Singleton class in coffeescript

Question

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.

You might look into running this code in a debugger, to see 'why' or 'how' this code does what it does. — Dean Radcliffe, Sep 17 '14 at 02:36

score 9 · Accepted Answer · edited May 23 '17 at 12:16

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 recommend 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.

Perfect!! This is exactly what i wanted to know. Thx. – suish Sep 17 '14 at 09:26 — suish, Sep 17 '14 at 09:26

score 0 · Answer 2 · answered Sep 16 '14 at 18:39

0

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.

answered Sep 16 '14 at 18:39

Dean Radcliffe

2,194
22
29

I'm not asking the definition of 'singleton pattern' but the reason why the code in the question could be singleton. – suish Sep 16 '14 at 23:34
I explained what singleton was, and why your code was singleton, and why you might choose singleton. If you're looking for more, maybe your question needs to reflect what you need. – Dean Radcliffe Sep 17 '14 at 02:30

Xavier S. · Answer 3 · 2014-09-16T20:21:22.377

-1

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()

edited Sep 16 '14 at 20:21

answered Sep 16 '14 at 20:01

Xavier S.

1,147
13
33

Singleton class in coffeescript

3 Answers3