6-singleton.js

/**
 * Create an example of a JavaScript Singleton.
 * After the first object is created, it will return additional 
 * references to itself
 */

let obj = (function () {
    let objInstance; //private variable
    function create() { //private function to create methods and properties
        let _isRunning = false;
        let start = function() {
            _isRunning = true;
        }
        let stop = function() {
            _isRunning = false;
        }
        let currentState = function() {
            return _isRunning;
        }
        return {
          start:start,
          stop: stop,
          currentState: currentState
        }
       
    }
    return {
        getInstance: function() {
            if(!objInstance) {
              objInstance = create();
            }
            return objInstance;
        }
    };
})();

// obj here is the same for both instances
let obj1 = obj.getInstance();
let obj2 = obj.getInstance();
console.log(obj1.currentState())
console.log(obj2.currentState())
obj1.start()
console.log(obj1.currentState())
console.log(obj2.currentState())