LibJS: Implement a naive String.prototype.localeCompare

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -253,6 +253,7 @@ namespace JS {
     P(length)                                \
     P(link)                                  \
     P(load)                                  \
+    P(localeCompare)                         \
     P(log)                                   \
     P(log1p)                                 \
     P(log2)                                  \

Userland/Libraries/LibJS/Runtime/StringPrototype.cpp

@@ -153,6 +153,7 @@ void StringPrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.strike, strike, 0, attr);
     define_native_function(vm.names.sub, sub, 0, attr);
     define_native_function(vm.names.sup, sup, 0, attr);
+    define_native_function(vm.names.localeCompare, locale_compare, 1, attr);
     define_native_function(*vm.well_known_symbol_iterator(), symbol_iterator, 0, attr);
 }
 
@@ -1202,4 +1203,25 @@ JS_DEFINE_NATIVE_FUNCTION(StringPrototype::sup)
     return create_html(global_object, vm.this_value(global_object), "sup", String::empty(), Value());
 }
 
+// 22.1.3.10 String.prototype.localeCompare ( that [ , reserved1 [ , reserved2 ] ] ), https://tc39.es/ecma262/#sec-string.prototype.localecompare
+// NOTE: This is the minimum localeCompare implementation for engines without ECMA-402.
+JS_DEFINE_NATIVE_FUNCTION(StringPrototype::locale_compare)
+{
+    auto string = ak_string_from(vm, global_object);
+    if (!string.has_value())
+        return {};
+
+    auto that_string = vm.argument(0).to_string(global_object);
+    if (vm.exception())
+        return {};
+
+    // FIXME: Actually compare the string not just according to their bits.
+    if (string == that_string)
+        return Value(0);
+    if (string.value() < that_string)
+        return Value(-1);
+
+    return Value(1);
+}
+
 }

Userland/Libraries/LibJS/Runtime/StringPrototype.h

@@ -69,6 +69,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(strike);
     JS_DECLARE_NATIVE_FUNCTION(sub);
     JS_DECLARE_NATIVE_FUNCTION(sup);
+    JS_DECLARE_NATIVE_FUNCTION(locale_compare);
 
     JS_DECLARE_NATIVE_FUNCTION(symbol_iterator);
 };

Userland/Libraries/LibJS/Tests/builtins/String/String.prototype.localeCompare.js

@@ -0,0 +1,39 @@
+test("basic functionality", () => {
+    expect(String.prototype.localeCompare).toHaveLength(1);
+
+    expect("".localeCompare("")).toBe(0);
+    expect("a".localeCompare("a")).toBe(0);
+    expect("6".localeCompare("6")).toBe(0);
+
+    function compareBoth(a, b) {
+        const aTob = a.localeCompare(b);
+        const bToa = b.localeCompare(a);
+
+        expect(aTob).toBe(1);
+        expect(aTob).toBe(-bToa);
+    }
+
+    compareBoth("a", "");
+    compareBoth("1", "");
+    compareBoth("a", "A");
+    compareBoth("7", "3");
+    compareBoth("0000", "0");
+
+    expect("undefined".localeCompare()).toBe(0);
+    expect("undefined".localeCompare(undefined)).toBe(0);
+
+    expect("null".localeCompare(null)).toBe(0);
+    expect("null".localeCompare(undefined)).not.toBe(0);
+    expect("null".localeCompare()).toBe(-1);
+
+    expect(() => {
+        String.prototype.localeCompare.call(undefined, undefined);
+    }).toThrowWithMessage(TypeError, "undefined cannot be converted to an object");
+});
+
+test("UTF-16", () => {
+    var s = "😀😀";
+    expect(s.localeCompare("😀😀")).toBe(0);
+    expect(s.localeCompare("\ud83d")).toBe(1);
+    expect(s.localeCompare("😀😀s")).toBe(-1);
+});