Classes and Constructors

Example 9-1 demonstrates one way to define a JavaScript class. It is not the idiomatic way to do so, however, because it did not define a constructor. A constructor is a function designed for the initialization of newly created objects. Constructors are invoked using the new keyword as described in Constructor Invocation. Constructor invocations using new automatically create the new object, so the constructor itself only needs to initialize the state of that new object. The critical feature of constructor invocations is that the prototype property of the constructor is used as the prototype of the new object. This means that all objects created with the same constructor inherit from the same object and are therefore members of the same class. Example 9-2 shows how we could alter the range class of Example 9-1 to use a constructor function instead of a factory function:

Example 9-2. A Range class using a constructor

// range2.js: Another class representing a range of values.  

// This is a constructor function that initializes new Range objects.
// Note that it does not create or return the object. It just initializes this.
function Range(from, to) {
    // Store the start and end points (state) of this new range object.
    // These are noninherited properties that are unique to this object.
    this.from = from; = to;

// All Range objects inherit from this object.
// Note that the property name must be "prototype" for this to work.
Range.prototype = {
    // Return ...

Get JavaScript: The Definitive Guide, 6th Edition now with the O’Reilly learning platform.

O’Reilly members experience live online training, plus books, videos, and digital content from nearly 200 publishers.