Node.js Tutorial - Node.js Class

Javascript classes are all declared as functions:


function Shape () {/*from  ww  w  .j a v  a 2  s. co  m*/
    this.X = 0;
    this.Y = 0;

    this.move = function (x, y) {
        this.X = x;
        this.Y = y;
    }
    this.distance_from_origin = function () {
        return Math.sqrt(this.X*this.X + this.Y*this.Y);
    }
}

var s = new Shape();
s.move(10, 10);
console.log(s.distance_from_origin());

You can add as many properties and methods to your classes, at any time:


var s = new Shape(15, 35);
s.FillColour = "red";

The function that declares the class is its constructor!

Prototypes and Inheritance

By default, all objects in JavaScript have a prototype object.

The prototype object is the mechanism through which they inherit properties and methods.

The following code shows how to use prototype to create Inheritance.

Change the Shape class so that all inheriting objects also get the X and Y properties, as well as the methods you have declared on it:


function Shape () {//  www .  j ava 2 s. c  o  m
}

Shape.prototype.X = 0;
Shape.prototype.Y = 0;

Shape.prototype.move = function (x, y) {
     this.X = x;
     this.Y = y;
}
Shape.prototype.distance_from_origin = function () {
     return Math.sqrt(this.X*this.X + this.Y*this.Y);
}
Shape.prototype.area = function () {
     throw new Error("I don't have a form yet");
}
var s = new Shape();
s.move(10, 10);
console.log(s.distance_from_origin());

function Square() {
}

Square.prototype = new Shape();
Square.prototype.__proto__ = Shape.prototype;
Square.prototype.Width = 0;

Square.prototype.area = function () {
   return this.Width * this.Width;
}
var sq = new Square();
sq.move(-5, -5);
sq.Width = 5;
console.log(sq.area());
console.log(sq.distance_from_origin());

The code above generates the following result.

You can further extend things with a new class called Rectangle , inheriting from the Square class:


function Rectangle () {/*  w  w  w.  j  a  v a2  s .  co  m*/
}

Rectangle.prototype = new Square();
Rectangle.prototype.__proto__ = Square.prototype;
Rectangle.prototype.Height = 0;

Rectangle.prototype.area = function () {
   return this.Width * this.Height;
}

var re = new Rectangle();
re.move(25, 25);
re.Width = 10;
re.Height = 5;
console.log(re.area());
console.log(re.distance_from_origin());

We can use the operator instanceof to check the inheritance.


console.log(sq instanceof Square);      // true
console.log(sq instanceof Shape);       // true
console.log(sq instanceof Rectangle);   // false
console.log(re instanceof Rectangle);   // true
console.log(sq instanceof Square);      // true
console.log(sq instanceof Shape);       // true
console.log(sq instanceof Date);        // false