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Singleton pattern ​

Purpose ​

The singleton pattern ensures that at most one instance of a class can be created.

For example, we might have a SmartHome class containing things like the name of the smart home, a list of devices, and so on. We would only want one of these objects to exist at any one time, otherwise we could have conflicting settings.

Implementation ​

In javascript, the singleton pattern is extremely simple. We can just create an object literal:

js
const smartHome = {
  homeName: 'Mojo Dojo',
  securityMode: 2
}

However, we can implement the singleton pattern in a traditional class-based way:

js
class SmartHome {
  static instance

  constructor(name, mode) {
    if (SmartHome.instance) {
      throw new Error('A smart home already exists.')
    } else {
      this.homeName = name
      this.securityMode = mode
      SmartHome.instance = this
    }
  }
}

Now, we are only able to create one instance of SmartHome.

js
// create an instance
const smartHome = new SmartHome('Mojo Dojo', 2)
console.log(smartHome)

// retrieve the instance again
console.log(SmartHome.instance)
console.log(smartHome === SmartHome.instance) // true

// try to make a new instance
const secondHome = new SmartHome('Casa House', 1) // error
console
SmartHome { homeName: 'Mojo Dojo', securityMode: 2 }
SmartHome { homeName: 'Mojo Dojo', securityMode: 2 }
true
file:///home/shai/Repos/corndel/smart-home/models/SmartHome.js:6
      throw new Error('A settings object already exists.')
            ^

Error: A settings object already exists.

This would allow us to benefit from class syntax such as private properties, etc.