See https://medium.com/@mnjul/private-class-members-in-es6-a-complete-guide-3d7f83c3af5

es6-private-class-members-1.js

// Apache License: https://www.apache.org/licenses/LICENSE-2.0

;(function (exports){

// classPrivateMethods -> WeakMap (instance -> members)
let privateMemberMap = new WeakMap();

function createInternalFunction(classPrivateMethods) {
  if (!privateMemberMap.has(classPrivateMethods)) {
    privateMemberMap.set(classPrivateMethods, new WeakMap());
  }

  function generatePrivateMemberObject(instance) {
    let obj = {};

    Object.entries(classPrivateMethods)
      .forEach(([name, func]) => {
        obj[name] = func.bind(instance);
      });

    return obj;  
  }

  return function(instance) {
    if (!privateMemberMap.get(classPrivateMethods).has(instance)) {
      privateMemberMap.get(classPrivateMethods).set(
        instance,
        generatePrivateMemberObject(instance)
      );
    }

    return privateMemberMap.get(classPrivateMethods).get(instance);   
  }
}

exports.createInternalFunction = createInternalFunction;

})(window);


;(function (exports){

const privateMethods = Object.seal({
  _privMethod1() {
    console.log(this.constructor.name, "At private method 1 on obj", _(this)._privProperty);
  },

  _privMethod2() {
    console.log(this.constructor.name, "At private method 2 on obj", _(this)._privProperty);
    _(this)._privMethod1();
  },

  _privMethod3() {
    console.log(this.constructor.name, "At private method 3 on obj", _(this)._privProperty);
    this.pubMethod4();
  },
});

let _ = createInternalFunction(privateMethods);

class Class1 {
  constructor(property) {
    _(this)._privProperty = property;
  }

  pubMethod1() {
    console.log(this.constructor.name, "At public method 1 on obj", _(this)._privProperty);
    _(this)._privMethod1();
  }

  pubMethod2() {
    console.log(this.constructor.name, "At public method 2 on obj", _(this)._privProperty);
    _(this)._privMethod2();
  }

  pubMethod3() {
    console.log(this.constructor.name, "At public method 3 on obj", _(this)._privProperty);
    _(this)._privMethod3();
  }

  pubMethod4() {
    console.log(this.constructor.name, "At public method 4 on obj", _(this)._privProperty);
  }  
}

exports.Class1 = Class1;

})(window);

let obj1 = new Class1('some');
obj1.pubMethod1();
obj1.pubMethod2();
obj1.pubMethod3();
obj1.pubMethod4();

let obj11 = new Class1('another');
obj11.pubMethod1();
obj11.pubMethod2();
obj11.pubMethod3();
obj11.pubMethod4();