I use singleton pattern class in coffeescript which shown below recently. It works perfectly but I don't know why this could be a singleton pattern. This might be a stupid question but thanks for your answering.

#coffeescript
class BaseClass
  class Singleton

  singleton = new Singleton()
  BaseClass = -> singleton

a = new BaseClass()
a.name = "John"
console.log a.name # "John"
b = new BaseClass()
b.name = "Lisa"
console.log b.name # "Lisa"
console.log a.name # "Lisa"

and code below is javascript which is produced by the code above

var BaseClass, a, b;

BaseClass = (function() {
  var Singleton, singleton;

  function BaseClass() {}

  Singleton = (function() {
    function Singleton() {}

    return Singleton;

  })();

  singleton = new Singleton();

  BaseClass = function() {
    return singleton;
  };

  return BaseClass;

})();

a = new BaseClass();

a.name = "John";

console.log(a.name);

b = new BaseClass();

b.name = "Lisa";

console.log(b.name);

console.log(a.name);

EDITED : I am not asking the definition of 'singleton pattern' nor how they are generally created but the reason why the code above always returns the same instance instead of creating new one.

I use singleton pattern class in coffeescript which shown below recently. It works perfectly but I don't know why this could be a singleton pattern. This might be a stupid question but thanks for your answering.

#coffeescript
class BaseClass
  class Singleton

  singleton = new Singleton()
  BaseClass = -> singleton

a = new BaseClass()
a.name = "John"
console.log a.name # "John"
b = new BaseClass()
b.name = "Lisa"
console.log b.name # "Lisa"
console.log a.name # "Lisa"

and code below is javascript which is produced by the code above

var BaseClass, a, b;

BaseClass = (function() {
  var Singleton, singleton;

  function BaseClass() {}

  Singleton = (function() {
    function Singleton() {}

    return Singleton;

  })();

  singleton = new Singleton();

  BaseClass = function() {
    return singleton;
  };

  return BaseClass;

})();

a = new BaseClass();

a.name = "John";

console.log(a.name);

b = new BaseClass();

b.name = "Lisa";

console.log(b.name);

console.log(a.name);

EDITED : I am not asking the definition of 'singleton pattern' nor how they are generally created but the reason why the code above always returns the same instance instead of creating new one.

• javascript
• coffeescript

• You might look into running this code in a debugger, to see 'why' or 'how' this code does what it does. – Dean Radcliffe Commented Sep 17, 2014 at 2:36

3 Answers 3

First of all, there is a good example of Singleton Pattern implementation in CoffeeScript Cookbook:

class Singleton
  instance = null

  class PrivateClass
    constructor: (@message) ->
    echo: -> @message

  @get: (message) ->
    instance ?= new PrivateClass(message)

You tried to do a similar thing, but messed with CoffeeScript syntax a little bit. Here is how it should look:

class BaseClass
  class Singleton

  singleton = new Singleton()

  constructor: ->
    return singleton

Note that I'm using an explicit return here. I'm doing it because CoffeeScript implicit return doesn't work for class constructors.

I would also remend you to take a look at Simplest/Cleanest way to implement singleton in JavaScrip question.

My favorite singleton implementation is the following one:

class BaseClass
  instance = null

  constructor: ->
    if instance
      return instance
    else
      instance = this
    # contructor code

It works like yours except for two things:

• it doesn't require an additional class definition;
• it creates first singleton instance only when it's needed.

From Wikipedia,

The singleton pattern is a design pattern that restricts the instantiation of a class to one object

Since in your example, a change in a is reflected in b, it shows that they are the same object, thus this is the Singleton Pattern.

You may use this if there is only supposed to be a single instance of something on a page, such as an AJAX control. In JavaScript, you can't prevent someone from calling your function with "new", as in var b = new BaseClass(), but you can ensure that only a single instance is ever returned using this pattern.

You just have to declare an instance variable that is used only to contain your instance.

  Singleton = ->
    self           = this

    self.foo = ->
      # stuff here

    self

  module.exports = @instance ?= new Singleton()