Intl.Locale.prototype.baseName

const myLoc = new Intl.Locale("fr-Latn-CA"); // Sets locale to Canadian French
console.log(myLoc.toString()); // "fr-Latn-CA-u-ca-gregory"
console.log(myLoc.baseName); // "fr-Latn-CA"

// Sets language to Japanese, region to Japan,
// calendar to Gregorian, hour cycle to 24 hours
const japan = new Intl.Locale("ja-JP-u-ca-gregory-hc-24");
console.log(japan.toString()); // "ja-JP-u-ca-gregory-hc-h24"
console.log(japan.baseName); // "ja-JP"

// Input string indicates language as Dutch and region as Belgium,
// but options object overrides the region and sets it to the Netherlands
const dutch = new Intl.Locale("nl-Latn-BE", { region: "NL" });

console.log(dutch.baseName); // "nl-Latn-NL"