Intl.Locale.prototype.caseFirst
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since September 2020.
The caseFirst accessor property of Intl.Locale instances returns whether case is taken into account for this locale's collation rules.
Description
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.
| 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. |
The caseFirst property's value is set at construction time, either through the kf key of the locale identifier or through the caseFirst option of the Intl.Locale() constructor. The latter takes priority if they are both present; and if neither is present, the property has value undefined.
The set accessor of caseFirst is undefined. You cannot change this property directly.
Examples
Like other locale subtags, the caseFirst value can be added to the Intl.Locale object via the locale string, or a configuration object argument to the constructor.
Adding a caseFirst value via the locale string
In the Unicode locale string spec, caseFirst is an "extension subtag". These subtags add additional data about the locale, and are added to locale identifiers using the -u extension key. To add the caseFirst value to the initial locale identifier string passed into the Intl.Locale() constructor, first add the -u extension key if it doesn't exist. Next, add the -kf extension to indicate that you are adding a value for caseFirst. Finally, add the caseFirst value.
const locale = new Intl.Locale("fr-Latn-FR-u-kf-upper");
console.log(locale.caseFirst); // "upper"
Adding a caseFirst value via the configuration object argument
The Intl.Locale() constructor has an optional configuration object argument, which can contain any of several extension types, including caseFirst. 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); // "lower"
Specifications
| Specification |
|---|
| ECMAScript® 2026 Internationalization API Specification # sec-Intl.Locale.prototype.caseFirst |
Browser compatibility
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