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Adds I18n (Internationalization) string support to a Fusion.js app.

This plugin looks for translations in the ./translations folder by default. Translations for each language are expected to be in a JSON file with a locale as a filename. For example, for U.S. English, translations should be in ./translations/en-US.json. Language tags are dictated by your browser, and likely follow the RFC 5646 specification.

If you're using React, you should use fusion-plugin-i18n-react instead of this package.

Table of contents


yarn add fusion-plugin-i18n


Simple middleware

The plugin provides a programmatic interface which exposes translate. See Service API for more details.

// src/some-middleware-plugin.js
import {createPlugin} from 'fusion-core';

export default createPlugin({
  deps: {I18n: I18nToken},
  middleware: ({I18n}) => {
    return (ctx, next) => {
      if (__NODE__ && ctx.path === '/some-path') {
        const i18n = I18n.from(ctx);
        ctx.body = {
          message: i18n.translate('test', {name: 'world'}), // hello world
      return next();

The default loader expects translation files to live in ./translations/{locale}.


  "HomeHeader": "Welcome!",
  "Greeting": "Hello, ${name}"


  "HomeHeader": "Bem-vindo!",
  "Greeting": "Olá, ${name}"


// src/main.js
import App from 'fusion-core';
import I18n, {
} from 'fusion-plugin-i18n';
import {FetchToken} from 'fusion-tokens';
import fetch from 'unfetch';

export default () => {
  const app = new App(<div />);

  app.register(I18nToken, I18n);
    ? app.register(I18nLoaderToken, createI18nLoader())
    : app.register(FetchToken, fetch);

  return app;


Registration API

import I18n from 'fusion-plugin-i18n-react';

The i18n plugin. Typically, it should be registered to I18nToken. Provides the i18n service

import {I18nToken} from 'fusion-plugin-i18n-react';

The canonical token for the I18n plugin. Typically, it should be registered with the I18n plugin.


import {I18nLoaderToken} from 'fusion-plugin-i18n';

A function that provides translations. Optional. Server-side only.

type I18nLoader = {
  from: (ctx: Context) => {locale: string | Locale, translations: {[string]: string},
  • loader.from: (ctx) => ({locale, translations}) -

    • ctx: FusionContext - Required. A FusionJS context object.
    • locale: Locale - A Locale
    • translations: {[string]: string} - A object that maps translation keys to translated values for the given locale
Default value

If no loader is provided, the default loader will read translations from ./translations/{locale}.json. See src/loader.js for more details.

import {HydrationStateToken} from 'fusion-plugin-i18n';

Sets the hydrated state in the client, and can be useful for testing purposes. Optional. Browser only.

type HydrationState = {
  chunks: Array<string | number>,
  translations: {[string]: string},
Default value

If no hydration state is provided, this will be an empty object ({}) and have no effect.

import {FetchToken} from 'fusion-tokens';

A fetch implementation. Browser-only.

type Fetch = (url: string, options: Object) => Promise<Response>;
  • url: string - Required. Path or URL to the resource you wish to fetch.
  • options: Object - Optional. You may optionally pass an init options object as the second argument. See Request for more details.
  • [return]: Promise<Request> - Return value from fetch. See [Response](A function that loads appropriate translations and locale information given an HTTP request context) for more details.
Default value

If no fetch implementation is provided, window.fetch is used.

Service API

const translation: string = i18n.translate(
  key: string,
  interpolations: {[string]: string}
  • key: string - A translation key. When using createI18nLoader, it refers to a object key in a translation json file.
  • interpolations: object - A object that maps an interpolation key to a value. For example, given a translation file {"foo": "${bar} world"}, the code i18n.translate('foo', {bar: 'hello'}) returns "hello world".
  • translation: string - A translation, or key if a matching translation could not be found.

Other examples

Custom translations loader example

// src/main.js
import App from 'fusion-core';
import I18n, {I18nToken, I18nLoaderToken} from 'fusion-plugin-i18n';
import {FetchToken} from 'fusion-tokens';
import fetch from 'unfetch';
import I18nLoader from './translations';

export default () => {
  const app = new App(<div></div>);

  app.register(I18nToken, I18n);
    ? app.register(I18nLoaderToken, I18nLoader);
    : app.register(FetchToken, fetch);

  // ....

  return app;

// src/translation-loader.js
import {Locale} from 'locale';

const translationData = {
  'en_US': {
    test: "hello ${name}"

export default (ctx) => {
  // locale could be determined in different ways,
  // e.g. from ctx.headers['accept-language'] or from a /en_US/ URL
  const locale = new Locale('en_US');
  const translations = translationData[locale];
  return {locale, translations};