Extending Classes In A Modular JavaScript Application Architecture Using RequireJS

Yesterday, I tried to apply some deep thinking to how dependencies should be managed in a modular JavaScript application architecture that is using RequireJS. The conclusion that I came to was that RequireJS should manage and load "definitions" while your application should manage and load "instances." This makes sense since instantiation is the domain of your business logic, not your organizational framework. When it comes to extending classes in a RequireJS context, the same principle holds true, but things seem a little bit more interesting. Base classes, which may have static functionality, are still just "definitions" that can be managed by the RequireJS framework.

NOTE: In JavaScript, most values are technically "instances" at the language level. When I refer to "instances" in this post, I am referring to objects that need to be instantiated using the "new" keyword.

To look at inheritance in a RequireJS context, I'm going to create a simple class, Friend. Friend has a single method, getName(). The Friend class inherits from the core Model class which provides the core method, getInstanceID().

Friend()

• getName()

Model()

• getInstanceID()

Before we look at how the Friend or the base Model class are coded, let's look at the demo that makes use of the two classes:

The main code demo relies only on the Friend module directly. The core Model module is a dependency of the Friend module and will be loaded automatically by the RequireJS framework. As you can see from the code, we are creating a few instances of Friend and then invoking both the sub-class method, getName(), and the core method, getInstanceID(). And, when we run the above code, we get the following console output:

Sarah : 1
Tricia : 2
Joanna : 3

So far, so simple - I think you get the idea.

Now, let's look at the core Model module. This class is intended to be sub-classed by all other model classes within the application:

model.js - Our Core Model Class

To me, this module is really interesting! Not only are we defining a constructor and then returning it (as the module definition), we are also providing public and private static functionality. The private, static functionality manages the internal counter for instance creation; the public, static functionality - Model.getInstanceCount() - provides a window into the private static data.

This kind of module can start to blur the line between "definition" and "instance" (in the context of this conversation). On the one hand, it defines the core Model class; however, on the other hand, it clearly provides data and behavior - the characteristics of an object that you would typically consider an "instance."

Using our previous conclusions about management responsibilities, we'll have to consider this module a "definition." Since nothing is being instantiated using the "new" keyword, we'll defer to RequireJS to load this class as a dependency is subsequent modules.

Now, let's take a look at the Friend class. Since Friend extends Model, Model is clearly a dependency for the full definition of Friend. And, since Model is, itself, a "definition" not an instance, we'll use RequireJS to manage and load the Model module for the Friend module.

friend.js - Our Friend Class

As you can see, we are using RequireJS to load the Model class for use within the Friend module definition.

With the various types of dependencies found within an application, it can sometimes get confusing as to who is responsible for loading which dependency. Mix in the object-based nature of JavaScript and the matter can become even more confusing. Modules that appear to provide both definition and functionality challenge our understanding of what we consider an "instance." In my dialogue, I refer to an instance as that which is explicitly instantiated by the application. As such, base classes - which provide static, instance-like functionally - are still "definitions" that should be managed and loaded by the RequireJS framework.

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