Privates and WeakMaps

gistfile1.js

// Based on a gist by @rwaldron
// https://gist.github.com/2897761

function privatize() {
  var map = new WeakMap();
  return function private(obj) {
    var data = map.get(obj);
    if (!data) {
      map.set(obj, data = {});
    }
    return data;
  };
}

var Person = (function () {

  var private = privatize();

  function Person(name) {
    private(this).name = name;
  }
  Person.prototype.say = function (msg) {
    return private(this).name + ' says: ' + msg;
  };

  return Person;

}());

As a side note, this is "class private". Which means you can access private data of other members of the same class. For example:

Person.prototype.stealName = function (someGuy) {
  return private(someGuy).name; // works if someGuy instanceof Person
};

Why not just:

var Person = (function () {

  var private = {};

  function Person(name) {
    private.name = name;
  }
  Person.prototype.say = function (msg) {
    return private.name + ' says: ' + msg;
  };

  return Person;

}());

Because that would share the private data with all instances of Person. It would be the same as:

var Person = (function () {
  var _name;

  function Person(name) {
    _name = name;
  }
  Person.prototype.say = function (msg) {
    return _name + ' says: ' + msg;
  };

  return Person;
}());

var bob = new Person('Bob');
var peter = new Person('Peter');
bob.say('Hi!'); // Peter says: Hi!

That means it needs to be a map. And it needs to be a WeakMap so that the private record is GC'ed when the class instance is GC'ed.

why not:

const _ = new WeakMap()
const private = (instance) => {
  return _.get(instance)
}

class Person {
  constructor (name) {
    private(this).name = name
  }
  say (msg) {
    return private(this).name + 'says: ' + msg
  }
}

?

EDIT:

Actually, private should be defined as const private = (instance) => _.has(instance) ? _.get(instance) : _.set(instance)

hey @juandopazo, i didn't get this
Because that would leak the data to anyone with a reference to the instance.

can you elaborate more