String.prototype.toLocaleLowerCase()

The toLocaleLowerCase() method returns the calling string value converted to lower case, according to any locale-specific case mappings.

str.toLocaleLowerCase()
str.toLocaleLowerCase(locale)
str.toLocaleLowerCase([locale, locale, ...])

locale Optional

The locale parameter indicates the locale to be used to convert to lower case according to any locale-specific case mappings. If multiple locales are given in an Array, the best available locale is used. The default locale is the host environment’s current locale.

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

• A RangeError ("invalid language tag: xx_yy") is thrown if a locale argument isn't a valid language tag.
• A TypeError ("invalid element in locales argument") is thrown if an array element isn't of type string.

The toLocaleLowerCase() method returns the value of the string converted to lower case according to any locale-specific case mappings. toLocaleLowerCase() does not affect the value of the string itself. In most cases, this will produce the same result as toLowerCase(), 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.

'ALPHABET'.toLocaleLowerCase(); // 'alphabet'

'\u0130'.toLocaleLowerCase('tr') === 'i';    // true
'\u0130'.toLocaleLowerCase('en-US') === 'i'; // false

let locales = ['tr', 'TR', 'tr-TR', 'tr-u-co-search', 'tr-x-turkish'];
'\u0130'.toLocaleLowerCase(locales) === 'i'; // true

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