JavaScript: The Definitive Guide, 6th Edition

Chapter 9. Classes and Modules

JavaScript objects were covered in Chapter 6. That chapter treated each object as a unique set of properties, different from every other object. It is often useful, however, to define a class of objects that share certain properties. Members, or instances, of the class have their own properties to hold or define their state, but they also have properties (typically methods) that define their behavior. This behavior is defined by the class and is shared by all instances. Imagine a class named Complex to represent and perform arithmetic on complex numbers, for example. A Complex instance would have properties to hold the real and imaginary parts (state) of the complex number. And the Complex class would define methods to perform addition and multiplication (behavior) of those numbers.

In JavaScript, classes are based on JavaScript’s prototype-based inheritance mechanism. If two objects inherit properties from the same prototype object, then we say that they are instances of the same class. JavaScript prototypes and inheritance were covered in Prototypes and Inheritance, and you must be familiar with the material in those sections to understand this chapter. This chapter covers prototypes in Classes and Prototypes.

If two objects inherit from the same prototype, this typically (but not necessarily) means that they were created and initialized by the same constructor function. Constructors have been covered in Object Creation Expressions, Creating Objects ...