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Implementing Inheritance

Figure 4-1

In UML, each box represents a class, indicated by the class name. Lines coming from the top of Triangle,

Rectangle, and Pentagon converge and point at Shape, indicating that each of these classes inherits from

Shape. Likewise, the arrow pointing from Square to Rectangle indicates the inheritance relationship

there.

If you are interested in learning more about UML, refer to Instant UML (Wrox Press, ISBN

1861000871).

Implementing Inheritance

In order to implement inheritance in ECMAScript, you start out with a base class from which to inherit.

All developer-defined classes are candidate base classes. As a security precaution, native or host objects

cannot be base classes; this prevents giving the public access to compiled browser-level code that could

potentially be used in a malicious way.

After the base class has been selected, you can proceed to create its subclasses. It’s completely up to you

whether or not the base class should be used at all. Sometimes, you may want to create a base class that

isn’t intended to be used directly. Instead, it only provides common functionality to subclasses. In this

case, the base class is considered

abstract

Shape

Polygon

Ellipse

Circle

Rectangle

Pentagon

Triangle

Square

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