LibJS: Implement Array.prototype.toReversed()

Userland/Libraries/LibJS/Runtime/ArrayPrototype.cpp

@@ -75,6 +75,7 @@ void ArrayPrototype::initialize(GlobalObject& global_object)
     define_native_function(vm.names.copyWithin, copy_within, 2, attr);
     define_native_function(vm.names.group, group, 1, attr);
     define_native_function(vm.names.groupToMap, group_to_map, 1, attr);
+    define_native_function(vm.names.toReversed, to_reversed, 0, attr);
 
     // Use define_direct_property here instead of define_native_function so that
     // Object.is(Array.prototype[Symbol.iterator], Array.prototype.values)
@@ -85,6 +86,7 @@ void ArrayPrototype::initialize(GlobalObject& global_object)
     // 23.1.3.35 Array.prototype [ @@unscopables ], https://tc39.es/ecma262/#sec-array.prototype-@@unscopables
     // With find from last proposal, https://tc39.es/proposal-array-find-from-last/#sec-array.prototype-@@unscopables
     // With array grouping proposal, https://tc39.es/proposal-array-grouping/#sec-array.prototype-@@unscopables
+    // With change array by copy proposal, https://tc39.es/proposal-change-array-by-copy/#sec-array.prototype-@@unscopables
     auto* unscopable_list = Object::create(global_object, nullptr);
     MUST(unscopable_list->create_data_property_or_throw(vm.names.at, Value(true)));
     MUST(unscopable_list->create_data_property_or_throw(vm.names.copyWithin, Value(true)));
@@ -100,6 +102,7 @@ void ArrayPrototype::initialize(GlobalObject& global_object)
     MUST(unscopable_list->create_data_property_or_throw(vm.names.groupToMap, Value(true)));
     MUST(unscopable_list->create_data_property_or_throw(vm.names.includes, Value(true)));
     MUST(unscopable_list->create_data_property_or_throw(vm.names.keys, Value(true)));
+    MUST(unscopable_list->create_data_property_or_throw(vm.names.toReversed, Value(true)));
     MUST(unscopable_list->create_data_property_or_throw(vm.names.values, Value(true)));
 
     define_direct_property(*vm.well_known_symbol_unscopables(), unscopable_list, Attribute::Configurable);
@@ -1781,4 +1784,38 @@ JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::group_to_map)
     return map;
 }
 
+// 1.1.1.4 Array.prototype.toReversed ( ), https://tc39.es/proposal-change-array-by-copy/#sec-array.prototype.toReversed
+JS_DEFINE_NATIVE_FUNCTION(ArrayPrototype::to_reversed)
+{
+    // 1. Let O be ? ToObject(this value).
+    auto* object = TRY(vm.this_value(global_object).to_object(global_object));
+
+    // 2. Let len be ? LengthOfArrayLike(O).
+    auto length = TRY(length_of_array_like(global_object, *object));
+
+    // 3. Let A be ? ArrayCreate(𝔽(len)).
+    auto* array = TRY(Array::create(global_object, length));
+
+    // 4. Let k be 0.
+    // 5. Repeat, while k < len,
+    for (size_t k = 0; k < length; ++k) {
+        // a. Let from be ! ToString(𝔽(len - k - 1)).
+        auto from = PropertyKey { length - k - 1 };
+
+        // b. Let Pk be ! ToString(𝔽(k)).
+        auto property_key = PropertyKey { k };
+
+        // c. Let fromValue be ? Get(O, from).
+        auto from_value = TRY(object->get(from));
+
+        // d. Perform ! CreateDataPropertyOrThrow(A, Pk, fromValue).
+        MUST(array->create_data_property_or_throw(property_key, from_value));
+
+        // e. Set k to k + 1.
+    }
+
+    // 6. Return A.
+    return array;
+}
+
 }

Userland/Libraries/LibJS/Runtime/ArrayPrototype.h

@@ -56,6 +56,7 @@ private:
     JS_DECLARE_NATIVE_FUNCTION(copy_within);
     JS_DECLARE_NATIVE_FUNCTION(group);
     JS_DECLARE_NATIVE_FUNCTION(group_to_map);
+    JS_DECLARE_NATIVE_FUNCTION(to_reversed);
 };
 
 ThrowCompletionOr<void> array_merge_sort(GlobalObject&, FunctionObject* compare_func, MarkedVector<Value>& arr_to_sort);

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -485,6 +485,7 @@ namespace JS {
     P(toPlainTime)                           \
     P(toPlainYearMonth)                      \
     P(toPrecision)                           \
+    P(toReversed)                            \
     P(toString)                              \
     P(total)                                 \
     P(toTemporalInstant)                     \

Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype-generic-functions.js

@@ -252,6 +252,7 @@ describe("ability to work with generic non-array objects", () => {
     });
 
     test("reverse", () => {
+        const o = { length: 5, 0: "foo", 1: "bar", 3: "baz" };
         expect(Array.prototype.reverse.call(o)).toEqual({
             length: 5,
             4: "foo",
@@ -341,4 +342,10 @@ describe("ability to work with generic non-array objects", () => {
         const trueResult = result.get(trueObject);
         expect(trueResult).toEqual(["bar", "baz"]);
     });
+
+    test("toReversed", () => {
+        const result = Array.prototype.toReversed.call(o);
+        expect(result).toEqual([undefined, "baz", undefined, "bar", "foo"]);
+        expect(result).not.toBe(o);
+    });
 });

Userland/Libraries/LibJS/Tests/builtins/Array/Array.prototype.toReversed.js

@@ -0,0 +1,39 @@
+describe("normal behavior", () => {
+    test("length is 0", () => {
+        expect(Array.prototype.toReversed).toHaveLength(0);
+    });
+
+    test("basic functionality", () => {
+        const a = [1, 2, 3, 4, 5];
+        const b = a.toReversed();
+        expect(a).not.toBe(b);
+        expect(a).toEqual([1, 2, 3, 4, 5]);
+        expect(b).toEqual([5, 4, 3, 2, 1]);
+    });
+
+    test("is unscopable", () => {
+        expect(Array.prototype[Symbol.unscopables].toReversed).toBeTrue();
+        const array = [];
+        with (array) {
+            expect(() => {
+                toReversed;
+            }).toThrowWithMessage(ReferenceError, "'toReversed' is not defined");
+        }
+    });
+});
+
+describe("errors", () => {
+    test("null or undefined this value", () => {
+        expect(() => {
+            Array.prototype.toReversed.call();
+        }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+
+        expect(() => {
+            Array.prototype.toReversed.call(undefined);
+        }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+
+        expect(() => {
+            Array.prototype.toReversed.call(null);
+        }).toThrowWithMessage(TypeError, "ToObject on null or undefined");
+    });
+});