Equivalent of `supportedLocalesOf` for letter-case conversion (`toLocaleLowerCase`, `toLocaleUpperCase`)

💡 Ideas
1

I originally asked on StackOverflow if there's any way of introspecting the supported locales supported by toLocaleLowerCase and toLocaleUpperCase, and the answer seems to be a resounding no.

In JavaScript, you can introspect support for various internationalization functionality by use of static supportedLocalesOf methods of the various classes namespaced under Intl. For example:

Intl.PluralRules.supportedLocalesOf(['pt', 'yue', 'hyw']) // ['pt', 'yue']
Intl.Segmenter.supportedLocalesOf(['pt', 'yue', 'hyw']) // ['pt']

I'm not talking about feature detection, which can be done on an ad-hoc basis if you know how to feature detect specific characteristics of the locale:

const turkishIsSupported = 'i'.toLocaleUpperCase('tr') === 'Ä°'

However, checking like this requires custom logic and knowing what features to test for. What would be useful is a simple way of checking that a given locale is supported, without prior knowledge of that locale's features.

I'm not sure where such a method would logically live, given that there is no namespaced class under Intl for letter-case conversion methods. Some options:

  • Add one (seems a bit overkill if it's only used for this one purpose):
    Intl.LetterCase.supportedLocalesOf(locales)
// or...
Intl.CaseMapper.supportedLocalesOf(locales)
  • Attach the method as a property of the string prototype methods (is there much precedent for this in the web platform?)
    String.prototype.toLocaleLowerCase.supportedLocalesOf(locales)
'xyz'.toLocaleUpperCase.supportedLocalesOf(locales)
  • A top-level method on Intl:
    Intl.supportedLocales('letter-case', locales)
Intl.supportedLocales('plural-rules', locales)
  • Something else?