Node easily does arbitrary sized numbers. This is achieved either using Node's own Buffer type, TypedArrays or a binding such as BigNum which provides an interface to the number functions in Node's openSSL binding.
No it doesn't. Having to use buffers is not easy and the BigNum OpenSSL stuff is slow and limited (only integers). I have personally had a hell of a time supporting PostgreSQL's numeric type in a Node web server. Can it be done with Node? Sure, but it is not easy or fast.
If dat was just for moving buffers around then it would probably be okay, but it is wanting to be the place for data transformations as well which is what concerns me.