ES6 singleton pattern: constructor returns an instance scoped to the module. Without using the `new` operator.

es6-singleton-module-scoped-instance.js

const singleton = Symbol();
const singletonEnforcer = Symbol();

class Singleton {
  constructor(enforcer) {
    if (enforcer !== singletonEnforcer) {
      throw new Error('Cannot construct singleton');
    }

    this._type = 'Singleton';
  }

  static get instance() {
    if (!this[singleton]) {
      this[singleton] = new Singleton(singletonEnforcer);
    }

    return this[singleton];
  }

  singletonMethod() {
    return 'singletonMethod';
  }

  static staticMethod() {
    return 'staticMethod';
  }

  get type() {
    return this._type;
  }

  set type(value) {
    this._type = value;
  }
}

export default Singleton;

// index.js
import Singleton from './Singleton';

// Instantiate
// console.log(new Singleton); // => Cannot construct singleton

// Instance
const instance2 = Singleton.instance;

// Prototype Method
console.log(instance2.type, instance2.singletonMethod());

// Getter/Setter
instance2.type = 'type updated';
console.log(instance2.type);

// Static method
console.log(Singleton.staticMethod());