Singleton instance scope confusion in JavaScript

Question

I am learning Design Patterns in JavaScript and I was going through the Singleton design pattern. Here is the code:

var SingletonTester = (function () {
    // options: an object containing configuration options for the singleton
    // e.g var options = { name: 'test', pointX: 5};
    function Singleton(options) {
        // set options to the options supplied or an empty object if none provided.
        options = options || {};
        //set the name parameter
        this.name = 'SingletonTester';
        //set the value of pointX
        this.pointX = options.pointX || 6;
        //set the value of pointY
        this.pointY = options.pointY || 10;
    }
            // this is our instance holder
    var instance;

    // this is an emulation of static variables and methods
    var _static = {
        name: 'SingletonTester',
        // This is a method for getting an instance
        // It returns a singleton instance of a singleton object
        getInstance: function (options) {
            if (instance === undefined) {
                instance = new Singleton(options);
            }
            return instance;
        }
    };
    return _static;
})();
var singletonTest = SingletonTester.getInstance({
    pointX: 5
});
var singletonTest1 = SingletonTester.getInstance({
    pointX: 15
});
console.log(singletonTest.pointX); // outputs 5
console.log(singletonTest1.pointX); // outputs 5

I do not understand why the variable instance gets some value when singletonTest1 is initiated.

Second that ^, but if you still want a Singleton emulation, then Google for "javascript singleton enforcer". There are some Git projects which do that. Makes no proper sense anyway. JS is JS. — Ingmars
– Ingmars, Commented Jan 20, 2016 at 13:02
@DavinTryon This is already a singleton pattern, it works fine. My confusion was, how come instance variable had value when singletonTest1 was initiated. At least read the question before comment. — Bharat Soni
– Bharat Soni, Commented Jan 21, 2016 at 13:26
@Ingmars question is not about I want singleton pattern, it is about something else. I wanted to ask why instance variable was still alive when singletonTest1 was initiated. — Bharat Soni
– Bharat Soni, Commented Jan 21, 2016 at 13:30
@BharatSoni the reason this works is because SingletonTester is a global var. — Davin Tryon
– Davin Tryon, Commented Jan 21, 2016 at 15:14

Davin Tryon · Accepted Answer · 2016-01-21 16:21:03Z

1

When the module SingletonTester is created, it is also called:

var SingletonTester = (function () {
    // ... stuff in here
    var instance;
})(); // <--- here

That last line is a function application ();. After that application the SingletonTester module contains all of it's enclosed state.

Since instance is a property closed over by the SingletonTester closure, instance is alive for the entire existence of SingletonTester.

Side note: Singleton pattern is primarily about creating a thread-safe static instance for sharing across a process. Since JavaScript is single-threaded, this is obviously not a problem. You can instead keep things simple and just use a global var.

answered Jan 21, 2016 at 16:21

Davin Tryon

67.4k15 gold badges149 silver badges139 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

Bharat Soni Over a year ago

Ah, now I get it, got confused. Of course its a closure. Thanks a lot Davin.

Collectives™ on Stack Overflow

Singleton instance scope confusion in JavaScript

1 Answer 1

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related