JSFiddle - React, Tailwind, and code Playground

JavaScript

// Using ES6 Classes.
class Singleton {
  static getInstance() {
    if (!this.instance) {
      this.instance = new Object('Test');
    }
    return this.instance;
  }
}

console.log(Singleton.getInstance() === Singleton.getInstance());

// Using functions.
function SingletonWrapper() {
  let instance;

  return {
    getInstance: function() {
      if (!instance) {
        instance = new Object('Test');
      }
      return instance;
    }
  }
}

const singletonWrapper = SingletonWrapper();

console.log(singletonWrapper.getInstance() === singletonWrapper.getInstance());