Add same functionality to two classes

[NF]

We are looking to extend the functionality of listGrid and treeGrid with the same functions -
The new functions are both for listGrid and treeGrid.
We know that treeGrid is inherited from listGrid.
As far as I know, we cant change listGrid class...
Is there a way to add the common functionality without writing the code twice?
There is a diagram which demonstrates our expected result -


Thanks.

[Isomorphic]

Java would make this hard but in JavaScript it's trivial: create a method that adds the common methods and properties to the target class, and call it twice, once for your ListGrid subclass and once for your TreeGrid subclass.

[NF]

Thanks, we used a slightly different approach for this problem, we used Mixing Pattern (with classes) to extend both myTreeGrid and myListGrid (with the commonListGrid Class).