Singleton using closure and IIFE

// singleton definition creation
const Singleton = (function () {
  let instance;
  
  function createInstance() {
      // returns an object or value that will hold Singleton data
    return {
      data: "I am the singleton instance"
    };
  }

  // creating method to get the instance via closure
  return {
    getInstance: function () {
      // runs when instance does not exist
      if (!instance) {
        instance = createInstance();
      }
      // returns the same instance when singleton exists
      return instance;
    }
  };
})(); // immediately invoking function here

// usage
const instance1 = Singleton.getInstance();
const instance2 = Singleton.getInstance();

console.log(instance1 === instance2); // true

Using Class

class Singleton {
  // Static property to hold the singleton instance
  static instance = null;

  constructor() {
    // handling multiple instantiation
    if (Singleton.instance) {
      return Singleton.instance;
    }

    // object itself as a singleton
    Singleton.instance = this;
    
    // some data to store or reference
    this.data = "Singleton instance data";
  }
}

const singleton1 = new Singleton();
const singleton2 = new Singleton();

console.log(singleton1 === singleton2); // true

Exporting to another file

// singleton.js
class Singleton {
  constructor() {
    if (Singleton.instance) {
      return Singleton.instance;
    }
    Singleton.instance = this;
    this.data = "Module Singleton Data";
  }
}

export default new Singleton();

// Usage in another file
import Singleton from './singleton.js';

const singleton1 = Singleton;
const singleton2 = Singleton;

console.log(singleton1 === singleton2); // true