intl

Intl is a built-in JavaScript object that provides language-sensitive functionality for internationalization and localization. It is defined by the ECMAScript Internationalization API (ECMA-402). The object offers constructors and related utilities that enable locale-aware formatting and comparison for numbers, dates and times, currencies, lists, and text, as well as locale display names.

Core constructors include Intl.NumberFormat, Intl.DateTimeFormat, and Intl.Collator. More recent additions include Intl.RelativeTimeFormat, Intl.ListFormat, Intl.DisplayNames, Intl.Segmenter, and Intl.Locale, along with utility methods such as Intl.getCanonicalLocales. These APIs allow developers to format data according to a specified locale (for example "en-US", "ja-JP" or "de-DE"), and to fall back gracefully when a locale is unavailable.

How it works: formatting and collation rules are driven by locale data, which is typically sourced from the Unicode Common Locale Data Repository (CLDR) and, in many environments, ICU data. This enables locale-aware rules for number styles, currency placement, date and time formats, string sorting, and more. The API is designed to be progressive, with environments often supporting a core subset first and expanding over time.

Usage considerations include varying levels of support across browsers and runtimes, potential differences in locale coverage, and the need for fallbacks when certain locales or features are not available. Developers may also use polyfills to provide consistent behavior in older environments.

See also ECMA-402 and Unicode CLDR for the standards and data underpinning Intl functionality.