Parse and provides the best language option from a Accept-Language header value.

getPreferredLanguages.md

getPreferredLanguages

Parses the Accept-Language header's value in an array of language directives.

Usage

const acceptLanguageValue = 'pt-BR, en;q=0.9, fr;q=0.8, it;q=0.7, *;q=0.5'

getPreferredLanguages(acceptLanguageValue)
/* will output: 
[
  { quality: 1, locale: 'pt-BR' },
  { quality: 0.9, locale: 'en' },
  { quality: 0.8, locale: 'fr' },
  { quality: 0.7, locale: 'it' },
  { quality: 0.5, locale: '*' }
]
*/

// by default, returns a wildcard language if no value has been provided
getPreferredLanguages() // outputs [ { quality: 1, locale: '*' } ]

References:

• Accept-Language reference at MDN site

License

MIT

getPreferredLanguages.ts

/*
  getPreferredLanguages.ts
  
  MIT License
  Copyright (c) 2023 Jan Cassio
  Permission is hereby granted, free of charge, to any person obtaining a copy
  of this software and associated documentation files (the "Software"), to deal
  in the Software without restriction, including without limitation the rights
  to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
  copies of the Software, and to permit persons to whom the Software is
  furnished to do so, subject to the following conditions:
  The above copyright notice and this permission notice shall be included in all
  copies or substantial portions of the Software.
  THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
  IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
  FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
  AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
  LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
  OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
  SOFTWARE.
*/

type LanguageDirective = {
  locale: string | "*"
  quality: number | 1
}

const toLanguageDirective = (token: string): LanguageDirective => {
  const [locale, q] = token.trim().split(";")
  const quality = Number(q?.replace(/q=/, "")) || 1
  return { quality, locale }
}

const byQuality = (a: LanguageDirective, b: LanguageDirective) => {
  if (a.quality > b.quality) {
    return -1
  }
  if (a.quality < b.quality) {
    return 1
  }
  return 0
}

/**
 * Parse the Accept-Language header and return an array of language directives.
 *
 * @param value A valid Accept-Language header value.
 * @returns An array of language directives. The array is sorted by quality, from the most to the least preferred. If no Accept-Language header was provided, the array will contain a single wildcard language directive.
 */
export default function getPreferredLanguages(
  value: string | undefined | null
): LanguageDirective[] {
  if (!value) return [{ quality: 1, locale: "*" }]
  return value.split(",").map(toLanguageDirective).sort(byQuality)
}