The Intl.Locale.prototype.caseFirst property is an accessor property that returns whether case is taken into account for the locale's collation rules.


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 Locale's caseFirst property.

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

caseFirst values

Value Description
upper Upper case to be sorted before lower case.
lower Lower case to be sorted before upper case.
false No special case ordering.


Setting the caseFirst value via the locale string

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 inital 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.

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

Setting the caseFirst value via the configuration object argument

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.

let caseFirstObj= new Intl.Locale("en-Latn-US", {caseFirst: "lower"});
console.log(us12hour.caseFirst); // Prints "lower"


Browser compatibilityUpdate compatibility data on GitHub

Chrome Edge Firefox Internet Explorer Opera Safari
caseFirst 74 79 75 No 62 14
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
caseFirst 74 74 No 53 14 11.0
caseFirst 12.0.0
Before version 13.0.0, only the locale data for en-US is available by default. See the Locale() constructor for more details.

See also

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