LibJS: Add an Iterator constructor and object

The Iterator object cannot be constructed directly but can be subclassed
or created with `Iterator.from` (not implemented here).

Userland/Libraries/LibJS/Runtime/IteratorConstructor.cpp

@@ -0,0 +1,59 @@
+/*
+ * Copyright (c) 2023, Tim Flynn <trflynn89@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/Intrinsics.h>
+#include <LibJS/Runtime/Iterator.h>
+#include <LibJS/Runtime/IteratorConstructor.h>
+#include <LibJS/Runtime/IteratorPrototype.h>
+#include <LibJS/Runtime/Realm.h>
+#include <LibJS/Runtime/VM.h>
+
+namespace JS {
+
+// 3.1.1.1 The Iterator Constructor, https://tc39.es/proposal-iterator-helpers/#sec-iterator-constructor
+IteratorConstructor::IteratorConstructor(Realm& realm)
+    : Base(realm.vm().names.Iterator.as_string(), realm.intrinsics().function_prototype())
+{
+}
+
+ThrowCompletionOr<void> IteratorConstructor::initialize(Realm& realm)
+{
+    MUST_OR_THROW_OOM(Base::initialize(realm));
+
+    auto& vm = this->vm();
+
+    // 3.1.1.2.1 Iterator.prototype, https://tc39.es/proposal-iterator-helpers/#sec-iterator.prototype
+    define_direct_property(vm.names.prototype, realm.intrinsics().iterator_prototype(), 0);
+
+    define_direct_property(vm.names.length, Value(0), Attribute::Configurable);
+
+    return {};
+}
+
+// 3.1.1.1.1 Iterator ( ), https://tc39.es/proposal-iterator-helpers/#sec-iterator
+ThrowCompletionOr<Value> IteratorConstructor::call()
+{
+    auto& vm = this->vm();
+
+    // 1. If NewTarget is undefined or the active function object, throw a TypeError exception.
+    return vm.throw_completion<TypeError>(ErrorType::ConstructorWithoutNew, "Iterator");
+}
+
+// 3.1.1.1.1 Iterator ( ), https://tc39.es/proposal-iterator-helpers/#sec-iterator
+ThrowCompletionOr<NonnullGCPtr<Object>> IteratorConstructor::construct(FunctionObject& new_target)
+{
+    auto& vm = this->vm();
+
+    // 1. If NewTarget is undefined or the active function object, throw a TypeError exception.
+    if (&new_target == this)
+        return vm.throw_completion<TypeError>(ErrorType::ClassIsAbstract, "Iterator");
+
+    // 2. Return ? OrdinaryCreateFromConstructor(NewTarget, "%Iterator.prototype%").
+    return TRY(ordinary_create_from_constructor<Iterator>(vm, new_target, &Intrinsics::iterator_prototype));
+}
+
+}