LibJS: Use MarkedValueList for internal own properties getter functions

Letting these create and return a JS::Array directly is pretty awkward
since we then need to go through the indexed properties for iteration.
Just use a MarkedValueList (i.e. Vector<Value>) for this and add a new
Array::create_from() function to turn the Vector into a returnable
Array as we did before.

This brings it a lot closer to the spec as well, which uses the
CreateArrayFromList abstract operation to do exactly this.

There's an optimization opportunity for the future here, since we know
the Vector's size we could prepare the newly created Array accordingly,
e.g. by switching to generic storage upfront if needed.

1 files changed, 9 insertions, 0 deletions

diff --git a/Userland/Libraries/LibJS/Runtime/Array.cpp b/Userland/Libraries/LibJS/Runtime/Array.cpp
index c04a65c1d8..27f6c835af 100644
--- a/Userland/Libraries/LibJS/Runtime/Array.cpp
+++ b/Userland/Libraries/LibJS/Runtime/Array.cpp
@@ -36,6 +36,15 @@ Array* Array::create(GlobalObject& global_object)
     return global_object.heap().allocate<Array>(global_object, *global_object.array_prototype());
 }
 
+// 7.3.17 CreateArrayFromList, https://tc39.es/ecma262/#sec-createarrayfromlist
+Array* Array::create_from(GlobalObject& global_object, const Vector<Value>& values)
+{
+    auto* array = Array::create(global_object);
+    for (size_t i = 0; i < values.size(); ++i)
+        array->define_property(i, values[i]);
+    return array;
+}
+
 Array::Array(Object& prototype)
     : Object(prototype)
 {