private state in ES6 classses

example.js

const privates = Privates();
  
class MyClass{
  
  constructor(a,b,c){
    Object.assign(privates(this),{a,b,c})
  }
  
  sum(){
    var {a,b,c} = privates(this);
    return a + b + c;
  }
  
}
  

Privates.js

function Privates() {

    var map = new WeakMap();

    return function (obj) {

        if (!map.has(obj))
            map.set(obj, {});

        return map.get(obj);
    };
}