frozen weakmap encapsulation

frozenWeakmapEncapsulation.js

/*
https://github.com/tc39/proposal-class-fields/blob/master/PRIVATE_SYNTAX_FAQ.md#how-can-you-model-encapsulation-using-weakmaps
"There's a potential pitfall with this approach, though"
...so prevent it!
*/
const Person = (function(){
  const privates = new WeakMap;
  let ids = 0;
  return class Person {
    constructor(name, makeGreeting) {
      this.name = name;
      privates.set(this,Object.freeze({id: ids++, makeGreeting}));//prevention
    }
    equals(otherPerson) {
      return privates.get(this).id === privates.get(otherPerson).id;
    }
    greet(otherPerson) {
      return privates.get(this).makeGreeting(otherPerson.name);
    }
  }
})();
let alice = new Person('Alice', name => `Hello, ${name}!`);
let bob = new Person('Bob', name => `Hi, ${name}.`);
alice.equals(bob); // false
alice.greet(bob); // === 'Hello, Bob!'
//At first glance this appears fine, but:

let mallory = new Person('Mallory', function(name) {this.id = 0; return `o/ ${name}`;});
mallory.greet(bob); // === 'o/ Bob'
mallory.equals(alice); // false. Phew!