[Javascript] Singleton Pattern

Source: https://javascriptpatterns.vercel.app/patterns/design-patterns/singleton-pattern

With the Singleton Pattern, we restrict the instantiation of certain classes to one single instance. This single instance is unmodifiable, and can be accessed globally throughout the application.

Create a Singleton in Javascritp:

Class:

let instance;

// 1. Creating the `Counter` class, which contains a `constructor`, `getInstance`, `getCount`, `increment` and `decrement` method. 
// Within the constructor, we check to make sure the class hasn't already been instantiated. 
class Counter {
  constructor() {
      if (instance) {
      throw new Error("You can only create one instance!");
    }
    this.counter = counter;
    instance = this;
  }

  getCount() {
    return this.counter;
  }

  increment() {
    return ++this.counter;
  }

  decrement() {
    return --this.counter;
  }
}

// 2. Setting a variable equal to the the frozen newly instantiated object, by using the built-in `Object.freeze` method. 
// This ensures that the newly created instance is not modifiable.
const singletonCounter = Object.freeze(new Counter());

// 3. Exporting the variable as the `default` value within the file to make it globally accessible.
export default singletonCounter;

2 points:

1. If there is an instance already, we want to throw error.
2. Use Object.freeze function to make it unmodifiable.

Object:

let counter = 0;

export default Object.freeze({
  getCount: () => counter,
  increment: () => ++counter,
  decrement: () => --counter,
})

Be carefully about using Singelton:

Depedency Hiding: When importing another module, it may not always be obvious that that module is importing a Singleton. This could lead to unexpected value modification within the Singleton, which would be reflected throughout the application.

Let's say Counter is a singelton. module A and B both import counter and call `increment` twice in each modules.

If in Moulde A, we import Module B. Then `increment` will be called 4 times. But in Module A, we only call twice, that might cause confusion when you just looking at Module A code.

Unnecessary: ES2015 Modules are singletons by default. We no longer need to explicitly create singletons to achieve this global, non-modifiable behavior.