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Lightweight simple translation module for node.js / express.js with dynamic json storage. Uses common __('...') syntax in app and templates.
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Latest commit 6c32145 @mashpie Merge pull request #179 from travnels/sprintf-documentation-and-examples
Sprintf documentation and examples


Lightweight simple translation module with dynamic json storage. Supports plain vanilla node.js apps and should work with any framework (like express, restify and probably more) that exposes an app.use() method passing in res and req objects. Uses common __('...') syntax in app and templates. Stores language files in json files compatible to webtranslateit json format. Adds new strings on-the-fly when first used in your app. No extra parsing needed.

Build Status NPM version Dependency Status


npm install i18n


npm test


// load modules
var express = require('express'),
    i18n = require("i18n");

now you are ready to use a global i18n.__('Hello'). Global assumes you share a common state of localization in any time and any part of your app. This is usually fine in cli-style scripts. When serving responses to http requests you'll need to make sure that scope is NOT shared globally but attached to your request object.


Minimal example, just setup two locales and a project specific directory

    locales:['en', 'de'],
    directory: __dirname + '/locales'

list of configuration options

    // setup some locales - other locales default to en silently
    locales:['en', 'de'],

    // fall back from Dutch to German
    fallbacks:{'nl': 'de'},

    // you may alter a site wide default locale
    defaultLocale: 'de',

    // sets a custom cookie name to parse locale settings from  - defaults to NULL
    cookie: 'yourcookiename',

    // where to store json files - defaults to './locales' relative to modules directory
    directory: './mylocales',

    // whether to write new locale information to disk - defaults to true
    updateFiles: false,

    // what to use as the indentation unit - defaults to "\t"
    indent: "\t",

    // setting extension of json files - defaults to '.json' (you might want to set this to '.js' according to webtranslateit)
    extension: '.js',

    // setting prefix of json files name - default to none '' (in case you use different locale files naming scheme (webapp-en.json), rather then just en.json)
    prefix: 'webapp-',

    // enable object notation
    objectNotation: false

    // setting of log level DEBUG - default to require('debug')('i18n:debug')
    logDebugFn: function (msg) {
        console.log('debug', msg);

    // setting of log level WARN - default to require('debug')('i18n:warn')
    logWarnFn: function (msg) {
        console.log('warn', msg);

    // setting of log level ERROR - default to require('debug')('i18n:error')
    logErrorFn: function (msg) {
        console.log('error', msg);

Example usage in global scope

In your app, when not registered to a specific object:

var greeting = i18n.__('Hello');

Example usage in express.js

In an express app, you might use i18n.init to gather language settings of your visitors and also bind your helpers to response object honoring request objects locale, ie:

// Configuration
app.configure(function() {


    // default: using 'accept-language' header to guess language settings


in your apps methods:

app.get('/de', function(req, res){
  var greeting = res.__('Hello');

in your templates (depending on your template engine)

<%= __('Hello') %>


For serving the same static files with different language url, you could:

app.use(express.static(__dirname + '/www'));
app.use('/en', express.static(__dirname + '/www'));
app.use('/de', express.static(__dirname + '/www'));

Examples for common setups

See tested examples inside /examples or browse these gists:



Translates a single phrase and adds it to locales if unknown. Returns translated parsed and substituted string.

// template and global (this.locale == 'de')
__('Hello'); // Hallo
__('Hello %s', 'Marcus'); // Hallo Marcus
__('Hello {{name}}', { name: 'Marcus' }); // Hallo Marcus

// scoped via req object (req.locale == 'de')
req.__('Hello'); // Hallo
req.__('Hello %s', 'Marcus'); // Hallo Marcus
req.__('Hello {{name}}', { name: 'Marcus' }); // Hallo Marcus

// passing specific locale
__({phrase: 'Hello', locale: 'fr'}); // Salut
__({phrase: 'Hello %s', locale: 'fr'}, 'Marcus'); // Salut Marcus
__({phrase: 'Hello {{name}}, locale: 'fr'}, { name: 'Marcus' }); // Salut Marcus


Plurals translation of a single phrase. Singular and plural forms will get added to locales if unknown. Returns translated parsed and substituted string based on count parameter.

// template and global (this.locale == 'de')
__n("%s cat", "%s cats", 1); // 1 Katze
__n("%s cat", "%s cats", 3); // 3 Katzen

// scoped via req object (req.locale == 'de')
req.__n("%s cat", "%s cats", 1); // 1 Katze
req.__n("%s cat", "%s cats", 3); // 3 Katzen

// passing specific locale
__n({singular: "%s cat", plural: "%s cats", locale: "fr"}, 1); // 1 chat
__n({singular: "%s cat", plural: "%s cats", locale: "fr"}, 3); // 3 chat

__n({singular: "%s cat", plural: "%s cats", locale: "fr", count: 1}); // 1 chat
__n({singular: "%s cat", plural: "%s cats", locale: "fr", count: 3}); // 3 chat


Setting the current locale (ie.: en) globally or in current scope.

setLocale(req, 'de');

To change the initial locale (when you set it on i18n.init()) for all the user session (eg.: you have a language selector on your web page to let the user select the preferred language), you have some options. You could set it via res.setLocale('de') on each loop before load the each page. Or you could manage it via any session middleware or by setting a cookie in the client and let i18n read it's value.

In the last case you will need to enable cookies (eg. for express will be app.use(express.cookieParser())) and then you can use the i18n.configure.cookie to let i18n which language must use. Simply use the same cookie name when setting it in the user preferred language, like here:

res.cookie('yourcookiename', 'de', { maxAge: 900000, httpOnly: true });

After this and until the cookie expires, i18n will get the value of the cookie and will set that language instead of default for every page.


Getting the current locale (ie.: en) from current scope or globally.

getLocale(); // --> de
getLocale(req); // --> de
req.getLocale(); // --> de


Returns a whole catalog optionally based on current scope and locale.

getCatalog(); // returns all locales
getCatalog('de'); // returns just 'de'

getCatalog(req); // returns all locales
getCatalog(req, 'de'); // returns just 'de'

req.getCatalog(); // returns all locales
req.getCatalog('de'); // returns just 'de'

Optionally manual attaching helpers for different template engines

In general i18n has to be attached to the response object to let it's public api get accessible in your templates and methods. As of 0.4.0 i18n tries to do so internally via i18n.init, as if you were doing it in app.configure on your own:

app.use(function(req, res, next) {
    // express helper for natively supported engines
    res.locals.__ = res.__ = function() {
        return i18n.__.apply(req, arguments);



Different engines need different implementations, so yours might miss or not work with the current default helpers. This one showing an example for mustache in express:

// register helper as a locals function wrapped as mustache expects
app.use(function (req, res, next) {
    // mustache helper
    res.locals.__ = function () {
      return function (text, render) {
        return i18n.__.apply(req, arguments);



You could still setup your own implementation. Please refer to Examples below, post an issue or contribute your setup.

Output parsing of expressions

As inspired by gettext there is currently support for sprintf-style expressions. You can also use mustache syntax for named parameters.

sprintf support

var greeting = __('Hello %s, how are you today?', 'Marcus');

this puts Hello Marcus, how are you today?. You might add endless arguments and even nest it.

var greeting = __('Hello %s, how are you today? How was your %s.', 'Marcus', __('weekend'));

which puts Hello Marcus, how are you today? How was your weekend.

You might need to have repeated references to the same argument, which can be done with sprintf.

var example = __('%1$s, %1$s, %1$s', 'repeat');

which puts

repeat, repeat, repeat

In some cases the argument order will need to be switched for different locales. The arguments can be strings, floats, numbers, etc.

var example = __('%2$d then %1$s then %3$.2f', 'First', 2, 333.333);

which puts

2 then First then 333.33

mustache support

You may also use mustache syntax for your message strings. To pass named parameters to your message, just provide an object as the second parameter. You can still pass unnamed parameters by adding additional arguments.

var greeting = __('Hello {{name}}, how are you today?', { name: 'Marcus' });

this puts Hello Marcus, how are you today?. You might also combine it with sprintf arguments and also nest it.

var greeting = __('Hello {{name}}, how was your %s.', { name: 'Marcus' }, __('weekend'));

which puts Hello Marcus, how was your weekend.

variable support

you might even use dynamic variables as they get interpreted on the fly. Better make sure no user input finds it's way to that point as they all get added to the en.js file if not yet existing.

var greetings = ['Hi', 'Hello', 'Howdy'];
for (var i=0; i < greetings.length; i++) {
    console.log( __(greetings[i]) );

which puts


basic plural support

two different plural forms are supported as response to count:

var singular = __n('%s cat', '%s cats', 1);
var plural = __n('%s cat', '%s cats', 3);

this puts 1 cat or 3 cats and again these could get nested:

var singular = __n('There is one monkey in the %%s', 'There are %d monkeys in the %%s', 1, 'tree');
var plural = __n('There is one monkey in the %%s', 'There are %d monkeys in the %%s', 3, 'tree');

putting There is one monkey in the tree or There are 3 monkeys in the tree


Will get modular support for different storage engines, currently just json files are stored in filesystem.

json file

the above will automatically generate a en.json by default inside ./locales/ which looks like

    "Hello": "Hello",
    "Hello %s, how are you today?": "Hello %s, how are you today?",
    "weekend": "weekend",
    "Hello %s, how are you today? How was your %s.": "Hello %s, how are you today? How was your %s.",
    "Hi": "Hi",
    "Howdy": "Howdy",
    "%s cat": {
        "one": "%s cat",
        "other": "%s cats"
    "There is one monkey in the %%s": {
        "one": "There is one monkey in the %%s",
        "other": "There are %d monkeys in the %%s"
    "tree": "tree"

that file can be edited or just uploaded to webtranslateit for any kind of collaborative translation workflow:

    "Hello": "Hallo",
    "Hello %s, how are you today?": "Hallo %s, wie geht es dir heute?",
    "weekend": "Wochenende",
    "Hello %s, how are you today? How was your %s.": "Hallo %s, wie geht es dir heute? Wie war dein %s.",
    "Hi": "Hi",
    "Howdy": "Hallöchen",
    "%s cat": {
        "one": "%s Katze",
        "other": "%s Katzen"
    "There is one monkey in the %%s": {
        "one": "Im %%s sitzt ein Affe",
        "other": "Im %%s sitzen %d Affen"
    "tree": "Baum"

Logging & Debugging

Logging any kind of output is moved to debug module. To let i18n output anything run your app with DEBUG env set like so:

$ DEBUG=i18n:* node app.js

i18n exposes three log-levels:

  • i18n:debug
  • i18n:warn
  • i18n:error

if you only want to get errors and warnings reported start your node server like so:

$ DEBUG=i18n:warn,i18n:error node app.js

Combine those settings with you existing application if any of you other modules or libs also uses debug

Using custom logger

You can configure i18n to use a custom logger. For example attach some simple console-logging:



    // setting of log level DEBUG - default to require('debug')('i18n:debug')
    logDebugFn: function (msg) {
        console.log('debug', msg);

    // setting of log level WARN - default to require('debug')('i18n:warn')
    logWarnFn: function (msg) {
        console.log('warn', msg);

    // setting of log level ERROR - default to require('debug')('i18n:error')
    logErrorFn: function (msg) {
        console.log('error', msg);

Object notation

In addition to the traditional, linear translation lists, i18n also supports hierarchical translation catalogs.

To enable this feature, be sure to set objectNotation to true in your configure() call. Note: If you can't or don't want to use . as a delimiter, set objectNotation to any other delimiter you like.

Instead of calling __("Hello") you might call __("greeting.formal") to retrieve a formal greeting from a translation document like this one:

"greeting": {
    "formal": "Hello",
    "informal": "Hi",
    "placeholder": {
        "formal": "Hello %s",
        "informal": "Hi %s"

In the document, the translation terms, which include placeholders, are nested inside the "greeting" translation. They can be accessed and used in the same way, like so __('greeting.placeholder.informal', 'Marcus').


Object notation also supports pluralization. When making use of it, the "one" and "other" entries are used implicitly for an object in the translation document. For example, consider the following document:

"cat": {
    "one": "Katze",
    "other": "Katzen"

When accessing these, you would use __n("cat", "cat", 3) to tell i18n to use both the singular and plural form of the "cat" entry. Naturally, you could also access these members explicitly with __("") and __("cat.other").


When starting a project from scratch, your translation documents will probably be empty. i18n takes care of filling your translation documents for you. Whenever you use an unknown object, it is added to the translation documents.

By default, when using object notation, the provided string literal will be inserted and returned as the default string. As an example, this is what the "greeting" object shown earlier would look like by default:

"greeting": {
    "formal": "greeting.formal",
    "informal": "greeting.informal"

In case you would prefer to have a default string automatically inserted and returned, you can provide that default string by appending it to your object literal, delimited by a :. For example:

__("greeting.placeholder.informal:Hi %s")


  • 0.5.0: feature release; added {{mustache}} parsing by #85, added "object.notation" by #110, fixed buggy req.__() implementation by #111 and closed 13 issues
  • 0.4.1: stable release; merged/closed: #57, #60, #67 typo fixes; added more examples and new features: #53, #65, #66 - and some more api reference
  • 0.4.0: stable release; closed: #22, #24, #4, #10, #54; added examples, clarified concurrency usage in different template engines, added i18n.getCatalog
  • 0.3.9: express.js usage, named api, jscoverage + more test, refactored configure, closed: #51, #20, #16, #49
  • 0.3.8: fixed: #44, #49; merged: #47, #45, #50; added: #33; updated: README
  • 0.3.7: tests by mocha.js, added this.locale to __ and __n
  • 0.3.6: travisCI, writeFileSync, devDependencies, jslint, MIT, fixed: #29, #9, merged: #25, #30, #43
  • 0.3.5: fixed some issues, prepared refactoring, prepared publishing to npm finally
  • 0.3.4: merged pull request #13 from Fuitad/master and updated README
  • 0.3.3: merged pull request from codders/master and modified for backward compatibility. Usage and tests pending
  • 0.3.2: merged pull request #7 from carlptr/master and added tests, modified fswrite to do sync writes
  • 0.3.0: added configure and init with express support (calling guessLanguage() via 'accept-language')
  • 0.2.0: added plurals
  • 0.1.0: added tests
  • 0.0.1: start

Licensed under MIT

Copyright (c) 2011-2014 Marcus Spiegel

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.


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