ICU Message Formatter

Format ICU message syntax strings from supplied parameters and your own configurable types.

This is a low-level API alternative to string formatting libraries like intl-messageformat (the underlying formatting library behind react-intl) which often blow out bundle sizes because of the sheer amount of formatting capabilities included by default. With this library, you add/configure your own formatters to keep your bundle size as light as it can possibly be.

Out of the box, the only formatting this library does is basic replacement of {tokens} with strings. Some features are part of this library, but must explicitly be configured. Everything else is BYO formatter (some examples below).

Installation

As a module in a JavaScript project:

npm install @ultraq/icu-message-formatter

As a script for the browser via the unpkg CDN:

An IIFE version of this library is available at: https://unpkg.com/@ultraq/icu-message-formatter/dist/icu-message-formatter.browser.min.js In this form, this module will then be present in the global scope as IcuMessageFormatter.

An ESM version of this library is available at: https://unpkg.com/@ultraq/icu-message-formatter/dist/icu-message-formatter.browser.es.min.js That URL can be used directly in ESM scripts made for the browser, and otherwise works like the NPM package.

Usage

Create a new MessageFormatter instance and configure it with any type handlers that you know you'll encounter in your format strings. Then, call format to process whatever string and data you throw at it for the locale you want it in:

import {MessageFormatter} from '@ultraq/icu-message-formatter';
// const {MessageFormatter} = IcuMessageFormatter; // If in a browser context using the IIFE bundle

let formatter = new MessageFormatter('en-NZ', {
  currency: ({value, currency}, options, locale, values) => {
     return new Intl.NumberFormat(locale, { style: 'currency', currency }).format(value);
  }
});

let message = formatter.format('Hey {name}, that\'s gonna cost you {amount, currency}!', {
  name: 'Emanuel',
  amount: {
    value: 2,
    currency: 'GBP'
  }
});

console.log(message); // "Hey Emanuel, that's gonna cost you £2.00!"

API

MessageFormatter

import {MessageFormatter} from '@ultraq/icu-message-formatter';

The main class for formatting messages.

new MessageFormatter(locale, typeHandlers = {})

Creates a new formatter that can work using any of the custom type handlers you register.

locale: the locale to use for formatting
typeHandlers: optional object where the keys are the names of the types to register, their values being the functions that will return a nicely formatted string for the data and locale they are given. Type handlers are passed 5 parameters:
- the object which matched the key of the block being processed
- any format options associated with the block being processed
- the locale to use for formatting
- the object of placeholder data given to the original format/process call
- and the process function itself (see below) so that sub-messages can be processed by type handlers

format(message, values = {})

Formats an ICU message syntax string using values for placeholder data and any currently-registered type handlers.

message: the ICU message string to format
values: object of placeholder data to fill out the message

process(message, values = {})

Process an ICU message syntax string using values for placeholder data and any currently-registered type handlers. The result of this method is an array of the component parts after they have been processed in turn by their own type handlers. This raw output is useful for other renderers, eg: React where components can be used instead of being forced to return raw strings.

message: the ICU message string to format
values: object of placeholder data to fill out the message

Type handlers available in this library

pluralTypeHandler

import {pluralTypeHandler} from '@ultraq/icu-message-formatter';

Handler for plural statements within ICU message syntax strings. See https://formatjs.io/docs/core-concepts/icu-syntax#plural-format for more details on how the plural statement works.

For the special # placeholder, it will be processed as if it were {key, number}, using the number handler that has been registered with the current message formatter instance. If none has been registered, then the fallback behaviour will be invoked (which is to emit the value of key).

selectTypeHandler

import {selectTypeHandler} from '@ultraq/icu-message-formatter';

Handler for select statements within ICU message syntax strings. See https://formatjs.io/docs/core-concepts/icu-syntax#select-format for more details on how the select statement works.

Other exported utilities

findClosingBracket(string, fromIndex)

import {findClosingBracket} from '@ultraq/icu-message-formatter';

Finds the index of the next closing curly bracket, }, including in strings that could have nested brackets. Returns the index of the matching closing bracket, or -1 if no closing bracket could be found.

string:
fromIndex:

splitFormattedArgument(block)

import {splitFormattedArgument} from '@ultraq/icu-message-formatter';

Split a {key, type, format} block into those 3 parts, taking into account nested message syntax that can exist in the format part. Returns an array with key, type, and format items in that order, if present in the formatted argument block.

block: