proto.js

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

var triangle = new Triangle(3,4);