User Guide

Object 553
function Book() {
this.setQuantity = function(numBooks:Number):Void {
this.books = numBooks;
};
this.getQuantity = function():Number {
return this.books;
};
this.getTitle = function():String {
return "Catcher in the Rye";
};
this.addProperty("bookcount", this.getQuantity, this.setQuantity);
this.addProperty("bookname", this.getTitle, null);
}
var myBook = new Book();
myBook.bookcount = 5;
trace("You ordered "+myBook.bookcount+" copies of "+myBook.bookname);
// output: You ordered 5 copies of Catcher in the Rye
The previous example works, but the properties bookcount and bookname are added to every
instance of the
Book object, which requires having two properties for every instance of the
object. If there are many properties, such as
bookcount and bookname, in a class, they could
consume a great deal of memory. Instead, you can add the properties to
Book.prototype so
that the
bookcount and bookname properties exist only in one place. The effect, however, is
the same as that of the code in the example that added
bookcount and bookname directly to
every instance. If an attempt is made to access either property in a Book instance, the
property's absence will cause the prototype chain to be ascended until the versions defined in
Book.prototype are encountered. The following example shows how to add the properties to
Book.prototype:
function Book() {}
Book.prototype.setQuantity = function(numBooks:Number):Void {
this.books = numBooks;
};
Book.prototype.getQuantity = function():Number {
return this.books;
};
Book.prototype.getTitle = function():String {
return "Catcher in the Rye";
};
Book.prototype.addProperty("bookcount", Book.prototype.getQuantity,
Book.prototype.setQuantity);
Book.prototype.addProperty("bookname", Book.prototype.getTitle, null);
var myBook = new Book();
myBook.bookcount = 5;
trace("You ordered "+myBook.bookcount+" copies of "+myBook.bookname);