PRDV402: Introduction to JavaScript II

In this article, we've described some of the basic features of class-based object-oriented programming as implemented in languages like Java and C++.

In the two previous articles, we looked at a couple of core JavaScript features: constructors and prototypes. These features certainly have some relation to some of the OOP concepts described above.

• constructors in JavaScript provide us with something like a class definition, enabling us to define the "shape" of an object, including any methods it contains, in a single place. But prototypes can be used here, too. For example, if a method is defined on a constructor's prototype property, then all objects created using that constructor get that method via their prototype, and we don't need to define it in the constructor.
• the prototype chain seems like a natural way to implement inheritance. For example, if we can have a Student object whose prototype is Person, then it can inherit name and override introduceSelf().

But it's worth understanding the differences between these features and the "classical" OOP concepts described above. We'll highlight a couple of them here.

First, in class-based OOP, classes and objects are two separate constructs, and objects are always created as instances of classes. Also, there is a distinction between the feature used to define a class (the class syntax itself) and the feature used to instantiate an object (a constructor). In JavaScript, we can and often do create objects without any separate class definition, either using a function or an object literal. This can make working with objects much more lightweight than it is in classical OOP.

Second, although a prototype chain looks like an inheritance hierarchy and behaves like it in some ways, it's different in others. When a subclass is instantiated, a single object is created which combines properties defined in the subclass with properties defined further up the hierarchy. With prototyping, each level of the hierarchy is represented by a separate object, and they are linked together via the __proto__ property. The prototype chain's behavior is less like inheritance and more like delegation. Delegation is a programming pattern where an object, when asked to perform a task, can perform the task itself or ask another object (its delegate) to perform the task on its behalf. In many ways, delegation is a more flexible way of combining objects than inheritance (for one thing, it's possible to change or completely replace the delegate at run time).

That said, constructors and prototypes can be used to implement class-based OOP patterns in JavaScript. But using them directly to implement features like inheritance is tricky, so JavaScript provides extra features, layered on top of the prototype model, that map more directly to the concepts of class-based OOP. These extra features are the subject of the next article.