类的一大应用就是多态。多态是一个面向对象编程的高级术语－－－－“一个东西可编程很多不同的东西，但不是任何东西”。［引自coffeescript深入浅出］
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
生成的javascript
// 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));