The biggest issue is precisely concerning the definition of "numerical properties". Here are some "numerical properties:"

    5
    -0
    "3"
    3.0
    4294967294

But these are not "numerical properties:"

    "-0"
    -1
    "3.0"
    4294967295

Which means that, when setting a property on an array, you must attempt to parse the object in a very peculiar way so that you can make a decision about iteration order, and whether to adjust the length property.