There are a lot of people who will tell you that a Singleton is an anti-pattern. Be that as it may, they are something you are going to come across at some point in your development travels and it is good to know about them.

A Singleton is pretty much what it sounds like. It is a single function/object that can't be replicated. Any time you instantiate another instance, instead of creating a copy, it returns the single instance of the object. This is somewhat of a controversial design pattern because anything can make changes to the object which results in similar problems to those you encounter with global scope: lack of encapsulation, mutable values, and side effects.

You may find, however, that the pros outweigh the cons. Let's imagine for a moment that we are creating the earth (eep). Our planet is going to have all the necessities of a world: sea, earth, and, of course, critters.

function World() {
    this.earth = []
    this.water = []
    this.creatures =[]
}

Now let's instantiate our world and add a critter:

const myWorld = new World()
myWorld.creatures.push("Kangaroo")

Wonderful. Our world is soon going to be brimming with life. Unfortunately, we run into a problem if elsewhere in our program, you instantiate the world again.

const yourWorld = new World()
console.log(yourWorld)

This is going to return a planet completely barren of critters, including the kangaroo we added earlier. This is not surprising given we instantiated a new world, but this is not what we want for our Singleton. Instantiating a new world should give us a reference to that same function we created earlier. So, let's implement our function as a Singleton.

const World = function () {
  if (World._instance) {
    return World._instance
  }
  World._instance = this
  this.earth = []
  this.water = []
  this.creatures = []
}

Now when we instantiate a World it is going to check if an instance of the world already exists. If it does, it'll return it, otherwise it will create one. Now, if we create myWorld and yourWorld, adding a critter to one will be reflected in the other:

const myWorld = new World()
const yourWorld = new World()

myWorld.creatures.push("Kangaroo")
console.log(myWorld)
// World { earth: [], water: [], creatures: [ 'Kangaroo' ] }
console.log(yourWorld)
// World { earth: [], water: [], creatures: [ 'Kangaroo' ] }

You have yourself a Singleton. You may not realize it but you are probably already using them. If you are using ESM import/export modules those are Singletons. Hopefully, this gives you a better understanding of how they work under the hood, and feel more comfortable using Singletons when the need arises.