Beginning Web Development, Silverlight, and ASP.NET AJAX

CHAPTER 14 ■ JAVASCRIPT PROGRAMMING WITH ASP.NET AJAX 341
else
return "Engine is already running";
},
openThrottle: function(){
if (this._currentSpeed0)
this._currentSpeed--;
}
The last step is to register the class in the namespace, declaring the class that it
inherits from is Vehicles.Boat:
Vehicles.SpeedBoat.registerClass('Vehicles.SpeedBoat', Vehicles.Boat);
Now you can declare SpeedBoat objects in your code, accessing their methods or the
base methods of the Boat class. For example, the getBoatDetails method is available on
the base class, so the SpeedBoat can access it like this:
var MySpeedBoat = new Vehicles.SpeedBoat('Intimidator', '10', 'Arnie', '100cc');
alert(MySpeedBoat.getBoatDetails());
Inheritance like this has limitations—the only methods that your class can use are
those in the class that it inherits from (and the class that its parent class inherits from,
and so on). In cases where you might want to implement common methods across different
classes, you cannot do it with inheritance. For example, the speedboat implements
throttle opening and closing, and you may want to implement these for a motorcycle
also. You can’t derive a motorcycle from a boat, so you’d end up having to implement
them twice, and can end up with differences as a result, making for an untidy API (i.e.,
Motorcycle could have ThrottleOpen while SpeedBoat has openThrottle), which isn’t desirable.
The concept of interfaces is defined to help avoid this. You’ll see how this works in
JavaScript in the next section.
Using Interfaces in JavaScript
An interface can be used to specify a function prototype that crosses different class types,
and is not dependent on an inheritance tree.
You can define functions that need to be in common across such classes—those that
inherit from different base classes—by building them as an interface and having the class
implement that interface.