Your logic here is based on a faulty assumption. In your second implementation, the constructor variable numberVar is never used. You have no code that can reach it and thus you are not using a private variable in the second code block.
Your methods in that second implementation are accessing an object property named numberVar which is publicly accessible as a property on the object which is different than the local variable of the same name in your constructor. You cannot have private variables in your second implementation because your prototype-declared methods are not declared in a private scope.
Here's what happens in your second code block:
var Bar = function Bar() { // this variable is never used as there is no code in this scope // that can reach this variable. In fact, it is garbage collected // immediately var numberVar = 0; }; Bar.prototype = { getNumber: function () { return this.numberVar; }, setNumber: function (val) { // sets a public property on the object this.numberVar = val; } }; var bar = new Bar(); bar.setNumber(10); console.log(bar.numberVar); // 10, this property is public
For a general discussion of methods declared in the constructor vs. prototype-defined methods, see this discussion:
Advantages of using prototype, vs defining methods straight in the constructor?
