javascript singleton -- is this correct

Question

I'm writing javascript singleton class and want to use singleton pattern like this:

function TestClass() {
    var self = TestClass.prototype;
    if (self.instance) return self.instance;
    self.instance = this;

    //methods and props declarations
}

var x = new TestClass;
var y = new TestClass;

console.log(x === y); // true

It seems to be worked as i expected, but i worry about memory leaks. So i decided to ask experts, if it is the correct solution

the `new` keyword creates a new instance, so you are not creating a singleton here — Ibu, Nov 20 '13 at 17:41
This thread http://stackoverflow.com/questions/1895635/javascript-singleton-question has some good solutions — Rob M., Nov 20 '13 at 17:46
Doesn't this JSFiddle suggest that this doesn't work? http://jsfiddle.net/BUqF5/ I was not aware that you could to singletons in JavaScript so this is still new to me — Jason Sperske, Nov 20 '13 at 17:48
Here's a good SO post about singletons: http://stackoverflow.com/questions/1635800/javascript-best-singleton-pattern — DrCord, Nov 20 '13 at 17:55

score 1 · Accepted Answer · answered Nov 20 '13 at 17:57

1

Not exactly. I generally do something like this when I need a singleton:

function TestClass() {
  if (TestClass.__singleton) {
    return TestClass.__singleton;
  }

  // begin construction ...

  this.a = function() { };
  this.b = 1;

  // ... end construction

  TestClass.__singleton = this;

} // TestClass

var x = new TestClass();  // creates a new TestClass and stores it
var y = new TestClass();  // finds the existing TestClass

console.log(x === y);  // true

y.b = 2;
x.c = 3;

console.log(x.b === y.b && x.c === y.c); // true

If my understanding is correct, subsequent TestClass instantiations, in this case, will create a small, partially-defined TestClass object, but will immediately mark them for garbage collection when TestClass.__singleton is returned.

answered Nov 20 '13 at 17:57

svidgen

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this is almost the way i do it. ) – Alex Cube Nov 20 '13 at 18:03
@AlexCube I'd be interested to see your variation as an answer here! – svidgen Nov 20 '13 at 18:05
My variation is at the topic ) The only difference is that i'm using prototype, but results are the same – Alex Cube Nov 20 '13 at 18:12
@AlexCube Ah, touche -- didn't even check the name on the OP! – svidgen Nov 20 '13 at 18:25

Peter Aron Zentai · Answer 2 · 2013-11-20T18:34:52.820

0

Your code is not wrong per se but not good either. you are not creating a singleton, just "hijack" the prototype. #svidgen gave a correct response.

edited Nov 20 '13 at 18:34

answered Nov 20 '13 at 17:56

Peter Aron Zentai

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Thanx for your answer. But i'm wondering why "hijacking" the prototype is bad? It works as i expected, but i still worry about memory leaks due to recursive linking. And is the example above (TestClass.__singleton) correct? – Alex Cube Nov 20 '13 at 18:10
Yes. That is a more straightforward way than mine. No leaking there, JavaScript has GC. – Peter Aron Zentai Nov 20 '13 at 18:26
The hijacked prototype is at minimum weird/uncommon. you dont want anything uncommon when dealing with JavaScript, as it is only the principles that keeps chaos from coming when it comes to large projects in JavaScript. – Peter Aron Zentai Nov 20 '13 at 18:28
Also, prototype is not meant to store instance data. except from very specific advanced scenarios prototype should only contain functions, data should be on an instance. – Peter Aron Zentai Nov 20 '13 at 18:32

javascript singleton -- is this correct

2 Answers2