LibWeb: Add support for IDL legacy platform objects

A legacy platform object is a non-global platform object that
implements a special operation. A special operation is a getter, setter
and/or deleter. This is particularly used for old collection types,
such as HTMLCollection, NodeList, etc.

This will be used to make these spec-compliant and remove their custom
wrappers. Additionally, it will be used to implement collections that
we don't have yet, such as DOMStringMap.

Userland/Libraries/LibWeb/Bindings/IDLAbstractOperations.cpp

@@ -0,0 +1,53 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#include <AK/NumericLimits.h>
+#include <LibJS/Runtime/AbstractOperations.h>
+#include <LibJS/Runtime/PropertyName.h>
+#include <LibWeb/Bindings/IDLAbstractOperations.h>
+
+namespace Web::Bindings::IDL {
+
+// https://heycam.github.io/webidl/#is-an-array-index
+bool is_an_array_index(JS::GlobalObject& global_object, JS::PropertyName const& property_name)
+{
+    // 1. If Type(P) is not String, then return false.
+    if (!property_name.is_number())
+        return false;
+
+    // 2. Let index be ! CanonicalNumericIndexString(P).
+    auto index = JS::canonical_numeric_index_string(global_object, property_name);
+
+    // 3. If index is undefined, then return false.
+    if (index.is_undefined())
+        return false;
+
+    // 4. If IsInteger(index) is false, then return false.
+    // NOTE: IsInteger is the old name of IsIntegralNumber.
+    if (!index.is_integral_number())
+        return false;
+
+    // 5. If index is −0, then return false.
+    if (index.is_negative_zero())
+        return false;
+
+    // FIXME: I'm not sure if this is correct.
+    auto index_as_double = index.as_double();
+
+    // 6. If index < 0, then return false.
+    if (index_as_double < 0)
+        return false;
+
+    // 7. If index ≥ 2 ** 32 − 1, then return false.
+    // Note: 2 ** 32 − 1 is the maximum array length allowed by ECMAScript.
+    if (index_as_double >= NumericLimits<u32>::max())
+        return false;
+
+    // 8. Return true.
+    return true;
+}
+
+}

Userland/Libraries/LibWeb/Bindings/IDLAbstractOperations.h

@@ -0,0 +1,15 @@
+/*
+ * Copyright (c) 2021, Luke Wilde <lukew@serenityos.org>
+ *
+ * SPDX-License-Identifier: BSD-2-Clause
+ */
+
+#pragma once
+
+#include <LibJS/Forward.h>
+
+namespace Web::Bindings::IDL {
+
+bool is_an_array_index(JS::GlobalObject&, JS::PropertyName const&);
+
+}

Userland/Libraries/LibWeb/CMakeLists.txt

@@ -4,6 +4,7 @@ set(SOURCES
     Bindings/EventTargetWrapperFactory.cpp
     Bindings/EventWrapperFactory.cpp
     Bindings/HTMLCollectionWrapperCustom.cpp
+    Bindings/IDLAbstractOperations.cpp
     Bindings/ImageConstructor.cpp
     Bindings/LocationObject.cpp
     Bindings/MainThreadVM.cpp