http-accept-language-kit

Parse, inspect, format, and lightly match HTTP Accept-Language headers.

http-accept-language-kit is a clean-room TypeScript package for apps that need the raw header turned into predictable data. It has no runtime dependencies, no mutable global language registry, and no Node-only APIs.

Demo

Try the browser demo: packages.wasta-wocket.fr/http-accept-language-kit.

Package quality

• TypeScript types are generated from the source.
• ESM-only package with no runtime dependencies.
• Marked as side-effect free for bundlers.
• CI runs npm ci, typecheck, build, and test.
• Tested on Node.js 20 and 22 with GitHub Actions.
• Browser-friendly implementation with no Node-only APIs.

Install

npm install http-accept-language-kit

Quick Start

import { formatAcceptLanguage, parseAcceptLanguage, pickAcceptedLanguage } from "http-accept-language-kit";

const parsed = parseAcceptLanguage("fr-CA, fr;q=0.8, en-US;q=0.6");

if (parsed.ok) {
  console.log(parsed.languages[0]);
  // { range: "fr-CA", quality: 1, order: 0, raw: "fr-CA", subtags: ["fr", "CA"] }
}

const selected = pickAcceptedLanguage(parsed.input, ["en-US", "fr-FR"]);
// "fr-FR"

const normalized = formatAcceptLanguage(parsed.languages);
// "fr-CA, fr;q=0.8, en-US;q=0.6"

Why This Package

Use this when you want to inspect the header itself before deciding what to do:

• stable diagnostics for malformed ranges, duplicate q values, and wildcard handling;
• sorted structured results while preserving original order and raw tokens;
• a tiny matcher for exact, base-language, and wildcard cases;
• browser-friendly code that also works in workers, CLIs, and server middleware.

It is not a full locale negotiation framework and does not try to implement every language matching algorithm from ECMA-402 or BCP 47.

By default, q-values follow HTTP qvalue precision and allow at most three decimal places. Import tools that need to accept looser legacy input can opt into allowExtendedQualityPrecision.

API

parseAcceptLanguage(input, options?)

Returns { ok, input, languages, diagnostics }. Valid languages are sorted by quality by default.

const result = parseAcceptLanguage("da, en-GB;q=0.8, en;q=0.7");

formatAcceptLanguage(languages, options?)

Formats parsed language items back to a compact header value.

formatAcceptLanguage(result.languages, { precision: 2 });

pickAcceptedLanguage(header, supportedLanguages, options?)

Returns the first supported language matched by quality order. Matching is intentionally small: exact match, base language match, then wildcard.

pickAcceptedLanguage("fr-CA, en;q=0.8", ["en-US", "fr-FR"]);

Parse Options

Option	Default	Description
allowWildcard	true	Accept * ranges while still reporting a wildcard diagnostic.
allowExtendedQualityPrecision	false	Accept q-values with more than three decimal places.
clampQuality	false	Clamp out-of-range q values instead of rejecting them.
keepInvalid	false	Include invalid items in languages for UI review flows.
sort	true	Sort by descending q value and then original order.

Format Options

Option	Default	Description
includeQualityOne	false	Emit ;q=1 for full-quality ranges.
precision	3	Maximum decimal places for q values.

Diagnostics

Diagnostics are stable strings intended for logs, UI hints, and tests:

• invalid-input
• empty-header
• empty-item
• invalid-language-range
• invalid-parameter
• invalid-quality
• quality-precision-exceeded
• duplicate-quality
• quality-clamped
• wildcard-range

License

MPL-2.0