WeakMap privacy

privacy.js

var User = (function() {

  var cache = new WeakMap();

  function User( name ) {
    cache.set( this, {} );
    this.name = name;
  }

  User.prototype = {

    set password( password ) {
      cache.get( this ).password = password;
    },

    auth: function( password ) {
      return password === cache.get( this ).password;
    }
  
  };

  return User;

}());


var user = new User('John');

user.password = 'P@ssw0rd!';
user.password; // undefined
user.auth('correcthorsebatterystaple'); // false
user.auth('P@ssw0rd!'); // true