Ah, I guess that explains it. Although that does not require the engine to have under-the-hood layout respect that, does it? They could just as easily choose to keep the memory layout in the same order.

Although I suppose you're already required to have two separate hidden classes to distinguish these two kinds of objects anyway.

> with a bizarre exception for arrays

Wow, you weren't joking with how bizarre this gets:

    var a = {};
    var b = {};
    a.a = 0;
    a.b = 1;
    a[0] = 0;
    a[1] = 1;
    b.b = 1;
    b.a = 0;
    b[1] = 1;
    b[0] = 0;
    Object.keys(a); // Array [ "0", "1", "a", "b" ]
    Object.keys(b); // Array [ "0", "1", "b", "a" ]

PS: Thanks for making a great shell :)