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call() method

this.newMethod();

delete this.newMethod;

}

The one downside to this is that if

ClassX

and

ClassY

have a property or method with the same name,

ClassY

’s takes priority because it is inherited from last. Besides that minor issue, multiple inheritance

with object masquerading is a breeze.

Because this method of inheritance caught on, the third edition of ECMAScript includes two new meth-

ods of the

Function

object:

call()

and

apply()

The call() method

The

call()

method is the method most similar to the classic object-masquerading method. Its first

argument is the object to be used for

this

. All other arguments are passed directly to the function itself.

For example:

function sayColor(sPrefix, sSuffix) {

alert(sPrefix + this.color + sSuffix);

};

var obj = new Object();

obj.color = “red”;

//outputs “The color is red, a very nice color indeed. “

sayColor.call(obj, “The color is “, “, a very nice color indeed. “);

In this example, the function

sayColor()

is defined outside of an object, and it references the

this

key-

word even though it is not attached to any object. The object

obj

is given a

color

property equal to

“red”

. When

call()

is, well, called, the first argument is

obj

, which indicates that the

this

keyword

sayColor()

should be assigned the value of

obj

. The second and third arguments are strings. They

are matched up with the

prefix

and

suffix

arguments of

sayColor()

, resulting in the message

“The

color is red, a very nice color indeed.”

being displayed.

To use this with the object masquerading method of inheritance, just replace the three lines that assign,

call, and delete the new method:

function ClassB(sColor, sName) {

//this.newMethod = ClassA;

//this.newMethod(sColor);

//delete this.newMethod;

ClassA.call(this, sColor);

this.name = sName;

this.sayName = function () {

alert(this.name);

};

}

Here, you want the

this

keyword in

ClassA

to be equal to the newly created

ClassB

object, so this is

passed in as the first argument. The second argument is the

color

argument, the only one for either

class.

107

Inheritance

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