JavaScript: The Definitive Guide, Fourth Edition

Object Properties and Methods

As we discussed earlier, all objects in JavaScript inherit from the Object class. While more specialized classes, such as the built-in String class or a user-defined Complex class, define properties and methods of their own, all objects, whatever their class, also support the properties and methods defined by the Object class. Because of their universality, these properties and methods are of particular interest.

The constructor Property

Starting with JavaScript 1.1, every object has a constructor property that refers to the constructor function used to initialize the object. For example, if I create an object o with the Complex( ) constructor, the property o.constructor refers to Complex:

var o = new Complex(1,2);
o.constructor == Complex;  // Evaluates to true

Each Complex object (or object of whatever type) does not have its own unique constructor property, of course; instead, this property is inherited from the prototype object. As discussed earlier in this chapter, JavaScript creates a prototype object for each constructor function you define and assigns that object to the prototype property of the constructor. What I did not reveal earlier, however, is that the prototype object is not initially empty. When created, it includes a constructor property that refers to the constructor function. That is, for any function f, f.prototype.constructor is always equal to f (unless we set it to something else).

Since the constructor function defines the ...