Methods like reverse and concat should preserve the holes (absent properties) in objects and arrays. For example, in any conformant implementation, v = [1,,3,4] print(1 in v) => false print(2 in v) => true w = v.reverse() print(w) => [4,3,,1] print(1 in w) => true print(2 in w) => false It is *not correct* to blindly copy properties from one array (or object) to the other, an "in" check is needed. Insult is heaped upon injury by the correct documentation being present in the source code along with the incorrect implementation.