xviii Introduction
Aggregation One object can reference another object.
Inheritance A newly created object has the same characteristics
as another object without explicitly duplicating its functionality.
Polymorphism One interface may be implemented by multiple
objects.
JavaScript has all these characteristics, though because the language
has no concept of classes, some aren’t implemented in quite the way you
might expect. At first glance, a JavaScript program might even look like
a procedural program you would write in C. If you can write a function
and pass it some variables, you have a working script that seemingly has
no objects. A closer look at the language, however, reveals the existence
of objects through the use of dot notation.
Many object-oriented languages use dot notation to access properties
and methods on objects, and JavaScript is syntactically the same. But in
JavaScript, you never need to write a class definition, import a package,
or include a header file. You just start coding with the data types that you
want, and you can group those together in any number of ways. You could
certainly write JavaScript in a procedural way, but its true power emerges
when you take advantage of its object-oriented nature. That’s what this
book is about.
Make no mistake: A lot of the concepts you may have learned in
more traditional object-oriented programming languages don’t neces-
sarily apply to JavaScript. While that often confuses beginners, as you
read, you’ll quickly find that JavaScript’s weakly typed nature allows
you to write less code to accomplish the same tasks as other languages.
You can just start coding without planning the classes that you need
ahead of time. Need an object with specific fields? Just create an ad hoc
object wherever you want. Did you forget to add a method to that object?
No problem—just add it later.
Inside these pages, you’ll learn the unique way that JavaScript
approaches object-oriented programming. Leave behind the notions
of classes and class-based inheritance and learn about prototype-based
inheritance and constructor functions that behave similarly. You’ll learn
how to create objects, define your own types, use inheritance, and other-
wise manipulate objects to get the most out of them. In short, you’ll learn
everything you need to know to understand and write JavaScript profes-
sionally. Enjoy!
Who This Book Is For
This book is intended as a guide for those who already understand object-
oriented programming but want to know exactly how the concept works in
JavaScript. Familiarity with Java, C#, or object-oriented programming in
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