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zInherit

the limit in an effort to create other ways of implementing inheritance. This section examines some of the

alternatives to the standard ECMAScript inheritance paradigms.

zInherit

Prototype chaining essentially copies all methods from an object to a class’s

prototype

object. But

what if there were a different way to accomplish this? There is. Using the zInherit library (available at

http://www.nczonline.net/downloads

), it’s possible to accomplish method inheritance without

using prototype chaining. This small library supports all modern browsers (Mozilla, IE, Opera, Safari)

as well as some older browsers (Netscape 4.x, IE/Mac).

In order to use the zInherit library, you must include zinherit.js using the

tag. Chapter 5,

“JavaScript in the Browser,” discusses including external JavaScript files in detail.

The zInherit library adds two methods to the

Object

class:

inheritFrom()

and

instanceOf()

. As

you may have guessed, the

inheritFrom()

method does the heavy lifting, copying the methods from

a given class. The following line uses prototype chaining to inherit methods from

ClassA

ClassB

ClassB.prototype = new ClassA();

This line can be replaced with the following:

ClassB.prototype.inheritFrom(ClassA);

The

inheritFrom()

method accepts one argument, which is the class from which to copy the methods.

Note that, as opposed to prototype chaining, this paradigm doesn’t actually create a new instance of the

class to inherit from, making it a little safer and freeing the developer from worrying about the construc-

tor arguments.

The

instanceOf()

method is a replacement for the

instanceof

operator. Because this paradigm

doesn’t use prototype chaining at all, this line of code won’t work:

ClassB instanceof ClassA

The

instanceOf()

method makes up for this loss, working with

inheritFrom()

to keep track of all

superclasses:

ClassB.instanceOf(ClassA);

Polygons revisited

The entire polygon example can be rewritten using the zInherit library by replacing just two lines

(highlighted):

The

inheritFrom()

method call must be used exactly where the prototype assign-

ment normally occurs in order to ensure proper inheritance.

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