JavaScript: The Definitive Guide, 6th Edition

Classes and Prototypes

In JavaScript, a class is a set of objects that inherit properties from the same prototype object. The prototype object, therefore, is the central feature of a class. In Example 6-1 we defined an inherit() function that returns a newly created object that inherits from a specified prototype object. If we define a prototype object, and then use inherit() to create objects that inherit from it, we have defined a JavaScript class. Usually, the instances of a class require further initialization, and it is common to define a function that creates and initializes the new object. Example 9-1 demonstrates this: it defines a prototype object for a class that represents a range of values and also defines a “factory” function that creates and initializes a new instance of the class.

Example 9-1. A simple JavaScript class

// range.js: A class representing a range of values.  

// This is a factory function that returns a new range object.
function range(from, to) {
    // Use the inherit() function to create an object that inherits from the
    // prototype object defined below.  The prototype object is stored as
    // a property of this function, and defines the shared methods (behavior)
    // for all range objects.
    var r = inherit(range.methods); 

    // Store the start and end points (state) of this new range object.
    // These are noninherited properties that are unique to this object.
    r.from = from;
    r.to = to;

    // Finally return the new object
    return r;
}

// This prototype object defines ...