A locale's collation rules are used to determine how strings are ordered in that locale. Certain locales use a character's case (UPPERCASE or lowercase) in the collation process. This additional rule can be expressed in a Intl.Locale object's caseFirst property.

There are 3 values that the caseFirst property can have, outlined in the table below.

In the Unicode locale string spec, the values that caseFirst represents correspond to the key kf. kf is treated as a locale string "extension subtag". These subtags add additional data about the locale, and are added to locale identifiers by using the -u extension key. Thus, the caseFirst value can be added to the initial locale identifier string that is passed into the Locale constructor. To add the caseFirst value, first add the -u extension key to the string. Next, add the -kf extension key to indicate that you are adding a value for caseFirst. Finally, add the caseFirst value to the string.

const locale = new Intl.Locale("fr-Latn-FR-u-kf-upper");
console.log(locale.caseFirst); // Prints "upper"

The Intl.Locale() constructor has an optional configuration object argument, which can be used to pass extension types. Set the caseFirst property of the configuration object to your desired caseFirst value, and then pass it into the constructor.

const locale = new Intl.Locale("en-Latn-US", { caseFirst: "lower" });
console.log(locale.caseFirst); // Prints "lower"

• Intl.Locale
• Unicode case first collation spec