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Prototype chaining

case, you must create a separate array to place the arguments into the correct order. You could also use

the

call()

method.

Prototype chaining

The form of inheritance actually intended for use in ECMAScript is prototype chaining. The last chapter

introduced the prototype paradigm for defining classes. Prototype chaining builds off this paradigm to

accomplish inheritance in an interesting way.

In the last chapter, you learned that the

prototype

object is the template upon which an object is based

when instantiated. To summarize: Any properties or methods on the

prototype

object will be passed

on all instances of that class. Prototype chaining uses this functionality to accomplish inheritance.

If the classes from the previous example are redefined using the prototype paradigm, they become the

following:

function ClassA() {

}

ClassA.prototype.color = “red”;

ClassA.prototype.sayColor = function () {

alert(this.color);

};

function ClassB() {

}

ClassB.prototype = new ClassA();

The magic in prototype chaining occurs in the highlighted previous line. Here, you are setting the

prototype

property of

ClassB

to be an instance of

ClassA

. This makes perfect sense because you want

all the properties and methods of

ClassA

, but you don’t want to have to assign each of them separately

ClassB

’s prototype property. What better way to do this than just to make the

prototype

into an

instance of

ClassA

Similar to object masquerading, all new properties and methods of the subclass must come after the

assignment of the

prototype

property because all methods assigned before will be deleted. Why?

Because the

prototype

property is being completely replaced with a new object; the original object to

which you would have added the methods is destroyed. So to add the name property and the

sayName()

method to

ClassB

, the code looks like this:

function ClassB() {

}

ClassB.prototype = new ClassA();

Note that no parameters are passed into the

ClassA

constructor call. This is standard

in prototype chaining. Be sure that your constructor functions properly without any

arguments.

109

Inheritance

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