just-t

The smallest practical i18n library: JSON in → string out.

Features

• Zero config: createJustT({ locale, messages })
• Interpolation: "Hello {name}"
• Optional, super simple plurals: one/other via count === 1
• Fail loudly: in dev it throws on missing keys/params, in prod it returns the key
• Type Safety:
  • Keys are derived from messages
  • Params are derived from {placeholders} (with as const; for plurals includes count: number)

Install

pnpm add just-t

Usage

import { createJustT } from "just-t";

const justT = createJustT({
  locale: "en",
  fallbackLocale: "de",
  messages: {
    en: {
      welcome: "Hello",
      hello_user: "Hello {name}",
      items: { one: "{count} item", other: "{count} items" }
    },
    de: {
      welcome: "Hallo",
      hello_user: "Hallo {name}",
      items: { one: "{count} Ding", other: "{count} Dinge" }
    }
  } as const
});

justT.t("welcome");
justT.t("hello_user", { name: "Max" });
justT.t("items", { count: 3 });

Messages from JSON

You can keep messages in one or multiple JSON files and load them into createJustT.

Browser (or any runtime): JSON string/object

import { createJustT } from "just-t";

const i18n = createJustT({
  locale: "en",
  json: `{
    "en": { "welcome": "Hello" },
    "de": { "welcome": "Hallo" }
  }`,
});

If your JSON is a single-locale message tree, provide jsonLocale:

const i18n = createJustT({
  locale: "en",
  jsonLocale: "en",
  json: { welcome: "Hello" }
});

Node.js: file/folder

// Node only
const i18n = await createJustT({
  locale: "en",
  folder: "./locales" // en.json, de.json, ...
});

API (Exports)

import {
  createJustT,
  type JustT,
  type CreateJustTOptions,
  type MessagesByLocale,
  type MessageValue,
  type PluralMessage,
  type Locale
} from "just-t";

Dev / Build

pnpm install
pnpm test
pnpm test:watch
pnpm test:types
pnpm build
pnpm typecheck

Note: pnpm build produces a bundled & minified dist/index.js via Rolldown and dist/index.d.ts via tsc.