Simplest/Cleanest way to implement singleton in JavaScript?

Question

What is the simplest/cleanest way to implement singleton pattern in JavaScript?

Answer 1

In es6:

class Singleton {
  constructor () {
    if (!Singleton.instance) {
      Singleton.instance = this
    }
    // Initialize object
    return Singleton.instance
  }
  // Properties & Methods
}

const instance = new Singleton()
Object.freeze(instance)

export default instance