Why does setting positions in a subclass of Array not change its length? Should I not subclass array?

Question

In the CoffeeScript program below, I create a subclass of Array which sets two positions in its constructor:

class SetPositionsArray extends Arr         


        

Answer 1

Array.length is a special property that doesn't work when subclassed.

If you look at the compiled javascript for your first example the way CoffeeScript builds classes is by creating a function and calling __extends(SetPositionsArray, Array);. This method can't 'copy' the concept of length into your new class.