3 files changed, 64 insertions, 0 deletions
diff --git a/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.cpp b/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.cpp
index 10e188eee6..4b16af0711 100644
--- a/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.cpp
@@ -5,6 +5,7 @@
  */
 
 #include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/Array.h>
 #include <LibJS/Runtime/GlobalObject.h>
 #include <LibJS/Runtime/Intl/AbstractOperations.h>
 #include <LibJS/Runtime/Intl/Segmenter.h>
@@ -28,6 +29,9 @@ void SegmenterConstructor::initialize(GlobalObject& global_object)
     // 18.2.1 Intl.Segmenter.prototype, https://tc39.es/ecma402/#sec-intl.segmenter.prototype
     define_direct_property(vm.names.prototype, global_object.intl_segmenter_prototype(), 0);
     define_direct_property(vm.names.length, Value(0), Attribute::Configurable);
+
+    u8 attr = Attribute::Writable | Attribute::Configurable;
+    define_native_function(vm.names.supportedLocalesOf, supported_locales_of, 1, attr);
 }
 
 // 18.1.1 Intl.Segmenter ( [ locales [ , options ] ] ), https://tc39.es/ecma402/#sec-intl.segmenter
@@ -83,4 +87,19 @@ ThrowCompletionOr<Object*> SegmenterConstructor::construct(FunctionObject& new_t
     return segmenter;
 }
 
+// 18.2.2 Intl.Segmenter.supportedLocalesOf ( locales [ , options ] ), https://tc39.es/ecma402/#sec-intl.segmenter.supportedlocalesof
+JS_DEFINE_NATIVE_FUNCTION(SegmenterConstructor::supported_locales_of)
+{
+    auto locales = vm.argument(0);
+    auto options = vm.argument(1);
+
+    // 1. Let availableLocales be %Segmenter%.[[AvailableLocales]].
+
+    // 2. Let requestedLocales be ? CanonicalizeLocaleList(locales).
+    auto requested_locales = TRY(canonicalize_locale_list(global_object, locales));
+
+    // 3. Return ? SupportedLocales(availableLocales, requestedLocales, options).
+    return TRY(supported_locales(global_object, requested_locales, options));
+}
+
 }
diff --git a/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.h b/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.h
index ee789b7de8..4c27e3908a 100644
--- a/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.h
+++ b/Userland/Libraries/LibJS/Runtime/Intl/SegmenterConstructor.h
@@ -23,6 +23,8 @@ public:
 
 private:
     virtual bool has_constructor() const override { return true; }
+
+    JS_DECLARE_NATIVE_FUNCTION(supported_locales_of);
 };
 
 }
diff --git a/Userland/Libraries/LibJS/Tests/builtins/Intl/Segmenter/Segmenter.supportedLocalesOf.js b/Userland/Libraries/LibJS/Tests/builtins/Intl/Segmenter/Segmenter.supportedLocalesOf.js
new file mode 100644
index 0000000000..bb0407147e
--- /dev/null
+++ b/Userland/Libraries/LibJS/Tests/builtins/Intl/Segmenter/Segmenter.supportedLocalesOf.js
@@ -0,0 +1,43 @@
+describe("correct behavior", () => {
+    test("length is 1", () => {
+        expect(Intl.Segmenter.supportedLocalesOf).toHaveLength(1);
+    });
+
+    test("basic functionality", () => {
+        // prettier-ignore
+        const values = [
+            [[], []],
+            [undefined, []],
+            ["en", ["en"]],
+            [new Intl.Locale("en"), ["en"]],
+            [["en"], ["en"]],
+            [["en", "en-gb", "en-us"], ["en", "en-GB", "en-US"]],
+            [["en", "de", "fr"], ["en", "de", "fr"]],
+            [["en-foobar"], ["en-foobar"]],
+            [["en-foobar-u-abc"], ["en-foobar-u-abc"]],
+            [["aa", "zz"], []],
+            [["en", "aa", "zz"], ["en"]],
+        ];
+        for (const [input, expected] of values) {
+            expect(Intl.Segmenter.supportedLocalesOf(input)).toEqual(expected);
+            // "best fit" (implementation defined) just uses the same implementation as "lookup" at the moment
+            expect(Intl.Segmenter.supportedLocalesOf(input, { localeMatcher: "best fit" })).toEqual(
+                Intl.Segmenter.supportedLocalesOf(input, { localeMatcher: "lookup" })
+            );
+        }
+    });
+});
+
+describe("errors", () => {
+    test("invalid value for localeMatcher option", () => {
+        expect(() => {
+            Intl.Segmenter.supportedLocalesOf([], { localeMatcher: "foo" });
+        }).toThrowWithMessage(RangeError, "foo is not a valid value for option localeMatcher");
+    });
+
+    test("invalid language tag", () => {
+        expect(() => {
+            Intl.Segmenter.supportedLocalesOf(["aaaaaaaaa"]);
+        }).toThrowWithMessage(RangeError, "aaaaaaaaa is not a structurally valid language tag");
+    });
+});