Design Patterns

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CHAPTER 4 ■ INHERITANCE
Instead of using a constructor function named Person to define the class structure, Person
is now an object literal. It is the prototype object for any other Person-like objects that you want
to create. Define all attributes and methods you want these objects to have, and give them
default values. For the methods, those default values will probably not be changed; for attributes,
they almost certainly will be:
var reader = clone(Person);
alert(reader.getName()); // This will output 'default name'.
reader.name = 'John Smith';
alert(reader.getName()); // This will now output 'John Smith'.
To create a new Person-like object, use the clone function (we go into more detail about
this function later in the section “The clone Function”). This provides an empty object with
the prototype attribute set to the prototype object. This means that if any method or attribute
lookup on this object fails, that lookup will instead look to the prototype object.
To create Author, you don’t make a subclass of Person. Instead you make a clone:
/* Author Prototype Object. */
var Author = clone(Person);
Author.books = []; // Default value.
Author.getBooks = function() {
return this.books;
}
The methods and attributes of this clone can then be overridden. You can change the
default values given by Person, or you can add new attributes and methods. That creates a new
prototype object, which you can then clone to create new Author-like objects:
var author = [];
author[0] = clone(Author);
author[0].name = 'Dustin Diaz';
author[0].books = ['JavaScript Design Patterns'];
author[1] = clone(Author);
author[1].name = 'Ross Harmes';
author[1].books = ['JavaScript Design Patterns'];
author[1].getName();
author[1].getBooks();
Asymmetrical Reading and Writing of Inherited Members
We mentioned before that in order to use prototypal inheritance effectively, you must forget
everything you know about classical inheritance. Here is one example of that. In classical
inheritance, each instance of Author has its own copy of the books array. You could add to it by
writing author[1].books.push('New Book Title'). That is not initially possible with the object
you created using prototypal inheritance because of the way prototype chaining works. A clone
is not a fully independent copy of its prototype object; it is a new empty object with its prototype