The Intl.RelativeTimeFormat.supportedLocalesOf() static method returns an array containing those of the provided locales that are supported in date and time formatting without having to fall back to the runtime's default locale.

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Intl.RelativeTimeFormat.supportedLocalesOf(locales, options)



A string with a BCP 47 language tag, or an array of such strings. For the general form and interpretation of the locales argument, see the parameter description on the Intl main page.

options Optional

An object that may have the following property:


The locale matching algorithm to use. Possible values are "lookup" and "best fit"; the default is "best fit". For information about this option, see the Intl page.

Return value

An array of strings representing a subset of the given locale tags that are supported in date and time formatting without having to fall back to the runtime's default locale.


Returns an array with a subset of the language tags provided in locales. The language tags returned are those for which the runtime supports a locale in date and time formatting that the locale matching algorithm used considers a match, so that it wouldn't have to fall back to the default locale.


Using supportedLocalesOf()

Assuming a runtime that supports Indonesian and German but not Balinese in date and time formatting, supportedLocalesOf returns the Indonesian and German language tags unchanged, even though pinyin collation is neither relevant to date and time formatting nor used with Indonesian, and a specialized German for Indonesia is unlikely to be supported. Note the specification of the "lookup" algorithm here — a "best fit" matcher might decide that Indonesian is an adequate match for Balinese since most Balinese speakers also understand Indonesian, and therefore return the Balinese language tag as well.

const locales = ["ban", "id-u-co-pinyin", "de-ID"];
const options = { localeMatcher: "lookup" };
  Intl.RelativeTimeFormat.supportedLocalesOf(locales, options).join(", "),
// "id-u-co-pinyin, de-ID"


ECMAScript Internationalization API Specification
# sec-Intl.RelativeTimeFormat.supportedLocalesOf

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