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Alternative Inheritance Paradigms

Polygon.call(this, 3);

this.base = iBase;

this.height = iHeight;

if (typeof Triangle._initialized == “undefined”) {

Triangle.prototype = new Polygon();

Triangle.prototype.getArea = function () {

return 0.5 * this.base * this.height;

};

Triangle._initialized = true;

}

The previous code illustrates both

Polygon

and

Triangle

defined using dynamic prototyping. The mis-

take is in the highlighted line, where

Triangle.prototype

is set. Logically, this is the correct location;

but functionally, it doesn’t work. Technically, by the time that code is run, the object is already instanti-

ated and tied to the original

prototype

object. Although changes to that prototype object are reflected

properly with very late binding, replacing the

prototype

object has no effect on that object. Only future

object instances reflect the change, making the first instance incorrect.

To correctly use dynamic prototyping with inheritance, you must assign the new

prototype

object out-

side of the constructor, like this:

function Triangle(iBase, iHeight) {

Polygon.call(this, 3);

this.base = iBase;

this.height = iHeight;

if (typeof Triangle._initialized == “undefined”) {

Triangle.prototype.getArea = function () {

return 0.5 * this.base * this.height;

};

Triangle._initialized = true;

}

Triangle.prototype = new Polygon();

This code works because the

prototype

object is assigned before any objects are instantiated.

Unfortunately, this means the code isn’t completely encapsulated in the constructor, which is the main

purpose of dynamic prototyping.

Alternative Inheritance Paradigms

Due to the limitations of ECMAScript inheritance (for instance, lack of a private scope and the inability

to easily access superclass methods), developers around the world have constantly pushed their code to

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Inheritance

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