Object-oriented JavaScript
inheritance and polymorphism

JavaScript’s built-in inheritance mechanism is unfortunately unclear (try to read the ECMA description of how it works). In the literature one sees lots of constructs meant to illustrate polymorphism, that either fail in some way to be polymorphic, or are too clumsy to use safely.

In JavaScript, functions are also of type Object (although the typeof operator applied to a function returns “function”), and can therefore carry members, including properties that reference other functions.

All JavaScript functions have a property prototype, which is a reference to an Object. The purpose of this property is to implement inheritance. It works as follows.

The technique is to set the prototype of a constructor to an object instance, which serves as the parent.

An object can then be created using operator new with the first constructor. For each public member of the parent, operator new gives the created object a property that refers to the parent’s member. An object created this way is called a child of the parent object, and the child is said to inherit the public members of the parent.

Any constructor may serve as a parent. For example:




	

There is nothing special about the parent constructor.

inheritance by prototype

There are two non-obvious constructs required to achieve inheritance: setting up the inheritance relation, and initializing the members of the parent class inherited by the child class. There is more than one way to do it, but this is the best I’ve found.





The setting of the prototype class variable to an instance of another class is the JavaScript analog of declaring that the one class is derived from the other. (The constructor of the other class is obtained automatically from the constructor member of this Object.)

Note that the private members of the parent are not directly accessible to the child. That is as it should be.

Thanks to Pablo Krause for pointing out a bug in a previous version that had led me down a complicated road.

polymorphism

Polymorphism in JavaScript is now very easy. One simply makes different constructors for different kinds of child objects inheriting from the same parent.

JavaScript polymorphism has a feature one does not find in all object-oriented languages. One can add and override members even on the fly, after an object has been created. An object can acquire or lose properties and behaviors over time. (This feature is typical of scripting languages, but in Java and C++ it is either absent or achieved only with difficulty.)

People argue about whether this is real object-oriented programming, but I personally like it, because often one wants to model things in the real world that also acquire and lose properties and behaviors over time.

member overrides

In JavaScript, one doesn’t so much override a parent’s member, as replace it. This is very easy, but there is a catch.

In contrast to the situation in class-based languages, one must take steps to save the parent’s method before the child’s constructor replaces it, if the method of the parent is ever to be called by a child.

See

JavaScript 1.5 Guide: Details of the Object Model

stack overflow: How to “properly” create a custom object in JavaScript?

stack overflow: How to get a constructor function to inherit from a constructor function in JavaScript?