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# @wrnexus/i18n
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> Per-request translations plus locale-aware number, date, and currency formatting for WrNexus apps.
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Part of the **WrNexus** framework — an SSR-first, Bun-native full-stack web framework.
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## Overview
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`@wrnexus/i18n` loads locale files from `app/locales/<lang>.json`, resolves the
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active language for each request (cookie → `Accept-Language` → default), and
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builds a `t(key, params)` translator used both in server code and in `.wrn`
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views. It also ships Intl-based formatting helpers and a tiny client runtime that
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wires up a language switcher. Translation lookup, language resolution, and HTML
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marker rewriting run server-side; only the small `I18N_RUNTIME` snippet runs in
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the browser.
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## Installation
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```bash
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bun add @wrnexus/i18n
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```
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> Private package — the machine must be authenticated to the `wrnexus` npm org
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> (a read token in `~/.npmrc`). Requires **Bun** (Node is not supported).
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## API
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### Loading & resolving
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| Export | Signature | Description |
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| ------------- | -------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------- |
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| `loadLocales` | `(dir: string) => Record<string, Messages>` | Reads every `<lang>.json` in `dir` into a `{ lang: messages }` map. Missing dir → `{}`; a bad file is warned and skipped. |
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| `resolveI18n` | `(messages: Record<string, Messages>, config?: I18nConfig) => ResolvedI18n` | Merges loaded messages + config into a resolved bundle (default lang, supported langs, messages). |
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| `resolveLang` | `(i18n: ResolvedI18n, cookieValue: string \| undefined, acceptLanguage: string \| null) => string` | Picks the active language: matching cookie → best `Accept-Language` tag (falls back to base tag, e.g. `en-US` → `en`) → `i18n.default`. |
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| `makeT` | `(i18n: ResolvedI18n, lang: string) => TFunction` | Builds a translator resolving current language → default → the key itself, with `{param}` interpolation. |
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### Types & constants
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| Export | Kind | Notes |
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| -------------- | ----------- | ------------------------------------------------------------------------------ |
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| `Messages` | `type` | `Record<string, unknown>` — a locale's messages (supports nested/dotted keys). |
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| `I18nConfig` | `interface` | `{ default?: string; locales?: string[] }`. |
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| `ResolvedI18n` | `interface` | `{ default: string; langs: string[]; messages: Record<string, Messages> }`. |
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| `LANG_COOKIE` | `const` | `"wire-lang"` — the cookie the language is read from / written to. |
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| `I18N_JS_HREF` | `const` | `"/__wrnexus/i18n.js"` — URL the client runtime is served at. |
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### HTML & client runtime
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| Export | Signature | Description |
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| ---------------- | ---------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
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| `translateHtml` | `(html: string, t: TFunction) => string` | Rewrites markers in rendered HTML: `t:<attr>="key"` → `<attr>="<translation>"` (attribute-escaped) and `<tag data-t="key">…</tag>` → element text becomes the translation (HTML-escaped). No-op unless a marker is present. |
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| `renderI18nData` | `(i18n: ResolvedI18n, lang: string) => string` | JS snippet setting `window.__wireI18n = { lang, langs, default }` for the client switcher. |
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| `I18N_RUNTIME` | `const string` | Browser IIFE that binds `[data-wire-lang-set="es"]` clicks and `select[data-wire-lang]` changes to set the `wire-lang` cookie and reload. Exposes `window.__wireLang.set(lang)`. |
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### Formatting helpers (re-exported from `./format.ts`)
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| Export | Signature | Example |
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| -------------------- | ------------------------------------------------------------------------------------------------- | ---------------------------------------------- |
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| `formatNumber` | `(value: number, lang: string, options?: Intl.NumberFormatOptions) => string` | `1234.5 → "1,234.5"` |
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| `formatCurrency` | `(value: number, currency: string, lang: string) => string` | `9.99, "USD" → "$9.99"` |
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| `formatDate` | `(value: Date \| number \| string, lang: string, options?: Intl.DateTimeFormatOptions) => string` | defaults to `{ dateStyle: "medium" }` |
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| `formatRelativeTime` | `(value: number, unit: Intl.RelativeTimeFormatUnit, lang: string) => string` | `-3, "day" → "3 days ago"` (`numeric: "auto"`) |
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| `plural` | `(count: number, forms: Partial<Record<Intl.LDMLPluralRule, string>>, lang: string) => string` | picks CLDR form; `#` is replaced by `count` |
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## Usage
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### Server: load, resolve, translate
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```ts
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import {
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loadLocales,
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resolveI18n,
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resolveLang,
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makeT,
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translateHtml,
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LANG_COOKIE,
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} from "@wrnexus/i18n";
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// app/locales/en.json, app/locales/es.json
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const messages = loadLocales("app/locales");
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const i18n = resolveI18n(messages, { default: "en", locales: ["en", "es"] });
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// Per request:
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const lang = resolveLang(i18n, req.cookies?.[LANG_COOKIE], req.headers.get("accept-language"));
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const t = makeT(i18n, lang);
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t("nav.home"); // dotted key → "Home"
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t("greeting", { name: "Ada" }); // "Hello, {name}" → "Hello, Ada"
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// After rendering a .wrn view, resolve translation markers in the HTML:
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const finalHtml = translateHtml(renderedHtml, t);
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```
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`app/locales/en.json`:
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```json
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{
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"nav": { "home": "Home" },
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"greeting": "Hello, {name}"
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}
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```
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### Views: translation markers
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```html
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<h1 data-t="nav.home">Home</h1>
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<input t:placeholder="search.placeholder" />
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```
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`translateHtml` replaces the element text for `data-t` and the attribute value for
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any `t:<attr>` (e.g. `t:placeholder`, `t:aria-label`).
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### Client: language switcher
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```ts
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import { renderI18nData, I18N_RUNTIME, I18N_JS_HREF } from "@wrnexus/i18n";
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// In the document <head>:
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const head = `
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<script>${renderI18nData(i18n, lang)}</script>
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<script src="${I18N_JS_HREF}"></script>
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`;
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// Serve I18N_RUNTIME at I18N_JS_HREF; then in markup:
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// <button data-wire-lang-set="es">Español</button>
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// <select data-wire-lang>…</select>
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```
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### Formatting
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```ts
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import {
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formatNumber,
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formatCurrency,
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formatDate,
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formatRelativeTime,
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plural,
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} from "@wrnexus/i18n";
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formatNumber(1234.5, lang); // "1,234.5"
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formatCurrency(9.99, "USD", lang); // "$9.99"
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formatDate(Date.now(), lang); // "Jul 4, 2026"
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formatRelativeTime(-3, "day", lang); // "3 days ago"
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plural(2, { one: "# item", other: "# items" }, lang); // "2 items"
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```
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## Configuration
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`resolveI18n` accepts an `I18nConfig`:
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- `default` — fallback language; used when nothing else matches. Ignored if it has
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no loaded messages, in which case the first supported language is used.
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- `locales` — explicit supported-language list; defaults to the loaded locale names.
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Language resolution order at request time (`resolveLang`): a supported `wire-lang`
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cookie value → the first matching `Accept-Language` tag (or its base subtag) → the
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resolved default.
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## Requirements / Notes
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- **Bun-only.** Locale loading uses `node:fs` (`existsSync`, `readdirSync`,
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`readFileSync`) and `node:path`; formatting relies on the platform `Intl` APIs.
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- Works with [`@wrnexus/core`](../core) — `TFunction` (the `t(key, params)` type)
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comes from core, and the resolved translator is exposed as `ctx.t` / `ctx.lang`
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in request handling.
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- Nested message objects are supported: keys are looked up whole first, then split
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on `.` to walk the object tree.
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