Yes. You can do this:

(function() { var private = "hi"; Tree.prototype.genus = function(){ return ((typeof this.name !== 'undefined') ? this.name : 'Hybridicus Maximus'); }; Tree.prototype.bulk = function(){ return ((typeof this.size !== 'undefined') ? this.size : '8') + ' ft'; }; })(); 

Now, that'll provide a private variable that those functions can see, but it'll be a private "class" variable - all instances will share the same variable, in other words. If you want a private variable per instance, you have to do that in the constructor (or "init" method, or whatever), meaning the methods that share those privates would also have to be created there. (You could of course put a function on the prototype that would create the instance methods at construction time.)

Yes. You can do this:

(function() { var private = "hi"; Tree.prototype.genus = function(){ return ((typeof this.name !== 'undefined') ? this.name : 'Hybridicus Maximus'); }; Tree.prototype.bulk = function(){ return ((typeof this.size !== 'undefined') ? this.size : '8') + ' ft'; }; })(); 

Now, that'll provide a private variable that those functions can see, but it'll be a private "class" variable - all instances will share the same variable, in other words. If you want a private variable per instance, you have to do that in the constructor (or "init" method, or whatever), meaning the methods that share those privates would also have to be created there. (You could of course put a function on the prototype that would create the instance methods at construction time.)

edit — One thing you could do is use a technique like this to build a mechanism like jQuery's ".data()", so that you'd have a class variable that acts as a place to keep per-instance values. It'd be kind-of clunky, but it'd be workable.