An array destructing specification choice

[Allen Wirfs-Brock]

In the following declaration, what should be the value of z?

let [z,y,z] = {0:0, 1:1, length: 2, 2:2};

should it be be 2 or undefined

undefined might be reasonable because it is an array pattern, and the source object is only has two "array-like" elements
2 might be reasonable because the source object actually has a property named "2"

Which alternative will be least surprising to JS programmers?

Allen