@sveltekit-i18n/base

Core i18n functionality for SvelteKit with support for custom message parsers. This package provides the foundation for sveltekit-i18n and can be used standalone when you need maximum flexibility with custom parsers.

When to use @sveltekit-i18n/base

Use this package if you:

• Need a custom message parser (like ICU, Fluent, or your own format)
• Want full control over message interpolation
• Are building a custom i18n solution

Use sveltekit-i18n if you:

• Want the quickest setup with sensible defaults
• Are happy with the default placeholder/modifier syntax
• Don't need custom parsers

Key Features

✅ Svelte 5 runes – One reactive instance, no stores
✅ Framework ready – Full SSR and CSR support
✅ Parser-agnostic – Use any message syntax you need
✅ Custom data sources – Load translations from anywhere (files, APIs, databases)
✅ Module-based – Translations load only for visited pages
✅ Route-aware – Automatic loading based on SvelteKit routes
✅ Translated pathnames – /cs/o-nas for /about, from one route tree
✅ Locale domains – example.cz in Czech, example.com in English, from one app
✅ Component-scoped – Multiple translation instances with custom definitions
✅ Extensible – Pipe the instance through extensions to reshape or augment its surface
✅ TypeScript – Locales inferred from your config, keys and payloads from a schema
✅ Zero dependencies – Lightweight and fast

Requirements

Svelte 5 or newer, and one of Node 22+, Bun 1.2+ or Deno 2+. The package is ESM-only and imports no node: module, so every runtime that runs your SvelteKit build runs it. The /kit subpath needs SvelteKit 2 or 3; the SvelteKit behaviour the docs describe is checked against 2.70 and 3.0.

Installation

npm install @sveltekit-i18n/base
# bun add @sveltekit-i18n/base
# deno add npm:@sveltekit-i18n/base

You'll also need a parser:

# Choose one:
npm install @sveltekit-i18n/parser-curly
npm install @sveltekit-i18n/parser-icu
npm install @sveltekit-i18n/parser-mf2
npm install @sveltekit-i18n/parser-i18next
npm install @sveltekit-i18n/parser-fluent

Quick Start

1. Create translation files

// src/lib/translations/en/common.json
{
  "greeting": "Hello, {{name}}!",
  "farewell": "Goodbye!"
}

2. Setup with a parser

// src/lib/i18n.js
import { defineI18n } from '@sveltekit-i18n/base/kit';
import parser from '@sveltekit-i18n/parser-curly';

export const config = {
  parser: parser({ onReport: console.warn, /* other parser options */ }),
  loaders: [
    {
      locale: ['en', 'cs'],
      namespace: 'common',
      loader: async ({ locale, namespace }) => (await import(`./translations/${locale}/${namespace}.json`)).default,
    },
  ],
};

export const { handle, load, use, get } = defineI18n(config);

3. Wire it into SvelteKit

#lib is the imports entry sv create scaffolds in a SvelteKit 3 app's package.json. A SvelteKit 2 app adds the same entry, "imports": { "#lib/*": "./src/lib/*" }, or imports from $lib/i18n — as it must on Vite 5 when i18n is a .ts file imported from a .js module or a plain <script>.

// src/hooks.server.js
export { handle } from '#lib/i18n.js';

// src/routes/+layout.server.js and src/routes/+layout.js — the same line in both
export { load } from '#lib/i18n.js';

<!-- src/routes/+layout.svelte -->
<script>
  import { use } from '#lib/i18n.js';

  let { data, children } = $props();

  use(() => data);
</script>

{@render children()}

<!-- src/app.html -->
<html lang="%lang%" dir="%dir%">

The server picks the visitor's locale from the Accept-Language header (or from a cookie, with preferredLocale), loads it per request and hands it to the browser, so nothing loads twice and no visitor sees another's locale. See SvelteKit for the details and Server-Side Rendering for wiring it by hand. Translated pathnames serve each page at a pathname of its locale, through the reroute hook defineI18n() returns, and domains serve a locale at a domain of its own.

4. Use in components

<script>
  import { get } from '#lib/i18n.js';

  const i18n = get();
</script>

<p>{i18n.t('common.greeting', { name: 'World' })}</p>

The call reads the reactive translation table and locale, so the text updates automatically when either changes — no stores, no $ prefix. Do NOT destructure the instance's value properties: reading them off the instance is what makes templates reactive. (t/l are functions and stay reactive even when destructured, since the tracked reads happen at call time. In a component, const { loading } = $derived(i18n) destructures value reads without losing reactivity.)

Using Different Parsers

ICU Message Format

import i18n from '@sveltekit-i18n/base';
import parser from '@sveltekit-i18n/parser-icu';

const config = {
  parser: parser({ onReport: console.warn }),
  loaders: [/* ... */],
};

{
  "items": "You have {count, plural, =0 {no items} one {# item} other {# items}}."
}

Custom Parser

import i18n from '@sveltekit-i18n/base';

const customParser = () => ({
  parse: (value, params) => {
    // Your custom interpolation logic; `parse` must not throw on a non-string value
    return typeof value === 'string'
      ? value.replace(/\{(\w+)\}/g, (_, key) => params[0]?.[key] ?? key)
      : value;
  },
});

const config = {
  parser: customParser(),
  loaders: [/* ... */],
};

Learn more about creating custom parsers.

Configuration Options

parser (required)

Message parser instance. See Parsers.

loaders

Array of loader configurations:

loaders: [
  {
    locale: 'en',           // Required: locale identifier
    namespace: 'common',    // Required: translation namespace
    loader: async () => {}, // Required: async function returning translations
    routes: ['/about'],     // Optional: load only for specific routes
  },
]

locale and namespace each take a list as well. Such a descriptor stands for one loader per locale and namespace pair, and the loader receives the pair it is loading, so one computed loader can replace a descriptor per file:

loaders: [
  {
    locale: ['en', 'cs'],
    namespace: ['common', 'nav'],
    loader: async ({ locale, namespace }) => (await import(`./${locale}/${namespace}.json`)).default,
  },
]

A named capture group in a RegExp route is a load parameter: its match reaches the loader as params, and the loader runs again when it changes, its new data replacing the old:

{
  locale: 'en',
  namespace: 'article',
  routes: [/^\/article\/(?<articleId>[^/]+)/],
  loader: async ({ locale, params }) => (await fetch(`${API_ORIGIN}/api/articles/${params.articleId}/i18n/${locale}`)).json(),
}

API_ORIGIN stands for an origin such as https://api.example.com: a loader runs on the server too, where fetch takes only an absolute URL. See route params for the rules.

A loader whose source does the caching itself — a SvelteKit remote query, an SWR layer, an HTTP cache — sets cache: false. It then runs on every load trigger that selects it, sharing a fetch of it already in flight for the same params and route, and config.cache does not apply to it; only a hydrated snapshot holds it back, for the locale and route it was rendered for, until another locale or route is requested or a preload() runs, and a call handed a preload() token shows what that preload fetched instead of running it again. See the loader's cache.

A loader that throws is logged, and the rest of the load lands without its data; it runs again on the next load trigger. SvelteKit's redirect() and an error() below 500 (told by their shape: an own status with a location or a body, on a value that is not an Error) are logged too, but they also reject the load, so a SvelteKit load awaiting the call hands them to SvelteKit; the rejected call is undone — what it replaced goes back — unless a later call that has not failed came in the meantime. See the loader.

Both loaders and a loader's routes accept readonly arrays, so a whole-config as const is fine.

basePath

The path the app is served under — SvelteKit's paths.base. Every route handed in loses it on the way in, on a segment boundary only (under /repo, /repo/about is /about), so loader routes name the app's own paths. Set both from one environment variable:

// SvelteKit 3, vite.config.js: sveltekit({ paths: { base: process.env.VITE_BASE_PATH ?? '' } })
// SvelteKit 2, svelte.config.js: kit: { paths: { base: process.env.VITE_BASE_PATH ?? '' } }
basePath: import.meta.env.VITE_BASE_PATH

See basePath.

translations

Synchronous translations, available immediately. They seed the tables: the loaders of a namespace they name still run and merge into it. Hand a server's state over with hydrate() instead:

translations: {
  en: {
    'app.name': 'My App',
  },
}

initLocale

The initial locale, used when nothing else decides:

initLocale: 'en'

With defineI18n() it is the locale a visitor gets when nothing they prefer is served, and it loads only when negotiation picks it (Which locale). With new I18n(config) the constructor loads it right away, so leave it out of a config whose instance you hydrate() by hand — its load starts before the hand-off can be applied.

fallbackLocale

Fallback when translation is missing:

fallbackLocale: 'en'

Note: This loads translations for both current locale and fallback locale, which may impact performance.

fallbackValue

Default return value when translation key is not found:

fallbackValue: '...' // Default: returns the key itself

sanitizeLocales

How locale identifiers are normalized before they key anything: true (default) resolves them to their ISO form through Intl ('en-us' is 'en-US'), false keeps them as authored, and a function normalizes them your way. See sanitizeLocales.

preprocess

Transform translations after loading:

preprocess: 'full' // 'full' | 'preserveArrays' | 'none' | custom function

• 'full' (default): Flattens all nested objects to dot notation
• 'preserveArrays': Flattens objects but preserves arrays
• 'none': No preprocessing
• Custom function: (input) => transformedOutput

schema

A map of translation key to the payload its message expects (never for a message that takes none). It types t/l — keys autocomplete, an unknown key is a type error, and the payload argument is checked. Register it once for the app, in a global script, and every instance whose config states no schema is typed by it:

// src/i18n-schema.d.ts — no top-level import or export
interface TranslationSchema {
  'common.greeting': { name: string };
  'common.farewell': never;
}

declare namespace SvelteKitI18n {
  interface Register {
    schema: TranslationSchema;
  }
}

Or state it per instance — only its type is read, so the value can stay empty at runtime, and a stated schema wins over the registry:

const i18n = new I18n({ ...config, schema: {} as TranslationSchema });

Hand-write it for a small set of messages, or generate it — @sveltekit-i18n/typegen, a separate package, generates one. A schema whose keys are not a closed set (schema: {}) types nothing and keeps the registry out, so keys stay plain strings. A library never registers. The registry needs base 3.1 or newer. See schema for the full rules.

cache

Time in milliseconds the loaded translations stay fresh for. By default, loaded translations never expire — each loader (but one with cache: false) runs once per locale and route params (a loader's routes decide whether a load trigger considers it, and the params their named groups capture decide when it runs again).

Set a finite value when your loaders fetch from a source that can change at runtime (e.g. a CMS):

cache: 3600000 // Translations older than 1 hour refetch on the next activating load

Expiry is evaluated by the next activating load trigger (setLocale, setRoute, loadTranslations) or preload(); a warm load — loadTranslations(…, { activate: false }) or loadNamespace() — fills the tables without evaluating it, and a call handed a preload() token leaves it to that preload. Set to 0 to treat translations as always stale (refetch on every activating load trigger). A loader with cache: false is outside the window. You can also drop the loaded state manually at any time with invalidate().

extensions

Pipes the constructed instance through extension functions, left to right. Each extension receives the surface produced so far (the raw instance for the first one) and returns the surface handed on — new I18n(config) evaluates to the last extension's output:

import stores from '@sveltekit-i18n/extension-stores';

const { t, locale, loading } = new I18n({
  ...config,
  extensions: [stores],
});

An extension may augment the instance in place, or replace the surface entirely (like the store adapter above). The official extensions live in the extensions repository:

• @sveltekit-i18n/extension-stores – replaces the instance with the Svelte-store surface of v2 ($t, $locale, $loading, …), the instance it received at instance
• @sveltekit-i18n/extension-html – adds a T component that renders the markup a message carries as elements and Svelte components, from an allowlist, with no {@html}
• @sveltekit-i18n/extension-markdown – adds a T component that renders the Markdown a message carries (CommonMark's inline syntax) as elements and Svelte components, with no {@html}
• @sveltekit-i18n/extension-typed-access – keys as members of t: i18n.t.home.title() beside i18n.t('home.title'), typed from the schema

Their order matters. stores returns no instance, so it goes after the others: [typedAccess, stores] hands out the tree as $t.home.title(), and with [html(…), stores] the component is at instance.T, as with markdown in html's place. html or markdown after typedAccess adds T beside the tree, while before it T is typed at instance.T only. Each of the two adds T, so an instance takes one of them: the second in a pipe throws at construction. Each package's README has the details, and lib's examples run each one.

A custom extension is just a function:

const withGreeting = (i18n) => Object.assign(i18n, {
  greet: (name) => i18n.t('common.greeting', { name }),
});

export const i18n = new I18n({ ...config, extensions: [withGreeting] });

i18n.greet('World');

Notes:

• Applied at construction time only — a later loadConfig() call ignores this property.
• When an extension returns a new object, the result is no longer instanceof I18n; the original instance stays reachable through whatever the extension exposes (the official extensions expose it as instance).

log

Logging configuration:

log: {
  level: 'warn',        // 'error' | 'warn' | 'debug'
  prefix: '[i18n]: ',   // Log prefix
  logger: console,      // Custom logger
}

API Reference

Reactive properties

• t(key, ...params) – translate for the active locale (reactive function)
• l(locale, key, ...params) – translate for an explicit locale
• locale – the ACTIVE locale; assignment is a fire-and-forget setLocale()
• locales – available locales
• loading – true while any activating load is in flight; a { activate: false } load counts only once an activating trigger joins it
• initialized – locale and route set, translations present
• translations / rawTranslations – the tables after and before preprocessing

Methods

Load-triggering methods return the promise of the matching load — concurrent duplicate triggers from one route share one in-flight load (and its promise) instead of fetching twice.

• loadTranslations(locale, route?, options?) – load translations for locale and route; route defaults to the current one, { activate: false } only fills the tables without switching to them, and { preloaded } takes a preload() token
• preload(locale, route?) – the request of a navigation that may never commit (a router's load, a hover's included): it judges freshness and runs cache: false loaders as an activating call would, shows nothing, and resolves to a token the commit's loadTranslations() or setRoute() takes as { preloaded } to show what it fetched instead of fetching it again
• loadNamespace(namespace, locale?) – load one namespace on demand, whatever its loaders' routes, without switching to it; it stays loaded across routes
• setLocale(locale) – request a locale; loads once a route is known
• setRoute(route, options?) – update the current route; { preloaded } takes a preload() token
• loadConfig(config) – (re)configure the instance
• addTranslations(translations) – seed synchronous translations; the loaders of their namespaces still run and merge into them
• snapshot(options?) – serialize what the active locale (and the fallback) holds; { records: true } returns the envelope hydrate() restores, with the loaders that delivered, the active locale and the route, and no argument returns the data alone, shaped like config.translations, for a plain hydrate({ translations })
• hydrate(envelope?) – restore a server's snapshot: its data, its load records (so those loaders do not run again — one with cache: false only for the locale and route it was rendered for, until a preload() runs or invalidate() covers it), its locale and its route; an envelope without records keeps the loaders of the namespaces its data names from running
• invalidate(locale?, namespace?) – mark loaded translations stale (one locale or all, one namespace or all); loaders run again on the next load trigger, and a loader still in flight for what was invalidated settles with whatever it returns or throws discarded — an activating trigger fetches it again before it activates, unless another loader of its load threw SvelteKit's control flow
• destroy() – detach a per-request or per-component instance: in-flight loads settle discarded, further load and mutation calls are ignored, reads keep working

Utilities

Helpers ship from a separate subpath, for the code around the instance that has to match the library's own behavior or decide which locale to ask for:

import { loggerFactory, matchLocale, resolveLoaders, sanitizeLocales, textDirection, toDotNotation } from '@sveltekit-i18n/base/utils';

• toDotNotation(input, preserveArrays?) – the flattening behind preprocess, for a custom preprocess that still wants dot notation
• resolveLoaders(loaders, sanitizeLocales?) – normalizes config.loaders the way the instance does, into one loader per locale and namespace pair, for code that reads a config from outside the instance
• sanitizeLocales(...locales) – normalizes a locale from a URL, cookie or Accept-Language header the way the instance does, so it can be compared against locale
• matchLocale(requested, available) – picks the configured locale a visitor asked for, from an Accept-Language header or navigator.languages, falling back from en-GB to en and answering undefined when nothing matches
• textDirection(locale) – 'ltr' or 'rtl' for a dir attribute, from the script the tag spells or the one Intl.Locale#maximize() adds, with no list of languages to keep
• loggerFactory(options?) – builds a logger from what log takes, as the instance builds its own, so a parser's reports can go where the library's messages go: onReport: ({ id, message }) => logger.warn(`${message} (${id})`)

SvelteKit

import { defineI18n } from '@sveltekit-i18n/base/kit';

export const { handle, load, use, get } = defineI18n(config, { preferredLocale });

• handle – the hooks.server.js hook; fills %lang% and %dir% in app.html's <html> tag
• load – the root layout's load, exported from +layout.server.js and +layout.js alike: negotiates the locale, loads it on the server per request and hands it to the one instance a browser tab keeps
• use(() => data) – called once in the root +layout.svelte; provides the instance, follows every navigation and keeps <html lang> and <html dir> in sync
• get() – the instance, in any component below the root layout

Full API documentation: docs/README.md

Documentation

• 🌐 sveltekit-i18n.github.io – The documentation site, with a live playground
• 📖 Full API Documentation – Complete reference
• 📚 Main Library Docs – Guides, tutorials, and best practices
• 🎨 Parsers – Available parsers and how to create your own
• 💡 Examples – Real-world usage examples

TypeScript Support

import { I18n, type Config } from '@sveltekit-i18n/base';
import parser, { type Parser } from '@sveltekit-i18n/parser-curly';

// The parser's params – the rest parameters of `t`/`l`. Annotate only when the
// config lives on its own; `new I18n({ ... })` infers them. Take the tuple from
// the parser rather than spelling it by hand.
type Params = Parser.Params;

const config: Config.T<Params> = {
  parser: parser({ onReport: console.warn }),
  loaders: [/* ... */],
};

Two more things are inferred from the config itself. schema types the keys and payloads of t/l, and every locale the config names — loader locales, initLocale, fallbackLocale and the keys of translations — completes the locale arguments and reads (setLocale, loadTranslations, loadNamespace, preload, invalidate, l, locale, locales):

const i18n = new I18n({ parser: parser({ onReport: console.warn }), initLocale: 'en', fallbackLocale: 'de' });

i18n.setLocale('en'); // 'en' | 'de' autocomplete here
i18n.setLocale('sv'); // still accepted — the union is a hint, not a constraint

The locales survive only when the config reaches the constructor as a literal — inline, as above, or a separate object with as const. An annotated or separately widened config, and any config with one dynamic locale source (loaders: locales.map(...)), leaves them plain string. See TypeScript for both.

Keys as members of t — i18n.t.home.title() beside i18n.t('home.title') — come from @sveltekit-i18n/extension-typed-access, an official extension typed from the same schema. It reads the keys nested by segment that @sveltekit-i18n/typegen registers beside the schema, as tree, and groups the keys itself without them; the core reads none of it.

Related Packages

• sveltekit-i18n – Complete solution, with the Curly Message Format parser included
• @sveltekit-i18n/parser-curly – Curly Message Format parser
• @sveltekit-i18n/parser-icu – ICU message format parser
• @sveltekit-i18n/parser-mf2 – Unicode MessageFormat 2 parser
• @sveltekit-i18n/parser-i18next – i18next interpolation and formatting syntax parser
• @sveltekit-i18n/parser-fluent – Fluent parser, Mozilla's localization system
• Extensions – Official extensions for the config.extensions pipe
• @sveltekit-i18n/typegen – Generates the schema type from your translation files

Contributing

For general contribution guidelines, see the Contributing Guide in the main library repository.

For issues specific to base functionality, create a ticket here.

Changelog

See Releases for version history.

Sponsor

You can support the maintenance of this package through GitHub Sponsors.

License

MIT