Privacy via WeakMap

0-privacy.js

let _ = new WeakMap;
  
export default {
  init: function (obj, map = {}) {
    _.set(obj, map);
  },
  get: function (obj, key) {
    return _.get(obj)[key];
  },
  set: function (obj, key, value) {
    return _.get(obj)[key] = value;
  }
};

1-employee.js

import {init, get, set} from 'privacy'

export default class Employee {
  constructor (name, title) {
    init(this, {name, title});
  }

  get name () { return get(this, 'name'); }
  
  get title () { return get(this, 'title'); }
  set title (value) { return set(this, 'title', value); }
}

2-main.js

import Employee from 'employee'

let a = new Employee('Malory', 'CEO');
let b = new Employee('Cyril', 'Accountant');
let c = new Employee('Carol', 'Secretary');

a.name;             // "Malory"
a.title;            // "CEO"

b.name;             // "Cyril"
b.title;            // "Accountant"
b.title = "VP";     // "VP"

c.name;             // "Carol"
c.title;            // "Secretary"
c.name = "Sheryl";  // Error (property is read-only)