private scope vs prototype

Foo_all_public.js

function FooPublic() {
    this._publicVariable;
}


FooPublic.prototype.increment = function() {
    return ++ this._publicVariable;
}


var f = new FooPublic();
f.increment(); // nice
f._publicVariable --; // now, I'm messing with the internals

Foo_encapsulated.js

function FooPrivate() {
 var privateVariable = 0;
 
 this.increment = function() {
     return ++privateVariable;
 }
    
}

var f = new FooPrivate();
f.increment(); // nice
f.privateVariable --; // f does not have member 'privateVariable'