Singleton Design Pattern with the help of car engine analogy.

SingletonDesignPatterns.js

// Design Patterns

// Singleton Design Pattern

const Singleton = (function () {
  let instance;

  function StartCar() {
    this.gear = 0;
    this.changeGear = function (gear) {
      this.gear = gear;
    };
    this.getGear = function () {
      return this.gear;
    };
  }

  function createInstance() {
    if (!instance) {
      instance = new StartCar();
    }

    return instance;
  }

  return {
    getInstance: createInstance
  };
})();

let instance1 = Singleton.getInstance();
let instance2 = Singleton.getInstance();

instance1.changeGear(2);
instance2.changeGear(5);

console.log(instance1.getGear());
console.log(instance2.getGear());
console.log(instance1 === instance2); // print true;