class Shape

    constructor: (@width) ->

    computeArea: -> throw new Error 'I am an abstract class!'

class Square extends Shape

    computeArea: -> Math.pow @width, 2

class Circle extends Shape

    radius: -> @width / 2

    computeArea: -> Math.PI * Math.pow @radius, 2

showArea = (shape) ->

    unless shape instanceof Shape

        throw new Error 'showArea requires a Shape instance!'

    console.log shape.computeArea()

showArea new Square(2) #4

showArea new Circle(2) #pi

// Generated by CoffeeScript 1.10.0

var Circle, Shape, Square, showArea,

  extend = function(child, parent) { for (var key in parent) { if (hasProp.call(parent, key)) child[key] = parent[key]; } function ctor() { this.constructor = child; } ctor.prototype = parent.prototype; child.prototype = new ctor(); child.__super__ = parent.prototype; return child; },

  hasProp = {}.hasOwnProperty;

Shape = (function() {

  function Shape(width) {

    this.width = width;

  }

  Shape.prototype.computeArea = function() {

    throw new Error('I am an abstract class!');

  };

  return Shape;

})();

Square = (function(superClass) {

  extend(Square, superClass);

  function Square() {

    return Square.__super__.constructor.apply(this, arguments);

  }

  Square.prototype.computeArea = function() {

    return Math.pow(this.width, 2);

  };

  return Square;

})(Shape);

Circle = (function(superClass) {

  extend(Circle, superClass);

  function Circle() {

    return Circle.__super__.constructor.apply(this, arguments);

  }

  Circle.prototype.radius = function() {

    return this.width / 2;

  };

  Circle.prototype.computeArea = function() {

    return Math.PI * Math.pow(this.radius, 2);

  };

  return Circle;

})(Shape);

showArea = function(shape) {

  if (!(shape instanceof Shape)) {

    throw new Error('showArea requires a Shape instance!');

  }

  return console.log(shape.computeArea());

};

showArea(new Square(2));

showArea(new Circle(2));