cat-singletone.js

const cat = {
   name: 'Snowball',
   talk() { 
     console.log('Мяу');
   },
   getInstance() {
    return this;
   }
}
const instance1 = cat.getInstance()
const instance2 = cat.getInstance()
console.log(instance1 === instance2) //true

//IIFE
var myCat = (function () {
    let instance;
 
    function createInstance() {
        const object = {
          name: 'Snowball',
          talk() { 
             console.log('Мяу');
          }
          return object;
        }
    } 
    return {
        getInstance: function () {
            if (!instance) {
                instance = createInstance();
            }
            return instance;
        }
    };
})();
const instance1 = cat.getInstance()
const instance2 = cat.getInstance()
console.log(instance1 === instance2) //true

//constructor + IIFE
const cat() = !function() {
   let intance;
   return function() {
       if (instance) {
           return instance
       }
       intance = {
          name: 'Snowball',
          talk() { 
             console.log('Мяу');
          }
          return object;
       }
       this = instance;
    }
}
const instance1 = new cat();
const instance2 = new cat();
console.log(instance1 === instance2) //true

//es6
class Cat {
    constructor() {
        if (Cat.instance) {
            return Cat.instance;
        }
        this.name = "Cнежок";
        Cat.instance = this;
    }

    talk() { 
        console.log('Мяу');
    }
}
const instance1 = new MyCat()
const instance2 = new MyCat()
console.log(instance1 === instance2) //true