fastify-i18n

Internationalization plugin for Fastify. Built upon node-polyglot.

Installation

Install fastify-i18n with your favorite package manager:

$ npm i fastify-i18n
# or
$ yarn add fastify-i18n
# or
$ pnpm i fastify-i18n
# or
$ bun add fastify-i18n

Usage

// esm
import i18n, { defineI18n, useI18n } from 'fastify-i18n';

// cjs
const { default: i18n, defineI18n, useI18n } = require('fastify-i18n');

Global Scope

import i18n from 'fastify-i18n';

fastify.register(i18n, {
  fallbackLocale: 'en',
  messages: {
    en: { text: 'Text' },
    ja: { text: 'テキスト' },
    ko: { text: '텍스트' },
    zh: { text: '文字' },
  },
});

/*
  curl --request GET \
    --url http://127.0.0.1:3000/api/i18n \
    --header 'accept-language: ja'
  */
fastify.get('/api/i18n', async (req, reply) => {
  return reply.send({ message: req.i18n.t('text') });
});

Local Scope

import type { FastifyInstance } from 'fastify';
import { defineI18n, useI18n } from 'fastify-i18n';

export default async (app: FastifyInstance) => {
  defineI18n(app, {
    en: { hello: 'Hello, World!' },
    ja: { hello: 'こんにちは世界！' },
    ko: { hello: '안녕하세요, 월드입니다!' },
    zh: { hello: '你好，世界！' },
  });

  /*
  curl --request GET \
    --url http://127.0.0.1:3000/api/hello-world \
    --header 'accept-language: ja'
  */
  app.get('/hello-world', async (req, reply) => {
    const i18n = useI18n(req);

    return reply.send({
      // global scope
      text: req.i18n.t('text'),

      // local scope
      hello: i18n.t('hello'),
    });
  });
};

Automatic Conversion

xx-XX to xx

fastify.register(i18n, {
  fallbackLocale: 'en',
  messages: {
    en: { text: 'Text' },
    ja: { text: 'テキスト' },
  },
});

$ curl --request GET \
       --url http://127.0.0.1:3000/api/i18n \
       --header 'Accept-Language: ja-JP'
# Output: { text: 'テキスト' } (ja)

xx to xx-XX

fastify.register(i18n, {
  fallbackLocale: 'en',
  messages: {
    'en-US': { text: 'Text' },
    'ja-JP': { text: 'テキスト' },
  },
});

$ curl --request GET \
       --url http://127.0.0.1:3000/api/i18n \
       --header 'Accept-Language: ja'
# Output: { text: 'テキスト' } (ja-JP)

Priority

fastify.register(i18n, {
  fallbackLocale: 'en',
  messages: {
    'en-US': { text: 'Text' },
    'zh-CN': { text: '文本' },
    'zh-TW': { text: '文字' },
  },
});

$ curl --request GET \
       --url http://127.0.0.1:3000/api/i18n \
       --header 'Accept-Language: zh'
# Output: { text: '文本' } (zh-CN)

fastify.register(i18n, {
  fallbackLocale: 'en',
  messages: {
    'en-US': { text: 'Text' },
    'zh-TW': { text: '文字' },
    'zh-CN': { text: '文本' },
  },
});

$ curl --request GET \
       --url http://127.0.0.1:3000/api/i18n \
       --header 'Accept-Language: zh'
# Output: { text: '文字' } (zh-TW)

Glob Import (Vite Only)

// global
fastify.register(i18n, {
  fallbackLocale: 'en-US',
  messages: import.meta.glob(['~/locales/*.ts'], { eager: true }),
});

// local scope
defineI18n(app, import.meta.glob(['./locales/*.ts'], { eager: true }));

Type-safe Resources

Coming soon...

V2 Migration Guide

This guide is intended to help with migration from fastify-i18n v1 to v2.

V2 no longer distinguishes between scopes; each scope's language will override the previous layer's key with the same scope. So, useI18n is no longer needed; you can now access the instance of node-polyglot from fastify.i18n or request.i18n.

app.get('/hello-world', async (req, reply) => {
- const i18n = useI18n(req);

  return reply.send({
    // global scope
    text: req.i18n.t('text'),

    // local scope
-   hello: i18n.t('hello'),
+   hello: req.i18n.t('hello'),
  });
});

Please note that i18n must be registered before your routes, otherwise the global scope will trigger only at the last layer unless you require it.

fastify.register(i18n);
fastify.register(router);

V3 Migration Guide

Support Fastify v5