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1.0.10 • Public • Published

Preferred Locale

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🎌 Get the users' most preferred locale/language from your app's available translations with zero dependencies


  • Uses the Intl.Locale API (backwards compatible)
  • Works on node & browsers
  • Zero dependencies

Guaranteed Node / Browser Support

Package Package
Node Chrome Firefox Safari Edge
preferred-locale ~600 bytes 10+ 69+ 68+ 12+ 18+


Many web applications that automatically detect the browser language and serve the relevent translation are fundamentally broken.

A browser that signals the user prefers the following locales (index 0 being most preferred) should never return content in Japanese (ja-JP) if the application has translations for Japanese and American English (en-US):

  • [ 'en-GB', 'en', 'ja-JP', 'en-US', 'ja' ]

Instead, many applications (e.g Epic Games' store, help and documentation) will instead serve their users content in Japanese as they do not provide translations for British English, only American English and only check for exact matches.

preferred-locale fixes this by traversing the supported node/browser languages in order of priority:

  1. If an exact match is found it uses that (e.g en-GB is translated).
  2. If the node/browser language is supported but the region is not (e.g Australian English), the canonical region is looked up and tested against (e.g en-AU becomes en-US),
  3. If only a language is provided (e.g en), the canonical region is looked up and tested against (e.g en becomes en-US)
  4. If no node/browser locale resolves to a translated locale, the fallback locale is returned

Live Demo

A step-by-step demonstration of how preferred-locale works with your own browser locales is available at eehz9.csb.app.

Example Step-By-Step

Application has translations for en-US and ja-JP

  1. Raw browser locales [ 'en-GB', 'en', 'ja-JP', 'en-US', 'ja' ]

  2. Unify the browser locales [ 'en-GB', 'en-US', 'ja-JP', 'en-US', 'ja-JP' ]

  3. Deduplicate the locales [ 'en-GB', 'en-US', 'ja-JP' ]

  4. Remove locales not translated [ 'en-US', 'ja-JP' ]

  5. User gets content in en-US


Yarn / NPM

yarn add preferred-locale
npm install preferred-locale
import preferredLocale from 'preferred-locale'      // ES Modules and Babel
const preferredLocale = require('preferred-locale') // CommonJS and Browserify
const translatedLocales = [ 'en-us', 'en-gb', 'fr-fr' ]
const fallback = 'en-us'
preferredLocale(translatedLocales, fallback, { lowerCaseRegion: true })
// Returns 'en-gb' if browser/node user language is [ 'en-gb', 'nl-nl' ]


<!-- jsDelivr -->
<script src='https://cdn.jsdelivr.net/npm/preferred-locale'></script>
<!-- unpkg -->
<script src='https://unpkg.com/preferred-locale'></script>
const translatedLocales = [ 'en-US', 'en-GB', 'fr-FR' ]
const fallback = 'en-US'
preferredLocale(translatedLocales, fallback)
// Returns 'en-GB' if browser/node user language is [ 'en-GB', 'fr-FR' ]


Table of Contents



Get the users' most preferred locale that is translated by your application.


  • translatedLocales Array<string> Translations provided by the application
  • fallback string Source locale of the application. Used when no browser locale matches translatedLocales
  • options Object Configuration options (optional, default {})
    • options.regionLowerCase boolean If true, returns en-us instead of en-US (optional, default false)
    • options.languageOnly boolean If true, returns en instead of en-US or en-us (optional, default false)

Returns string Preferred locale


npm i preferred-locale

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