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3.7.0 • Public • Published

Tag MessageFormat

Formats ICU Message strings with tags, number, date, plural, and select placeholders to create localized messages.

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This is a fork of intl-messageformat

Differences from the original package:

  • It uses the tag-messageformat-parser
  • Tags are supported in messages - this is not part of the ICU message "spec"
  • The other option defaults to required for plural, select and selectordinal - as defined in the ICU "spec"
  • Whitespace in plural messages is preserved
  • . is permitted to be used in argument and tag names

What is a tag?

A tag enables style placeholders to be included in the translation message without including any of the style information in the translation message.

This provides 3 benefits:

  1. It decouples the styling of the text from the translations, allowing the styling to change independently of translations.
  2. It allows translation messages to retain context for text that will be styled
  3. Tags can be named to provide hints to translators

A tag must adhere to the following conventions:

  • begin with <x:
  • The tag name can include only numbers, ascii letters, underscore and dot ..
  • must be closed, self-closing tags are supported but should be used sparingly as they can be confusing for translators
  • Valid tag examples:
    • <x:0>hello</x:0>
    • <x:link>click me</x:link>
    • <x:emoji />

Here's an simple example:

import TagMessageFormat from 'tag-messageformat';
var enNumPhotos = new TagMessageFormat('By signing up you agree to our <x:link>terms and conditions</x:link>', 'en-US');
var output = enNumPhotos.format({
  link: (content) => `<a href="#">${content}</a>`
console.log(output); // => "By signing up you agree to our <a href="#">terms and conditions</a>"

Using descriptive names for tag names can provide hints to translators about the purpose of the tags. In the above example, the text terms and conditions will be used to display a link the user can click on.

Tags and arguments can be used in combination in ICU message formats.

This example uses a {name} argument in a tag.

var enNumPhotos = new TagMessageFormat('Welcome back <x:bold>{name}</x:bold>', 'en-US');
var output = enNumPhotos.format({
  bold: (content) => `<span class="boldText">${content}</span>`,
  name: 'Bob'
console.log(output); // => "Welcome back <span class="boldText">Bob</span>"



This package aims to provide a way for you to manage and format your JavaScript app's string messages into localized strings for people using your app. You can use this package in the browser and on the server via Node.js.

This implementation is based on the [Strawman proposal][strawman], but there are a few places this implementation diverges.

Note: This TagMessageFormat API may change to stay in sync with ECMA-402, but this package will follow semver.

How It Works

Messages are provided into the constructor as a String message, or a pre-parsed AST object.

var msg = new TagMessageFormat(message, locales, [formats]);

The string message is parsed, then stored internally in a compiled form that is optimized for the format() method to produce the formatted string for displaying to the user.

var output = msg.format(values);

Common Usage Example

A very common example is formatting messages that have numbers with plural labels. With this package you can make sure that the string is properly formatted for a person's locale, e.g.:

var MESSAGES = {
    'en-US': {
        NUM_PHOTOS: 'You have {numPhotos, plural, ' +
            '=0 {no photos.}' +
            '=1 {one photo.}' +
            'other {# photos.}}'
    'es-MX': {
        NUM_PHOTOS: 'Usted {numPhotos, plural, ' +
            '=0 {no tiene fotos.}' +
            '=1 {tiene una foto.}' +
            'other {tiene # fotos.}}'
var output;
var enNumPhotos = new TagMessageFormat(MESSAGES['en-US'].NUM_PHOTOS, 'en-US');
output = enNumPhotos.format({numPhotos: 1000});
console.log(output); // => "You have 1,000 photos."
var esNumPhotos = new TagMessageFormat(MESSAGES['es-MX'].NUM_PHOTOS, 'es-MX');
output = esNumPhotos.format({numPhotos: 1000});
console.log(output); // => "Usted tiene 1,000 fotos."

Message Syntax

The message syntax that this package uses is not proprietary, in fact it's a common standard message syntax that works across programming languages and one that professional translators are familiar with. This package uses the ICU Message syntax and works for all CLDR languages which have pluralization rules defined.


  • Uses industry standards: ICU Message syntax and CLDR locale data.

  • Supports plural, select, and selectordinal message arguments.

  • Formats numbers and dates/times in messages using Intl.NumberFormat and Intl.DateTimeFormat, respectively.

  • Optimized for repeated calls to an TagMessageFormat instance's format() method.

  • Supports defining custom format styles/options.

  • Supports escape sequences for message syntax chars, e.g.: "\\{foo\\}" will output: "{foo}" in the formatted output instead of interpreting it as a foo argument.


Intl Dependency

This package assumes that the Intl global object exists in the runtime. Intl is present in all modern browsers and there's work happening to integrate Intl into Node.js.

Luckly, there's the Intl.js polyfill! You will need to conditionally load the polyfill if you want to support runtimes which Intl is not already built-in.

Loading Intl.js Polyfill in a browser

If the browser does not already have the Intl APIs built-in, the Intl.js Polyfill will need to be loaded on the page along with the locale data for any locales that need to be supported:

<script src="intl/Intl.min.js"></script>
<script src="intl/locale-data/jsonp/en-US.js"></script>

Note: Modern browsers already have the Intl APIs built-in, so you can load the Intl.js Polyfill conditionally, by for checking for window.Intl.

Loading Intl.js Polyfill in Node.js

Conditionally require the Intl.js Polyfill if it doesn't already exist in the runtime. As of Node <= 0.10, this polyfill will be required.

if (!global.Intl) {

Note: When using the Intl.js Polyfill in Node.js, it will automatically load the locale data for all supported locales.

Loading Tag MessageFormat in a browser

<script src="tag-messageformat/tag-messageformat.min.js"></script>

By default, Tag MessageFormat ships with the locale data for English (en) built-in to the library's runtime. When you need to format data in another locale, include its data; e.g., for French:

<script src="tag-messageformat/locale-data/fr.js"></script>

Note: All 200+ languages supported by this package use their root BCP 47 language tag; i.e., the part before the first hyphen (if any).

Loading Intl MessageFormat in Node.js

Simply require() this package:

var TagMessageFormat, { stringBuilderFactory, arrayBuilderFactory } = require('tag-messageformat');

Note: in Node.js, the data for all 200+ languages is loaded along with the library.

Public API

TagMessageFormat Constructor

To create a message to format, use the TagMessageFormat constructor. The constructor takes four parameters:

  • message - {String | AST} - String message (or pre-parsed AST) that serves as formatting pattern.

  • locales - {String | String[]} - A string with a BCP 47 language tag, or an array of such strings. If you do not provide a locale, the default locale will be used. When an array of locales is provided, each item and its ancestor locales are checked and the first one with registered locale data is returned. See: Locale Resolution for more details.

  • [formats] - {Object} - Optional object with user defined options for format styles.

  • [options] - {Object}

    • requireOther: boolean - Optional object with option to prevent ICU message other option arguments. Set this to false for backward compatibility with react-intl.
    • stringFormatFactory: function - Optional factory function for creating StringFormat instances - used to format simple argument values.
var msg = new TagMessageFormat('My name is {name}.', 'en-US');
// Allow plural and select ICU messages to be defined without an `other` option
var msg = new TagMessageFormat('My name is {name}.', 'en-US', {}, { requireOther: false });

Locale Resolution

TagMessageFormat uses a locale resolution process similar to that of the built-in Intl APIs to determine which locale data to use based on the locales value passed to the constructor. The result of this resolution process can be determined by call the resolvedOptions() prototype method.

The following are the abstract steps TagMessageFormat goes through to resolve the locale value:

  • If no extra locale data is loaded, the locale will always resolved to "en".

  • If locale data is missing for a leaf locale like "fr-FR", but there is data for one of its ancestors, "fr" in this case, then its ancestor will be used.

  • If there's data for the specified locale, then that locale will be resolved; i.e.,

    var mf = new TagMessageFormat('', 'en-US');
    assert(mf.resolvedOptions().locale === 'en-US'); // true
  • The resolved locales are now normalized; e.g., "en-us" will resolve to: "en-US".

Note: When an array is provided for locales, the above steps happen for each item in that array until a match is found.

resolvedOptions() Method

This method returns an object with the options values that were resolved during instance creation. It currently only contains a locale property; here's an example:

var msg = new TagMessageFormat('', 'en-us');
console.log(msg.resolvedOptions().locale); // => "en-US"

Notice how the specified locale was the all lower-case value: "en-us", but it was resolved and normalized to: "en-US".

format(values?: Object, formatOptions?: FormatOptions) Method

Once the message is created, formatting the message is done by calling the format() method on the instance and passing a collection of values:

valid options:

  • messageBuilderFactory used to customize the format of the message. By default, a stringBuilderFactory is used.
  • messsageBuilderContext the context passed to each messageBuilder instance
import { stringBuilderFactory, arrayBuilderFactory } from 'tag-messageformat';
var output = msg.format({name: "Eric"}, { messageBuilderFactory: stringBuilderFactory });
console.log(output); // => "My name is Eric."

Note: A value must be supplied for every argument in the message pattern the instance was constructed with.

User Defined Formats

Define custom format styles is useful you need supply a set of options to the underlying formatter; e.g., outputting a number in USD:

var msg = new TagMessageFormat('The price is: {price, number, USD}', 'en-US', {
    number: {
        USD: {
            style   : 'currency',
            currency: 'USD'
var output = msg.format({price: 100});
console.log(output); // => "The price is: $100.00"

In this example, we're defining a USD number format style which is passed to the underlying Intl.NumberFormat instance as its options.


Plural Label

This example shows how to use the ICU Message syntax to define a message that has a plural label; e.g., "You have 10 photos":

You have {numPhotos, plural,
    =0 {no photos.}
    =1 {one photo.}
    other {# photos.}
var MESSAGES = {
    photos: '...', // String from code block above.
var msg = new TagMessageFormat(MESSAGES.photos, 'en-US');
console.log(msg.format({numPhotos: 0}));    // => "You have no photos."
console.log(msg.format({numPhotos: 1}));    // => "You have one photo."
console.log(msg.format({numPhotos: 1000})); // => "You have 1,000 photos."

Note: how when numPhotos was 1000, the number is formatted with the correct thousands separator.

Big Thanks

Cross-browser Testing Platform and Open Source <3 Provided by Sauce Labs


This software is free to use under the Yahoo! Inc. BSD license. See the LICENSE file for license text and copyright information.

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