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A translation and localization library for Node.js and the browser.
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A translation and localization library for Node.js and the browser. The project is inspired by Ruby's famous I18n gem.


  • translation and localization
  • interpolation of values to translations (sprintf-style with named arguments)
  • pluralization (CLDR compatible)


Install via npm:

% npm install counterpart


Require the counterpart module to get a reference to the translate function:

var translate = require('counterpart');

This function expects a key and options as arguments and translates, pluralizes and interpolates a given key using a given locale, scope, and fallback, as well as interpolation values.


Translation data is organized as a nested object using the top-level key as namespace. E.g., the damals package ships with the translation:

  damals: {
    about_x_hours_ago: {
      one:   'about one hour ago',
      other: 'about %(count)s hours ago'
    /* ... */

Translations can be looked up at any level of this object using the key argument and the scope option. E.g., in the following example, a whole translation object is returned:

translate('damals')  // => { about_x_hours_ago: { one: '...', other: '...' } }

The key argument can be either a single key, a dot-separated key or an array of keys or dot-separated keys. So these examples will all look up the same translation:

translate('')          // => 'about one hour ago'
translate(['damals', 'about_x_hours_ago', 'one'])  // => 'about one hour ago'
translate(['damals', ''])     // => 'about one hour ago'

The scope option can be either a single key, a dot-separated key or an array of keys or dot-separated keys. Keys and scopes can be combined freely. Again, these examples will all look up the same translation:

translate('', { scope: 'damals' })
translate('one', { scope: 'damals.about_x_hours_ago' })
translate('one', { scope: ['damals', 'about_x_hours_ago'] })

The separator option allows you to change what the key gets split via for nested objects. It also allows you to stop counterpart splitting keys if you have a flat object structure:

translate('', { separator: '*' })
// => 'missing translation: en*'

Since we changed what our key should be split by counterpart will be looking for the following object structure:

  '': 'about one hour ago'

The setSeparator function allows you to globally change the default separator used to split translation keys:

translate.setSeparator('*') // => '.' (returns the previous separator)

There is also a getSeparator function which returns the currently set default separator.


Translations can contain interpolation variables which will be replaced by values passed to the function as part of the options object, with the keys matching the interpolation variable names.

E.g., with a translation { foo: 'foo %(bar)s' } the option value for the key bar will be interpolated into the translation:

translate('foo', { bar: 'baz' }) // => 'foo baz'


Translation data can contain pluralized translations. Pluralized translations are provided as a sub-object to the translation key containing the keys one, other and optionally zero:

  x_items: {
    zero:  'No items.',
    one:   'One item.',
    other: '%(count)s items.'

Then use the count option to select a specific pluralization:

translate('x_items', { count: 0  })  // => 'No items.'
translate('x_items', { count: 1  })  // => 'One item.'
translate('x_items', { count: 42 })  // => '42 items.'

The name of the option to select a pluralization is always count, don't use itemsCount or anything like that.

Note that this library currently only supports an algorithm for English-like pluralization rules (see locales/en.js. You can easily add pluralization algorithms for other locales by adding custom translation data to the "counterpart" namespace. Pull requests are welcome.

As seen above, the count option can be used both for pluralization and interpolation.


If for a key no translation could be found, translate returns an error string of the form "translation missing: %(key)s".

To mitigate this, provide the fallback option with an alternate text. The following example returns the translation for "baz" or "default" if no translation was found:

translate('baz', { fallback: 'default' })

Dealing with missing translations

When a translation key cannot be resolved to a translation, regardless of whether a fallback is provided or not, translate will emit an event you can listen to:

translate.onTranslationNotFound(function(locale, key, fallback, scope) {
  // do important stuff here...

Use translate.offTranslationNotFound(myHandler) to stop listening to missing key events.

As stated in the Fallbacks section, if for a key no translation could be found, translate returns an error string of the form "translation missing: %(key)s".

You can customize this output by providing your own missing entry generator function:

translate.setMissingEntryGenerator(function(key) {
  // console.error('Missing translation: ' + key);
  return 'OMG! No translation available for ' + key;


The default locale is English ("en"). To change this, call the setLocale function:

translate.getLocale()     // => 'en'
translate.setLocale('de') // => 'en' (returns the previous locale)
translate.getLocale()     // => 'de'

Note that it is advised to call setLocale only once at the start of the application or when the user changes her language preference. A library author integrating the counterpart package in a library should not call setLocale at all and leave that to the developer incorporating the library.

In case of a locale change, the setLocale function emits an event you can listen to:

translate.onLocaleChange(function(newLocale, oldLocale) {
  // do important stuff here...
}, [callbackContext]);

Use translate.offLocaleChange(myHandler) to stop listening to locale changes.

You can temporarily change the current locale with the withLocale function:

translate.withLocale(otherLocale, myCallback, [myCallbackContext]);

withLocale does not emit the locale change event. The function returns the return value of the supplied callback argument.

Another way to temporarily change the current locale is by using the locale option on translate itself:

translate('foo', { locale: 'de' });

There are also withScope and withSeparator functions that behave exactly the same as withLocale.

Fallback Locales

You can provide entire fallback locales as alternatives to hard-coded fallbacks.

translate('baz', { fallbackLocale: 'en' });

Fallback locales can also contain multiple potential fallbacks. These will be tried sequentially until a key returns a successful value, or the fallback locales have been exhausted.

translate('baz', { fallbackLocale: [ 'foo', 'bar', 'default' ] })

Globally, fallback locales can be set via the setFallbackLocale method.

translate.setFallbackLocale([ 'bar', 'en' ]) // Can also take multiple fallback locales

Adding Translation Data

You can use the registerTranslations function to deep-merge data for a specific locale into the global translation object:

translate.registerTranslations('de', require('counterpart/locales/de'));
translate.registerTranslations('de', require('./locales/de.json'));

The data object to merge should contain a namespace (e.g. the name of your app/library) as top-level key. The namespace ensures that there are no merging conflicts between different projects. Example (./locales/de.json):

  "my_project": {
    "greeting": "Hallo, %(name)s!",
    "x_items": {
      "one":   "1 Stück",
      "other": "%(count)s Stücke"

The translations are instantly made available:

translate('greeting', { scope: 'my_project', name: 'Martin' })  // => 'Hallo, Martin!'

Note that library authors should preload their translations only for the default ("en") locale, since tools like webpack or browserify will recursively bundle up all the required modules of a library into a single file. This will include even unneeded translations and so unnecessarily bloat the bundle.

Instead, you as a library author should advise end-users to on-demand-load translations for other locales provided by your package:

// Execute this code to load the 'de' translations:
require('counterpart').registerTranslations('de', require('my_package/locales/de'));

Registering Default Interpolations

Since v0.11, Counterpart allows you to register default interpolations using the registerInterpolations function. Here is an example:

translate.registerTranslations('en', {
  my_namespace: {
    greeting: 'Welcome to %(app_name)s, %(visitor)s!'

translate.registerInterpolations({ app_name: 'My Cool App' });

translate('my_namespace.greeting', { visitor: 'Martin' })
// => 'Welcome to My Cool App, Martin!'

translate('my_namespace.greeting', { visitor: 'Martin', app_name: 'The Foo App' })
// => 'Welcome to The Foo App, Martin!'

As you can see in the last line of the example, interpolations you give as options to the translate function take precedence over registered interpolations.

Using a key transformer

Sometimes it is necessary to adjust the given translation key before the actual translation is made, e.g. when keys are passed in mixed case and you expect them to be all lower case. Use setKeyTransformer to provide your own transformation function:

translate.setKeyTransformer(function(key, options) {
  return key.toLowerCase();

Counterpart's built-in key transformer just returns the given key argument.


The counterpart package comes with support for localizing JavaScript Date objects. The localize function expects a date and options as arguments. The following example demonstrates the possible options.

var date = new Date('Fri Feb 21 2014 13:46:24 GMT+0100 (CET)');

translate.localize(date)                       // => 'Fri, 21 Feb 2014 13:46'
translate.localize(date, { format: 'short' })  // => '21 Feb 13:46'
translate.localize(date, { format: 'long' })   // => 'Friday, February 21st, 2014 13:46:24 +01:00'

translate.localize(date, { type: 'date' })                  // => 'Fri, 21 Feb 2014'
translate.localize(date, { type: 'date', format: 'short' }) // => 'Feb 21'
translate.localize(date, { type: 'date', format: 'long' })  // => 'Friday, February 21st, 2014'

translate.localize(date, { type: 'time' })                  // => '13:46'
translate.localize(date, { type: 'time', format: 'short' }) // => '13:46'
translate.localize(date, { type: 'time', format: 'long' })  // => '13:46:24 +01:00'

translate.registerTranslations('de', require('counterpart/locales/de'));
translate.localize(date, { locale: 'de' })  // => 'Fr, 21. Feb 2014, 13:46 Uhr'

Sure, you can integrate custom localizations by adding to or overwriting the "counterpart" namespace. See locales/en.js and locales/de.js for example localization files.

As an instance

You can invoke an instance of Counterpart should you need various locales displayed at once in your application:

var Counterpart = require('counterpart').Instance;

var instance = new Counterpart();

instance.registerTranslations('en', { foo: 'bar' });

Error handling

When a translation fails, translate will emit an event you can listen to:

translate.onError(function(err, entry, values) {
  // do some error handling here...

Use translate.offError(myHandler) to stop listening for errors.


Here's a quick guide:

  1. Fork the repo and make install.
  2. Run the tests. We only take pull requests with passing tests, and it's great to know that you have a clean slate: make test.
  3. Add a test for your change. Only refactoring and documentation changes require no new tests. If you are adding functionality or are fixing a bug, we need a test!
  4. Make the test pass.
  5. Push to your fork and submit a pull request.


Released under The MIT License.

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