Question

I have a method toString() which is not being inherited from Shape. Why?

function Shape(){
  this.name = 'shape';
  this.toString = function() {return this.name;};
}

function TwoDShape(){
  this.name = '2D shape';
}

function Triangle(side, height) {
  this.name = 'Triangle';
  this.side = side;
  this.height = height;
  this.getArea = function(){return this.side * this.height / 2;};
}

TwoDShape.prototype = TwoDShape;
Triangle.prototype = Triangle;

TwoDShape.prototype.constructor = TwoDShape;
Triangle.prototype.constructor = Triangle;

var my = new Triangle(5, 10);

document.write("my getarea: " + my.getArea() + "my name is: " + my.toString()  + "<br>");​

Demo jsFiddle.

Answer 1

Triangle‘s prototype must be a Shape so that it inherits its methods:

Triangle.prototype = new Shape();

More specifically, since you have multiple levels of inheritance:

TwoDShape.prototype = new Shape();
TwoDShape.prototype.constructor = TwoDShape;

Triangle.prototype = new TwoDShape();
Triangle.prototype.constructor = Triangle;

That is, TwoDShape inherits Shape, and Triangle inherits from TwoDShape.

Generically, if Foo inherits Bar, you’d have:

Foo.prototype = new Bar(); // inherit Bar
Foo.prototype.constructor = Foo; // Fix constructor which now points to Bar

DEMO.

References:

• MDN: Introduction to Object-Oriented JavaScript: Inheritance
• OOP in JavaScript: Inheritance