An i18n library that allows integration with express and jade

express, jade, i18n, language, translation
npm install @cheevr/lang@1.0.11



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This module is deigned to make multiple language versions of text strings available in an easy to maintain way. Language files for multiple versions are kept in various formats on disk and can be selected by setting the right locale. The library includes helpers to mak eit easy to use with systems such as express.


Use the standard installation method:

node i @cheevr/lang

Simple Example

There are multiple ways you can use the library the simplest example does not require any servers to be used.

To get started create a file named lang/en-US.json:

    "Greeting": "Hello",
    "Person": "World"

To make use of your language strings you simple:

const lang = require('@cheevr/lang');
const example = '{Greeting}, {Person}!';

console.log(lang.process(example, 'en-US');
// Prints "Hello, World!"

You can now create additional file matching the locale that you want to support. The library supports both .json and .js files as your source.

Express Example

There are multiple functions written specifically for express and handling http requests. Assuming You've set up the same language files as in the simple example:

const lang = require('@cheevr/lang');
const express = require('express');

const app = express();

The library will now look for headers and parameters that specify the language. The header values can be overridden and the parameter name is configurable. By default the library will look for a parameter named lang in GET, query, session, cookie and POST data (in that order) for overrides of the header values.

Once the language has been parsed all subsequent method calls will default to using that locale and the request object will have a local property you can use to see the detected language.

You can now go ahead and process files with that information:

app.get('/test', (req, res) => res.send(lang.process(example, req.locale)));

Assuming you want to render a jade template this is the setup:

// Use the dictionary while rendering a jade template
app.get('/jade', (req, res) => res.render('index', { dict: lang.dictionary }) );

A jade file could now look like this and should fill in the right translations based on the request parameters:

        div #{dict.Greeting}, #{dict.Person}!

Caution is advised though if you use asynchronous operations between the middleware call and accessing the dictionary since other requests might have changed the locale in between. If you experience issues you can force the right language like this:

app.get('/jade', (req, res) => {
    lang.locale = req.locale;
    res.render('index', { dict: lang.dictionary })


This is a list of all the methods and properties available through the library. Note that some functionality is accessible through a file configuration using @cheevr/config.


This method will return a middleware handler that will look for language information on the incoming request. The standard header called accept-language is what the default source is but the middleware allows to override the detecting language by providing an extra (configurable) lang parameter. The parameter is looked for in GET, query, session, cookie and POST data in that order.

The language format follows standard locale definitions and supports both short form (en) and long form (en-US), as well as language priorities (en-US;q=0.8 or en;q=0.7).


Once a locale has been detected the identified matching result can be accessed on all subsequent request objects.

Lang.process({string} source, {string} locale, {string} [id], {bool} [force])

Allows to process any string with the given locale and return the processed string with any translations found. This method also supports caching results via the id parameter. If an id was passed in you can still force circumventing the in memory cache by setting the last parameter to true.

Lang.errorHandler({ClientRequest} req, {ServerResponse} res, {Error} e, {function} next)

If the middleware causes any issues this error handler is called, which by default responds with an error to the client request. If you want to handle errors yourself you can overwrite this method with a custom implementation.

Lang.locale {string}

Allows to force a locale before accessing properties on the lang object. Note that when using the middleware, this value will be set to whatever the incoming request specified. If you want to force a locale for a request set this value after the middleware has processed the request, or omit the middleware completely.

Lang.dictionary {Map<string,*>}

Readonly property that returns the dictionary with for the current locale. The locale can be set via middleware or the Lang.locale property.


Will trigger a reload of tha language files and read them from disk again.

Lang.extend({string} dir, {string} ...paths)

Allows to add a directory from which to load language files from. This is particularly useful if you're writing a module to be used in other projects, where there will be different lang directories configured than what your library expects. With this method you can access your modules directories without interfering with the root project. To make life easier the method support passing in multiple path components that will be joined automatically.

Relative paths will be resolved to either the current working directory of the process or to whatever the NODE_CWD environment variable is set to.


The lang library makes use of @cheevr/config to define its behaviour. To change the settings create a lang section in your configuration files. An example would be in config/default.json:

    "lang": {

Here's a list of options and what they do:

paramName {string} = "lang"

This option allows you to configure the parameter the middleware will look for when overriding the locale detected via the accept-language header

default {string} = "en-US"

When no language can be detected or a requested language is not available the library will fall back to a default language. If you want to use something different than en-US you can specify that here.

paths {string[]} = ["lang"]

Language files are by default placed in the lang folder of the root directory of the project Should you want to specify one or more different directories you can do it with this option. Relative paths are always in reference to cwd, but absolute paths are also supported if you want to force a certain directory.

localeDefaults {Map<string,string>} = { en: 'en-US', de: 'de-DE', ... }

Since locales consist of 2 components and both aren't always available there need to be a mapping from simple to full locale specification. This map will tell the library how to map the short forms to actual languages. Note that right now now no all variations have been mapped so you might want to set your own mapping if it hasn't been included here yet.

Future Features for Consideration

  • Support for YAML, CSV, XML, ...
  • Configurable placeholders (instead of {})
  • Make the interface for forcing a dictionary language more intuitive potentially adding a reference to the dictionary on the req object