JavaScript : Object-Oriented Programming

Creating instances in JavaScript

We already created instances with the simple Rectangle constructor function. We just needed to use the new keyword and the constructor function name. We then need to specify the required arguments enclosed in parentheses and assign the result to a variable.

The following lines declare a new version of the Rectangle constructor function that adds a calculateArea function to the blueprint:

function Rectangle(width, height) {
    console.log("I'm creating a new Rectangle");
    this.width = width;
    this.height = height;

    this.calculateArea = function() {
        return this.width * this.height;
    }
}

The new constructor function adds a parameterless calculateArea method to the instance. The following lines of code create a new Rectangle ...