Q:

How to get properties of Javascript object prototype?

I'd like to get _firstName and _birthDate from : I tried https://jsfiddle.net/0xLqhufd/
var Person = (function () {
function Person(firstName, birthDate) {
    this._firstName = firstName;
    this._birthDate = birthDate;
}
return Person;
}());

alert(Object.keys(Person.Prototype));

A:

You cannot get an object's properties before creating a new instance.
Object.keys(new Person)

Also for your understanding, you have properties with your current instance not in prototype. At this moment your prototype will be empty and it inherits the Object's, The core one, that holds toString etc. And those are non enumerable, so you cannot get those by using Object.keys().
DEMO

