Design pattern - Singleton pattern in JavaScript

Question

I'm following this article. I understood what singleton pattern is but I'm not be able to understand the code.

let Singleton = (function(){
    let instance = null;

    function createInstance() {
        let obj = new Object("I am the new instance");
        return obj;
    }

    return {
        getInstance:function() {
            console.log(instance);
            if(instance === null) {
                instance = createInstance();
            }
            return instance;
        }
    }
})();

function test() {
    let instance1 = Singleton.getInstance();
    let instance2 = Singleton.getInstance();
    console.log(Object.is(instance1,instance2));
}
test();

why instance1 and instance2 are same, we are always initializing the instance to null whenever we call Singleton.

Amadan · Accepted Answer · 2021-08-19 04:00:54Z

1

we are always initializing the instance to null whenever we call Singleton.

You do not. Singleton is the object in the return statement, which does not include the statement let instance = null. This statement is only executed once; this variable is captured under closure in getInstance method of the returned object (Singleton.getInstance). The code is equivalent to

let instance = null;

function createInstance() {
    let obj = new Object("I am the new instance");
    return obj;
}

let Singleton = {
    getInstance: function() {
        console.log(instance);
        if (instance === null) {
            instance = createInstance();
        }
        return instance;
    }
}

except that instance and createInstance are hidden inside the IIFE's scope.

You can verify how many times (and when) let instance = null executes by inserting more logging statements.

answered Aug 19, 2021 at 4:00

Amadan

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1 Comment

dhruv Over a year ago

Got it, I was interpreting Singleton as a function but is actually an object with getInstance property. Thank you Amadan.

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