String.prototype.toLocaleUpperCase()

locales Optional

A string with a BCP 47 language tag, or an array of such strings. Indicates the locale to be used to convert to upper case according to any locale-specific case mappings. For the general form and interpretation of the locales argument, see the parameter description on the Intl main page.

Unlike other methods that use the locales argument, toLocaleUpperCase() does not allow locale matching. Therefore, after checking the validity of the locales argument, toLocaleUpperCase() always uses the first locale in the list (or the default locale if the list is empty), even if this locale is not supported by the implementation.

A new string representing the calling string converted to upper case, according to any locale-specific case mappings.

The toLocaleUpperCase() method returns the value of the string converted to upper case according to any locale-specific case mappings. toLocaleUpperCase() does not affect the value of the string itself. In most cases, this will produce the same result as toUpperCase(), but for some locales, such as Turkish, whose case mappings do not follow the default case mappings in Unicode, there may be a different result.

Also notice that conversion is not necessarily a 1:1 character mapping, as some characters might result in two (or even more) characters when transformed to upper-case. Therefore the length of the result string can differ from the input length. This also implies that the conversion is not stable, so for example the following can return false: x.toLocaleLowerCase() === x.toLocaleUpperCase().toLocaleLowerCase()

• String.prototype.toLocaleLowerCase()
• String.prototype.toLowerCase()
• String.prototype.toUpperCase()