i wrote an article on the same topic of Object Oriented JavaScript in Mozilla
https://developer.mozilla.org/en/Introduction_to_Object-Oriented_JavaScript

In JavaScript, when you create a subclass from an existing class, only the *public* and *privileged* members are passed on (public/privileged members are created using the this keyword). You can also implement encapsulation with private members using closures.

In your example, this.speed is public and accessible, you can change it after instantiating the object. To truly protect speed, you\’d want to do something like this (I hope the spacing will be preserved):

var Car = function(mph) {
		//private attribute
		var speed;

		//privileged methods
		this.getSpeed = function() {
		return speed;
	}
	this.setSpeed = function(mph) {
		speed = mph;
	}

	this.setSpeed(mph);
} ;
Car.prototype = {
	//public, non-privileged methods go here
	whatever: function () {}
};

speed is private, you can only access it using the public getter and setter.

You can subclass Car and still access speed, but only through the privileged methods (getSpeed and setSpeed) because the privileged methods will be passed on (they are publicly accessible via the this keyword). No instance methods in the subclass will have *direct* access to speed, though – you have to go through the existing privileged methods (getSpeed and setSpeed).

\"JavaScript supports single class inheritance.\"

JavaScript also has prototypal inheritance which uses objects instead of defining a class structure and technically, you can have multiple inheritance via augmentation/mixins.