Format.JS

Format.JS is a modular collection of JavaScript libraries for internationalization (i18n) that provides industry-standard Unicode CLDR-based formatting for dates, times, numbers, currencies, relative time, units, lists, and pluralization across React, Vue, Angular, Ember, and vanilla JavaScript applications. Originally developed at Yahoo in 2014 (as part of the Yahoo Internation team led by Eric Ferraiuolo and Long Ho), Format.JS is now an independent open-source project maintained by FormatJS contributors with over 14,000 GitHub stars across its repositories. The core library, IntlMessageFormat, implements the ICU MessageFormat specification for parameterized message strings, supporting complex patterns like plurals (one, other, zero, few, many), select statements for gender-aware messages, nested arguments, number and date formatting within messages, and custom formatters. React Intl (@formatjs/intl) provides React-specific bindings including the IntlProvider component for locale configuration, FormattedMessage, FormattedDate, FormattedTime, FormattedNumber, FormattedRelativeTime, FormattedPlural components, the useIntl hook for function components, imperative API for server-side rendering, and message extraction tools for build-time compilation of translation messages to ICU format. The libraries support 150+ locales through CLDR (Common Locale Data Repository) data, handle right-to-left languages, time zone-aware date formatting via Intl API, currency formatting with proper symbols and grouping, compact number notation (1K, 1M, 1B), unit formatting (meters, kilometers, pounds), and list formatting with locale-specific conjunctions and disjunctions. Format.JS is used by major companies including Yahoo, Stripe, Twitch, Dropbox, and Lyft.