",      fr: "Exemple de contenu renvoyé en français",      "es-ES": "Ejemplo de contenido devuelto en español (España)",      "es-MX": "Ejemplo de contenido devuelto en español (México)",    }),  },} satisfies Dictionary;export default indexContent;

Your content declarations can be defined anywhere in your application as soon as they are included into the contentDir directory (by default, ./src). And match the content declaration file extension (by default, .content.{json,ts,tsx,js,jsx,mjs,mjx,cjs,cjx}).

For more details, refer to the content declaration documentation.

Express Application Setup

Setup your Express application to use express-intlayer:

import express, { type Express } from "express";import { intlayer, t, getDictionary, getIntlayer } from "express-intlayer";import dictionaryExample from "./index.content";const app: Express = express();// Load internationalisation request handlerapp.use(intlayer());// Routesapp.get("/t_example", (_req, res) => {  res.send(    t({      en-GB: "Example of returned content in English",      en: "Example of returned content in English",      fr: "Exemple de contenu renvoyé en français",      "es-ES": "Ejemplo de contenido devuelto en español (España)",      "es-MX": "Ejemplo de contenido devuelto en español (México)",    })  );});app.get("/getIntlayer_example", (_req, res) => {  res.send(getIntlayer("index").exampleOfContent);});app.get("/getDictionary_example", (_req, res) => {  res.send(getDictionary(dictionaryExample).exampleOfContent);});// Start serverapp.listen(3000, () => console.log(`Listening on port 3000`));

Compatibility

express-intlayer is fully compatible with:

• react-intlayer for React applications
• next-intlayer for Next.js applications
• vite-intlayer for Vite applications It also works seamlessly with any internationalisation solution across various environments, including browsers and API requests. You can customise the middleware to detect locale through headers or cookies:

import { Locales, type IntlayerConfig } from "intlayer";const config: IntlayerConfig = {  // ... Other configuration options  middleware: {    headerName: "my-locale-header",    cookieName: "my-locale-cookie",  },};export default config;

By default, express-intlayer will interpret the Accept-Language header to determine the client's preferred language.

For more information on configuration and advanced topics, visit our documentation.

Configure TypeScript

express-intlayer leverages the robust capabilities of TypeScript to enhance the internationalisation process. TypeScript's static typing ensures that every translation key is accounted for, reducing the risk of missing translations and improving maintainability.

Ensure the autogenerated types (by default at ./types/intlayer.d.ts) are included in your tsconfig.json file.

{  // ... Your existing TypeScript configurations  "include": [    // ... Your existing TypeScript configurations    ".intlayer/**/*.ts", // Include the auto-generated types  ],}

VS Code Extension

To improve your development experience with Intlayer, you can install the official Intlayer VS Code Extension.

Install from the VS Code Marketplace

This extension provides:

• Autocompletion for translation keys.
• Real-time error detection for missing translations.
• Inline previews of translated content.
• Quick actions to easily create and update translations.

For more details on how to use the extension, refer to the Intlayer VS Code Extension documentation.

Git Configuration

It is recommended to ignore the files generated by Intlayer. This allows you to avoid committing them to your Git repository.

VS Code Extension

To improve your development experience with Intlayer, you can install the official Intlayer VS Code Extension.

Install from the VS Code Marketplace

This extension provides:

• Autocompletion for translation keys.
• Real-time error detection for missing translations.
• Inline previews of translated content.
• Quick actions to easily create and update translations.

For more details on how to use the extension, refer to the Intlayer VS Code Extension documentation.

Git Configuration

It is recommended to ignore the files generated by Intlayer. This allows you to avoid committing them to your Git repository.

To do this, you can add the following instructions to your .gitignore file:

# Ignore the files generated by Intlayer.intlayer

Doc History

• 5.5.10 - 2025-06-29: Init history