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Methods of inheritance

Although ECMAScript doesn’t strictly define abstract classes as some other languages do, it sometimes

creates certain base classes that aren’t supposed to be used. Usually these are simply documented as

abstract.

The subclasses you create inherit all properties and methods from the superclass, including the construc-

tor and method implementations. Remember, all the properties and methods are public, so subclasses

may access these directly. Subclasses may add new properties and methods not present in the superclass

or override properties and methods of the superclass with new implementations.

Methods of inheritance

As usual with ECMAScript, you have more than one way to implement inheritance. This is because

inheritance in JavaScript isn’t explicit; it’s emulated. This means that the interpreter doesn’t handle all

the inheritance details. It is up to you, as the developer, to handle inheritance in a way that is most

appropriate for your situation.

Object masquerading

Object masquerading was never intended when the original ECMAScript was conceived. Instead, it

evolved as developers began to understand exactly how functions worked and, specifically, how to use

the

this

keyword in the context of functions.

The reasoning goes like this: A constructor assigns all properties and methods (with the Constructor

Paradigm of class declaration) using the

this

keyword. Because a constructor is just a function, you can

make the constructor of

ClassA

into a method of

ClassB

and call it.

ClassB

then receives the properties

and methods defined in

ClassA

’s constructor. For example,

ClassA

and

ClassB

are defined in this way:

function ClassA(sColor) {

this.color = sColor;

this.sayColor = function () {

alert(this.color);

};

}

function ClassB(sColor) {

}

As you remember, the

this

keyword references the currently created object in a constructor; in a method,

however,

this

points to the owning object. The theory is that treating

ClassA

as a regular function instead

of as a constructor establishes a type of inheritance. This can be done in the constructor

ClassB

like so:

function ClassB(sColor) {

this.newMethod = ClassA;

this.newMethod(sColor);

delete this.newMethod;

}

In this code, the method named

newMethod

is assigned to

ClassA

(remember, the name of a function is

just a pointer to it). Then, the method is called, passing the

color

argument from the

ClassB

construc-

tor. The final line of code deletes the reference to

ClassA

so that it cannot be called later on.

105

Inheritance

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