Which means following is not possible in Es6 classes.

class Cache{  
   let instance = null;
}

In order to avoid re-instantiation of same class , we need to maintain it’s state inside a variable which itself is not dependent upon the same class.

Singleton class in es6

Following is my approach to have a single instance of a class , no matter how many time you reinitiate it.

/* 
  * Setting up block level variable to store class state
  * , set's to null by default.
*/
let instance = null;

class Cache{  
    constructor() {
        if(!instance){
              instance = this;
        }

        // to test whether we have singleton or not
        this.time = new Date()

        return instance;
      }
}

Testing singleton

On above class we have defined a time property to make sure we have singleton working properly.

 let cache = new Cache()
 console.log(cache.time);

 setTimeout(function(){
   let cache = new Cache();
   console.log(cache.time);
 },4000);

If it prints the same time for both the instances, it means we have a working singleton.