singleton.js

// Hello.js

// this one creates a new object, `priv` holds a unique reference that is not
// exported outside this file
// this is a really important variable, it needs to be privately scoped, it
// will be used to check if constructor is actually called properly
const priv = {};

export default class Hello {
  static getInstance() {
    return instance;
  }

  constructor(privCheck) {
    // it's not possible to call this constructor outside this module, because
    // 'priv' variable is not exported
    if (privCheck !== priv) {
      throw new Error("Hello is a singleton. Please use `getInstance` method instead");
    }
  }

  getGreeting() {
    return "Hello, world!";
  }
}

const instance = new Hello(priv);