Unicode MessageFormat for JavaScript

This library provides a formatter and other tools for Unicode MessageFormat 2.0 (MF2), the new standard for localization developed by the MessageFormat Working Group.

This includes a formatter that can be used as a polyfill for the proposed ECMA-402 Intl.MessageFormat formatter.

The API provided by this library is current as of the LDML 48 (October 2025) version of the MF2 specification.

npm install --save messageformat

import { MessageFormat } from 'messageformat';
import { DraftFunctions } from 'messageformat/functions';

const msg = 'Today is {$today :datetime dateStyle=medium}';
const mf = new MessageFormat('en', msg, { functions: DraftFunctions });

mf.format({ today: new Date('2022-02-02') });
// 'Today is Feb 2, 2022'

The library also provides a number of other tools and utilities for MF2, such as:

• MF2 data model conversion tools

  import { parseMessage, stringifyMessage } from 'messageformat';
  

• MF2 data model validation and transformation tools

  import { validate, visit } from 'messageformat';
  

• Concreate Syntax Tree (CST) tools for MF2

  import { parseCST, messageFromCST, stringifyCST } from 'messageformat/cst';
  

• Utilities for building custom function handlers for MF2

  import {
    DraftFunctions,
    asPositiveInteger,
    asString
  } from 'messageformat/functions';
  

In addition to supporting MF2 syntax, compilers and formatting function runtimes are also provided for ICU MessageFormat and Fluent messages:

• @messageformat/icu-messageformat-1
• @messageformat/fluent

For more information on the types and functions provided by this package, see the documentation site.