Passive localization JavaScript library
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l10n.js is a JavaScript library that enables passive localization through native JavaScript methods, gracefully degrading if the library is not present. You can make Ajax applications, JavaScript libraries, etc. that can be localized but not require l10n.js to function. There is already a placeholder method for all API calls as specified in the ECMAScript specification and is present in all JavaScript engines, so when l10n.js isn't present, your application works fine.


This repository includes a simple demo that you can try out on your webserver.

If you know a language that isn't currently supported in the demo, I encourage you to contribute a localization by sending me your own localizations, either through GitHub or directly. The following strings would need to be localized:

  • %title to {Locale} - l10n.js demo in the locale.
  • %info to You are viewing a {locale} localization of this page. in the locale.
  • Optionally, %locale.dir to rtl if the locale uses right-to-left directionality.

Supported Browsers

  • Internet Explorer 5+
  • Firefox 2+
  • Opera 9+
  • Google Chrome 1+
  • Safari 4+

Getting Started

  1. Download l10n.js.
  2. Localize strings used in your JavaScript application. See the demo localizations file for an example localizations file. You can also specify external localizations in your main localizations file by assigning a URL string to a language code, such as "en-us": "localizations/en-us.json".
  3. Include the appropriate link elements, as described in the usage section, anywhere in your document. I recommend putting it in the document's <head>.
  4. Place <script type="text/javascript" src="path/to/l10n.js"></script> anywhere after the <link> tag.
  5. Call toLocaleString() on any strings you wish to localize.


Localizing strings

Calling toLocaleString() on every localizable string can create a lot of extra typing and bloat for sending your JavaScript down the wire. I recommend using the following helper function to localize strings. The reason I don't define this in l10n.js is to not introduce any new globals, which keeps l10n.js a one of the JavaScript libraries least-prone to conflicts with other libraries.

var l = function (string) {
    return string.toLocaleString();

With this helper function, you can start writing l("Your localizable string") instead of "Your localizable string".toLocaleString(). I chose l instead of _ (an underscore), because it's easier to spot so you can quickly skim your code to see which strings are localizable.

Variable replacement

If you don't mind requiring l10n.js for your JavaScript application or library to function, I suggest using short variable strings instead of default strings. It saves bandwidth by decreasing the size of localization files, and it enables you to write nice, short code as such in the following.

  • document.title = l("")
    • Example results: "Search - Acme, Inc."
  • confirm(l("%confirm.deleteAccount"))
    • Example results: "Are you sure you want to delete your account?"
  • link.href = "" + l("%locale.tld")
    • Example results: ""

Often, string concatenation is used instead of replacement in JavaScript. With l10n.js, to make localization easier, you may have to use replacements instead. You might want to use a JavaScript library that implements something similar to C++'s sprintf(). A nice JavaScript implementation I'd recommend is php.js's sprintf().

When localizations are downloaded

If you are using single localization URLs (<link rel="localization" hreflang="..." href="..." type="application/vnd.oftn.l10n+json"/>), they will only be downloaded when needed. If you are using multiple localizations in one (<link rel="localizations" href="..." type="application/vnd.oftn.l10n+json"/>), then the file will be downloaded right away, but externally linked localizations in the localization file will not be. If you provide an interface for your users to change locales, any non-loaded localization files will be loaded when necessary.

Including localizations with link elements

Multiple localizations can be included with one localization JSON file, with all of the top properties being language codes. Instead of putting all of the localized strings directly in the file, you may want to assign a specifc localization JSON URL to each locale, as to save bandwidth by only downloading locales the user needs.

The following is an example localization file for <link rel="localizations" href="path/to/localizations.json" type="application/vnd.oftn.l10n+json"/>.

  "en-US": {
      "What is your favourite colour?": "What is your favorite color?"
  "fr": "path/to/french-localization.json"

Using localization files is the same as calling String.toLocaleString() witht the JSON localizations object as the first parameter.

You can also include single localizations by specifying the standard HTML5 hreflang link element attribute and using a rel of localization instead of localizations with an 's', as shown in the following.

<link rel="localization" hreflang="en-US" href="american-english.json" type="application/vnd.oftn.l10n+json"/>

The JSON file for the localization might look like the following.

    "What is your favourite colour?": "What is your favorite color?"


Strong and emphasized text has titles (which can be viewed by hovering your cursor over them) containing their type if they are not functions or return type if they are.


If localizations is an object, it is added to the localizations.
If localizations is a string, it is requested as JSON and then added to the localizations.
If localizations is false, then all localizations are reset.
If localizations is an object, and a locale is false, then all localizations for that locale are reset.

The string representation of the String contructor is returned, to maintain backwards compatibility with any code using this method to actually get it.

    Loading a localizations JSON file:
    <pre><code>String.toLocaleString(<strong title="String">"path/to/localizations.json"</strong>)</code></pre>
    Defining localizations directly:
      The nearest locale to the user's locale that has the string being localized is
      used in localization.
"es": { // Spanish
    "Hello, world!": "¡Hola, mundo!"
    // more localizations...
"en-US": { // American English
    "Hello, world!": "Hello, America!" // Locale-specific message
    // more localizations...
"en-GB": false, // resetting British English localizations
// Specifying external localization JSON for Japanese:
// The URL isn't requested unless the user's locale is Japanese
"ja": "localizations/ja.json"


  • Resetting all localizations:
  • aString.toLocaleString()
    Returns the localized version of aString in the user's locale, if available. Otherwise, it returns the same string.


    A configurable string which represents the language code of the locale to use for localization. It defaults to the user's own locale.
    A configurable string which represents the language code of the default locale to use for localization if no localizations are available in the user's locale. By default this is not configured, and may be ignored if you are using l10n.js for passive-only localizations.

    Default locale

    The "" (empty string) locale is the default locale, where you can specify default or fallback strings.