Singleton in JavaScript

Posted on December 5th, 2018

Singleton is one of the better-known patters in programming. While, by some, seen as an anti-pattern, it is worth knowing something about it.

Creating such class isn't really hard, but has some caveats. Firstly, its constructor has to return the instance. Secondly, such class cannot be extended nor modified by any ancestor. Generally speaking, extending will only link to the initial instance.

So, how to write a singleton class? First, we start like normal:

class SingletonClass {
    constructor() {}
}

This is a standard class notation in ES2015 format. Please note the name and remember it. While creating anything, and especially a singleton class, choose the name wisely. Now I know that naming things is hard, but here we will be using the name rather that the this.

Second step is to define the instance key within the class. Please note that this is static value, it refers to the class, not to the instance of the class.

class SingletonClass {
    constructor() {
        if (!!SingletonClass.instance) {
            return SingletonClass.instance;
        }
        
        SingletonClass.instance = this;
        
        return this;
    }
}

Let's do some explaining. The constructor starts with checking whether SingletonClass.instance exists. Why not this? Like I said earlier, we refer to the class. This is its static value, not related to an instance. If the condition is met, it means that the class has been already created sometime ago and that old instance can be returned instead creating new one.

Next we're assigning SingletonClass.instance to this, meaning, we are binding current instance to the class, coupling it.

Lastly, we return this. This can be confusing, but look up. We've returned SingletonClass.instance before. Without this return, it would also work, but it's a good practice to keep consist return from a method.

Okay, this is all fine and well, but how can we prove that creating new instances is not possible? Well, good question. Let's enhance our class with something to do, for example, let it return the name we give it (yeah, programming!).

class SingletonClass {
    constructor(name = "") {
        if (!!SingletonClass.instance) {
            return SingletonClass.instance;
        }
        
        SingletonClass.instance = this;
        
        this.name = name;
        
        return this;
    }
    
    getName() {
        return this.name;
    }
}

Let's create some instances now:

const instanceOne = new SingletonClass("One");
const instanceTwo = new SingletonClass("Two");
const instanceThree = new SingletonClass();

Okay, simple as that. So, now we can log it:

console.log(`Name of instanceOne is "${instanceOne.getName()}"`);
console.log(`Name of instanceTwo is "${instanceTwo.getName()}"`);
console.log(`Name of instanceThree is "${instanceThree.getName()}"`);

Can you guess, what those will log out?

Name of instanceOne is "One"
Name of instanceTwo is "One"
Name of instanceThree is "One"

Why is that? Because it's singleton class! It always uses the one instance that was created in the beginning. Try changing the order, move instanceThree above instanceOne. What are those console.logs saying now?

Another thing is extending. This is a very popular feature of object-oriented programming. Popular, misused, abused etc. Singletons, technically, can't be extended, they don't have any ancestors. But, seeing is believing. Let's create a new class that will extend the old one:

class Extending extends SingletonClass {
    shoutName() {
        return this.name.toUpperCase();
    }
}

So, normally Extending should have two — getName derived from SingletonClass, and shoutName, it's own. Let's see:

const A = new Extending();

console.log("getName" in A);
console.log("shoutName" in A);

What do you see in the console?

true
false

Why? Because the instance was created sometime ago, while we defined instanceOne. The extension process can't even start, because SingletonClass constructor returns the instance first thing.

The only way to extend a singleton class is to do it before any instance is initiated. But this is an extreme anti-pattern, because you can't be sure that someone will not use the base class before you do the extending. Sure, you can extend it right after the declaration, but... why?


So, we now know how to create a class that has only one instance. Is it useful? If you want to use the benefits of a class but without allowing to use it freely. Sounds sarcastic, but isn't. Think of a logger. Why would you need to create multiple loggers? You should just use one, and it can be build using singleton. Think of caching database. You want all the data to be available without thinking of sharing the state somewhere.


The entire code is available at my CodePen, along with an additional check for multiple instances for the non-believers.

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