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scala-java-locales is a clean-room BSD-licensed implementation of the java.util.Locale API and related classes as defined on JDK8, mostly for Scala.js usage. It enables the locale API in Scala.js projects and supports usage requiring locales like number and dates formatting.


Simply add the following line to your sbt settings:

libraryDependencies += "io.github.cquiroz" %%% "scala-java-locales" % "1.2.0"

If you have a crossProject, the setting must be used only in the JS part:

lazy val myCross = crossProject.
    libraryDependencies += "io.github.cquiroz" %%% "scala-java-locales" % "1.2.0"

Requirement: you must use a host JDK8 to build your project, i.e., to launch sbt. scala-java-locales does not work on earlier JDKs.

Work in Progress / linking errors

This library is a work in progress and there are some unimplemented methods. If you use any of those on your Scala.js code, you will get linking errors.


The API follows the Java API for Locales, any major difference should be considered a bug.

The JVM includes a large locales database derived from CLDR. That includes things like date formats, region names, etc. Having the full db on js is possible but expensive in terms of space and for most applications only a few locales are needed, thus it is simpler to have a subset of them using some of the provided locale dbs or even better via sbt-locales sbt-locales lets you build a custom db with the minimal amount you need. There is a slight size benefit and a larger speed improvement doing so as scala.js has less code to optimize

For the common cases that you just need date formatting in angling you can just include

libraryDependencies += "io.github.cquiroz" %%% "locales-minimal-en-db" % "1.2.0"

Default Locale

Starting on 0.6.0 it is no longer necessary to register locales but only a minimal locale based on English is provided. You may want to use sbt-locales to generate a custom locale database.

For example see: gemini-locales

It is highly recommended to set your default Locale at the start of your application


The Java API requires a default Locale though it doesn't mandate a specific one, instead, the runtime should select it depending on the platform.

While the Java Locales use the OS default locale, on Scala.js platforms like browsers or node.js, it is harder to identify the default locale . scala-java-locales will try to guess the locale but if it can't or it is not not the locales db it sets en (English) as the default locale. This is a design decision to support the many API calls that require a default locale. It seems that Scala.js de facto uses en for number formatting.


java.util.Locale is a relatively simple class and by itself it doesn't provide too much functionality. The key for its usefulness is on providing data about the locale especially in terms of classes like java.text.DecimalFormatSymbols, java.text.DateFormatSymbols, etc. The Unicode CLDR project is a large repository of locale data that can be used to build the supporting classes, e.g. to get the DecimalFormatSymbols for a given locale.

Starting on Java 8, CLDR is also used by the JVM, for comparisons the java flag -Djava.locale.providers=CLDR should be set.

Note: Java 8 ships with an older CLDR version, specifically version 21. scala-java-locales uses the latest available version, hence there are some differences between the results and there are new available locales in scala-java-locales.


Locales and the CLDR specifications are vast subjects. The locales in this project are as good as the data and the interpretation of the specification is. While the data and implementation has been tested as much as possible, it is possible and likely that there are errors. Please post an issue or submit a PR if you find such errors.

In general the API attempts to behave be as close as possible to what happens on the JVM, e.g. the numeric system in Java seems to default to latn unless explicitly requested on the locale name.


A very simple Scala.js project is available at demo


scala-java-locales explicitly doesn't have any dependencies.



Copyright © 2020 Carlos Quiroz

scala-java-locales is distributed under the BSD 3-Clause license.