constructor defines methods

counterclass.js

> var Counter = function(from) {
>     this.from = from;
>     this.next = function() {
>         this.from += 1;
>         return this.from;
>     };
> }
> var count_from_7 = new Counter(7);
> var count_from_3 = new Counter(3);
> count_from_7.next();
8
> count_from_3.next();
4
> count_from_7.next();
9