singleton_2_1.js

let singleton = Symbol();
let singletonEnforcer = Symbol();

class Singleton {

  constructor(enforcer) {
    if (enforcer !== singletonEnforcer)
       throw "Instantiation failed: use Singleton.getInstance() instead of new.";
    // код конструктора
  }

  static get instance() {
    if (!this[singleton])
        this[singleton] = new Singleton(singletonEnforcer);
    return this[singleton];
  }
  
  static set instance(v) { throw "Can't change constant property!" }
}

export default Singleton;

singleton_2_2.js

import Singleton from 'singleton';

// Test constructor
try { var obj0 = new Singleton() } catch(c) { console.log(c) }
console.log('obj0', obj0 );


// Create and get object
let obj1 = Singleton.instance;
let obj2 = Singleton.instance;



console.log(obj2.foo === 123 );
obj1.foo = 456;
console.log('obj2', obj2 );
console.log('obj1 === obj2',  obj1 === obj2 );

try { var obj3 = new Singleton() } catch(c) { console.log(c) }
console.log('obj3', obj3);