Usefall

Purpose

Many things of a kind may exist.

I want to ensure I always work with the same single one of them.

Lazy Wolf

Ah my house is a mess!

Maybe it’s because I use a new dish every time I eat…

class LazyWolf {
  eat(dish) {
    console.log("Nom nom nom");
  }
}

class Dish {}

const lazyWolf = new LazyWolf();

let breakfastDish = new Dish();
breakfastDish.food = "apples";
lazyWolf.eat(breakfastDish);

let dinnerDish = new Dish();
dinnerDish.food = "grapes";
lazyWolf.eat(dinnerDish);

Angry House

I am angry because you made a mess.

Just make it impossible to use another dish in the house!

Lazy Wolf

How do I do that?

Also you if you are angry while you are smiling!?

Angry House

This is my angry face.

Make it impossible for yourself to call the new Dish() constructor directly.

Create a static factory method that always gives you the same dish instance.

class Dish {
  static isBeingCreatedByFactory = false;
  static singleInstance = null;

  constructor() {
    if (!Dish.isBeingCreatedByFactory) {
      throw new Error("Not allowed to create Dish.");
    }
  }

  static getInstance() {
    if (!Dish.singleInstance) {
      Dish.isBeingCreatedByFactory = true;
      Dish.singleInstance = new Dish();
      Dish.isBeingCreatedByFactory = false;
    }

    return Dish.singleInstance;
  }
}

const lazyWolf = new LazyWolf();

const breakfastDish = Dish.getInstance();
breakfastDish.food = "apples";
lazyWolf.eat(breakfastDish);

const dinnerDish = Dish.getInstance();
dinnerDish.food = "bananas";
lazyWolf.eat(dinnerDish);

Lazy Wolf

Wow, It’s impossible create a new dishes now.

There is only one single dish instance in the house now.

I keep getting back and using the same single dish for my food.

My house is finally clean!

Angry House

If you mess me up again, I will mess you up.

You are welcome.

Solution

Make it so only a single instance of a kind may exist (a singleton).

Make instance creation only possible via a function that returns the same single instance.