Use

When extending can-map, if a prototype property is not a function, it is used as a default value on instances of the extended Map. For example:

var Paginate = Map.extend({
    limit: 20,
    offset: 0,
    next: function(){
        this.attr("offset", this.attr("offset")+this.attr("limit"))
    }
});

var paginate = new Paginate({limit: 30});

paginate.attr("offset") //-> 0
paginate.attr("limit")  //-> 30

paginate.next();

paginate.attr("offset") //-> 30