Implementation looks fine, apart from the fact that it has quadratic time complexity due to the constant array copying. Either fix this or add a big fat warning; I'd expect a pre-order traversal to have linear time complexity. Ideally you would use agenerator instead of pushing to a list IMO.

The tests seem somewhat excessive for such a simple function (my rule of thumb roughly is 1:1 implementation to tests, 1:2 or 1:3 at most). You could also greatly shorten the tests by writing a utility constructor - something likeconst T = (left, value, right) => {left: left, value: value, right: right}.

I also think we should start consolidating (binary) tree algorithms in a (binary= tree class. A class would be a good idea to document the expected tree format and to group together related binary tree algorithms, including postorder and inorder traversal.

Finally, this is missing a doc comment; generally that would be fine for such a simple function ("self-documenting"), but I think that at least the tree format should be documented here.