JavaScript: The Definitive Guide, Fourth Edition

Constructors

We saw previously that you can create and initialize a new object in JavaScript by using the new operator in conjunction with a predefined constructor function such as Object( ), Date( ), or Function( ). These predefined constructors and the built-in object types they create are useful in many instances. However, in object-oriented programming, it is also common to work with custom object types defined by your program. For example, if you are writing a program that manipulates rectangles, you might want to represent rectangles with a special type, or class, of object. Each object of this Rectangle class would have a width property and a height property, since those are the essential defining characteristics of rectangles.

To create objects with properties such as width and height already defined, we need to write a constructor to create and initialize these properties in a new object. A constructor is a JavaScript function with two special features:

• It is invoked through the new operator.

• It is passed a reference to a newly created, empty object as the value of the this keyword, and it is responsible for performing appropriate initialization for that new object.

Example 8-1 shows how the constructor function for a Rectangle object might be defined and invoked.

Example 8-1. A Rectangle object constructor function

// Define the constructor. // Note how it initializes the object referred to by "this". function Rectangle(w, h) { this.width = w; this.height = h; ...