5.0.0 • Public • Published

    Build Status Coverage Status npm Published on webcomponents.org


    Instant and Modular I18N engine for lit-html and Polymer

    html`` tagged template literal API is provided by i18n-element

    Compatible Versions

    • Polymer library (@polymer/polymer NPM package) is configured as a peer dependency since 4.0.0
    • Polymer elements using I18nBehavior must depend on @polymer/polymer NPM package themselves
    i18n-behavior i18n-element Polymer lit-html
    5.x 5.x 3.x (optional) 2.x
    4.x 4.x 3.x (optional) 1.x
    3.x 3.x 3.x (mandatory) 1.x
    2.x 2.x 1.x-2.x -
    1.x - 1.x -

    Browser Compatibility

    TODO: To be updated

    Browser Chrome Firefox Edge 13+ IE 11 Safari 10+ Chrome Android Mobile Safari Opera
    • Polyfilled by @webcomponents/webcomponentsjs/webcomponents-{bundle|loader}.js

    Conceptual Workflow

    • demo/gulpfile.js provides support for extracting UI strings from html tagged template literals in JavaScript sources as well as Polymer HTML templates in HTML Imports


        npm install i18n-behavior

    Import for Polymer elements

        import { I18nBehavior } from 'i18n-behavior/i18n-behavior.js'

    Quick Tour

    I18N-ready pwa-starter-kit

        npm install -g polymer-cli
        git clone https://github.com/t2ym/pwa-starter-kit
        cd pwa-starter-kit
        npm ci
        # Add Locales
        gulp locales --targets="de es fr ja zh-Hans"
        # I18N Process
        # Translate XLIFF ./xliff/bundle.*.xlf
        # Merge Translation
        # Dev build on http://localhost:8080
        polymer serve
        # Static build
        polymer build
        # Static build on http://localhost:8080
        cd build/{esm-unbundled|esm-bundled|es6-bundled|es5-bundled}
        python -m SimpleHTTPServer -p 8080

    Usage in Polymer legacy syntax

    Run-time Automatic I18N

    Apply BehaviorsStore.I18nBehavior or imported I18nBehavior

    Source Code

        // Legacy Polymer syntax
          importMeta: import.meta,
          is: 'custom-element',
          _template: html`
            <span id="label">UI text label</span> <!-- no need to touch UI text strings -->
          behaviors: [
            I18nBehavior // Add this line for I18N

    I18N-ready preprocessed DOM at element registration

    Hard-coded UI text strings are automatically extracted and replaced with annotations bound to text object.

    lang attribute specifies the current locale. By default, <html lang> attribute is observed and mirrored to those for I18N-ready element instances.

        <html lang="en"><!-- html lang attribute is observed and mirrored -->
          <custom-element lang="en">
            <span id="label">{{text.label}}</span><!-- UI texts are bound to text object -->

    If the containing element of the target text has id attribute, the string id is named with the id value. If not, the string id is automatically generated. It is recommended to put meaningful id to each string for robustness. When attaching id attribute is too much for the containing element, text-id attribute can be used instead.

        <span text-id="label">{{text.label}}</span>

    text dynamic property

    this.text dynamic object property represents an object with UI text strings for the current locale.

        this.text = {
          "label": "UI Text Label"

    this.text dynamic object is SHARED among all the instances of the same custom element.

    model dynamic property

    this.model is deepcopied from this.text.model per instance to store I18N target attribute values. UI text strings in I18N target attributes are automatically extracted and replaced with annotations according to the shared repository (i18n-attr-repo) of I18N target attributes per elements (like placeholder attribute of input element).

    On lang-updated event, this.text.model is updated but this.model is NOT automatically updated and needs explicit update like this.

        listeners: {
          'lang-updated': '_langUpdated'
        _langUpdated: function (event) {
          if (Polymer.dom(event).rootTarget === this) {
            this.model = deepcopy(this.text.model);

    <json-data> for manual text definitions

    Optionally, any JSON data can be manually added to I18N target strings via <json-data> element. This option is effective for manual extraction of hard-coded UI text strings in JavaScript literals.

      <dom-module id="my-element">
          ... <!-- ordinary template for rendering -->
          <template><!-- containing template element to avoid rendering -->
            <json-data id="items">[
              "Responsive Web App boilerplate",
              "Iron Elements and Paper Elements",
              "End-to-end Build Tooling (including Vulcanize)",
              "Unit testing with Web Component Tester",
              "Routing with Page.js",
              "Offline support with the Platinum Service Worker Elements"
            <json-data id="sender">{ "name": "Sam", "gender": "male" }</json-data>
        this.text.items[0] === 'Responsive Web App boilerplate'
        this.text.sender.name === 'Sam'

    Localized text strings fetched from JSON

    While default text strings are extracted from the hard-coded strings in HTML template, localized text strings are asynchronously fetched from JSON files under locales directory at the server.


    Build-time Automatic I18N

    gulp-i18n-preprocess filter performs build-time automatic I18N and embeds UI texts as JSON.

    I18N-ready Source Code preprocessed by gulp-i18n-preprocess:

        <dom-module id="custom-element">
          <template localizable-text="embedded">
            <span id="label">{{text.label}}</span>
            <template id="localizable-text">
                "label": "UI Text Label"

    Default text values are immediately extracted from the embedded JSON without overheads of run-time traversal into the whole template.




    npm i i18n-behavior

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    • t2ym