Fast Plural Rules

Evaluates locale-specific plural rules to identify the right plural form for a cardinal number, which represents an item count. Internationalization libraries can utilize it to choose the right localized string.

Focused and complete - nothing but the rule evaluation is included, but still supporting almost 150 languages.
Tiny and fast - 7.27 kB, 4.8 kB minified, 1.32 kB, gzipped. Using plain hand-coded functions as plural rules to pick plural forms using language locales.
Standard and documented - written using the Translate Project documentation and the Mozilla documentation.
Reliable and correct - contains the full test suite comparing the actual results with the Mozilla specification, run in both Node.js and the browser.
Universal and modern - supports both plural form index (0-5) and plural form rules (zero, one, two, few, many and other) and includes declarations for TypeScript.

If you are looking for a library compiling and executing the declarative CLDR plural rules, see plural-rules. Generated programmatically for better reliability, but bigger and slower.

Synopsis

import { getPluralFormForCardinalByLocale } from 'fast-plural-rules'

// Returns index of the plural form for the specified locale and cardinal.
getPluralFormForCardinalByLocale('en', 1) // Returns 0; "1 file"
getPluralFormForCardinalByLocale('en', 2) // Returns 1; "2 files"
getPluralFormForCardinalByLocale('en', 5) // Returns 1; "5 files"
getPluralFormForCardinalByLocale('cs', 1) // Returns 0; "1 soubor"
getPluralFormForCardinalByLocale('cs', 2) // Returns 1; "2 soubory"
getPluralFormForCardinalByLocale('cs', 5) // Returns 2; "5 souborů"

// Returns a localized message for the specified locale and cardinal.
localizeMessage('en', 'fileCount', 3) // Returns "3 files"
localizeMessage('cs', 'fileCount', 3) // Returns "3 soubory"

// Returns a localized message for the specified locale and cardinal.
function localizeMessage (locale, messageKey, cardinal) {
  const pluralForm = getPluralFormForCardinalByLocale(locale, cardinal)
  const messageFormat = messages[locale][messageKey][pluralForm]
  return messageFormat.replace('{0}', cardinal)
}

// A language pack with a testing message.
const messages = {
  en: {
    fileCount: [
      "{0} file", // 0 - singular
      "{0} files" // 1 - plural
    ],
  }
  cs: {
    fileCount: [
      "{0} soubor",  // 0 - singular
      "{0} soubory", // 1 - plural for 2-4 items
      "{0} souborů"  // 2 - plural for 5+ items
    ]
  }
}

There is another full example using plural form names instead of numeric indexes like this:

// Localized messages organized by locales and message keys.
const messages = {
  en: {
    fileCount: {
      one:   '{0} file', // singular
      other: '{0} files' // plural
    }
  },
  cs: {
    fileCount: {
      one:   '{0} soubor',  // singular
      few:   '{0} soubory', // plural for 2-4 items
      other: '{0} souborů'  // plural for 5 and more items
    }
  }
}

Installation and Getting Started

This module can be installed in your project using NPM or Yarn. Make sure, that you use Node.js version 6 or newer.

$ npm i fast-plural-rules --save

$ yarn add fast-plural-rules

Functions are exposed as named exports, for example:

import { getPluralFormForCardinalByLocale } from 'fast-plural-rules'

You can read more about the module loading in other environments, like with ESM or in web browsers. Usage scenarios demonstrate applications of this library in typical real-world situations. Design concepts explain the approach to the correct internationalization of messages with cardinals taken by this library. Translators will read about plural rules for supported languages to be able to write the right plural forms to language packs. Finally, the API reference lists all functions with a description of their functionality.

Library Integrations

Extended Day.js - relativeTime plugin formats time durations to the past and to the future.

Contributing

In lieu of a formal styleguide, take care to maintain the existing coding style. Add unit tests for any new or changed functionality. Lint and test your code using Grunt.

License

Licensed under the MIT license.