TypeScript icon, indicating that this package has built-in type declarations

2.1.0 • Public • Published


Build Coverage Downloads Size

Parse and stringify BCP 47 language tags.


What is this?

This is a package that can parse BCP 47 language tags to an object representing them, and serialize those objects back into language tags. It supports a forgiving mode to handle incorrect BCP 47 tags and can emit warnings about problems in incorrect tags.

When should I use this?

You can use this package if you need to access the data stored in BCP 47 language tags. You can also use this package if you want to check (lint) or manipulate tags.


This package is ESM only. In Node.js (version 12.20+, 14.14+, or 16.0+), install with npm:

npm install bcp-47

In Deno with Skypack:

import * as bcp47 from 'https://cdn.skypack.dev/bcp-47@2?dts'

In browsers with Skypack:

<script type="module">
  import * as bcp47 from 'https://cdn.skypack.dev/bcp-47@2?min'


import {parse, stringify} from 'bcp-47'

const schema = parse('hy-Latn-IT-arevela')



{ language: 'hy',
  extendedLanguageSubtags: [],
  script: 'Latn',
  region: 'IT',
  variants: ['arevela'],
  extensions: [],
  privateuse: [],
  irregular: null,
  regular: null }


This package exports the following identifiers: parse and stringify. There is no default export.

parse(tag[, options])

Parse a BCP 47 tag into a language schema. Note that the algorithm is case insensitive.


Whether to normalize legacy tags when possible (boolean, default: true). For example, i-klingon does not match the BCP 47 language algorithm but is considered valid by BCP 47 nonetheless. It is suggested to use tlh instead (the ISO 639-3 code for Klingon). When normalize is true, passing i-klingon or other deprecated tags, is handled as if their suggested valid tag was given instead.


By default, when an error is encountered, an empty object is returned. When in forgiving mode, all found values up to the point of the error are included (boolean, default: false). So, for example, where by default en-GB-abcdefghi an empty object is returned (as the language variant is too long), in forgiving mode the language of schema is populated with en and the region is populated with GB.


When given, warning is called when an error is encountered (Function).


Parsed BCP 47 language tag (Schema).


When tag is null or undefined.


Compile a schema to a BCP 47 language tag.


BCP 47 language tag (string).


A schema represents a language tag. A schema is deemed empty when it has neither language, irregular, regular, nor privateuse (where an empty privateuse array is handled as no privateuse as well).


Two or three character ISO 639 language code, four character reserved language code, or 5 to 8 (inclusive) characters registered language subtag (string). For example, en (English) or cmn (Mandarin Chinese).


Selected three-character ISO 639 codes(Array<string>), such as yue in zh-yue-HK (Chinese, Cantonese, as used in Hong Kong SAR).


Four character ISO 15924 script code (string), such as Latn in hy-Latn-IT-arevela (Eastern Armenian written in Latin script, as used in Italy).


Two alphabetical character ISO 3166-1 code or three digit UN M49 code (string). For example, CN in cmn-Hans-CN (Mandarin Chinese, Simplified script, as used in China) or 419 in es-419 (Spanish as used in Latin America and the Caribbean).


5 to 8 (inclusive) character language variants (Array<string>), such as rozaj and biske in sl-rozaj-biske (San Giorgio dialect of Resian dialect of Slovenian).


List of extensions (Array<Object>), each an object containing a one character singleton, and a list of extensions (string). singleton cannot be x (case insensitive) and extensions must be between two and eight (inclusive) characters. For example, an extension would be u-co-phonebk in de-DE-u-co-phonebk (German, as used in Germany, using German phonebook sort order), where u is the singleton and co and phonebk are its extensions.


List of private-use subtags (Array<string>), where each subtag must be between one and eight (inclusive) characters.


One of the regular tags (string): tags that are seen as something different by the algorithm. Valid values are:

  • art-lojban
  • cel-gaulish
  • no-bok
  • no-nyn
  • zh-guoyu
  • zh-hakka
  • zh-min
  • zh-min-nan
  • zh-xiang

One of the irregular tags (string): tags that are seen as invalid by the algorithm). Valid values are:

  • en-GB-oed
  • i-ami
  • i-bnn
  • i-default
  • i-enochian
  • i-hak
  • i-klingon
  • i-lux
  • i-mingo
  • i-navajo
  • i-pwn
  • i-tao
  • i-tay
  • i-tsu
  • sgn-BE-FR
  • sgn-BE-NL
  • sgn-CH-DE

function warning(reason, code, offset)

Called when an error occurs.

  • reason (string) — English reason for failure
  • code (number) — code for failure
  • offset (number) — index-based place where the error occurred in the tag
code reason
1 Too long variant, expected at most 8 characters
2 Too long extension, expected at most 8 characters
3 Too many extended language subtags, expected at most 3 subtags
4 Empty extension, extensions must have at least 2 characters of content
5 Too long private-use area, expected at most 8 characters
6 Found superfluous content after tag


This package is fully typed with TypeScript. It exports additional Schema, Extension, Warning, and Options types that model their respective interfaces.


This package is at least compatible with all maintained versions of Node.js. As of now, that is Node.js 12.20+, 14.14+, and 16.0+. It also works in Deno and modern browsers.


This package is safe.



Yes please! See How to Contribute to Open Source.


MIT © Titus Wormer

Package Sidebar


npm i bcp-47

Weekly Downloads






Unpacked Size

24.6 kB

Total Files


Last publish


  • wooorm