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Privates and WeakMaps

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// 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;
 
}());

Why not just a global function private?

Owner

Because that would leak the data to anyone with a reference to the instance.

Donde hay un resumen de las nuevas features de ES6 que ya se puedan usar en Node/V8?

Owner

Esa es una buena pregunta. Hay que ver en cada versión de Node qué build de V8 usaron. Es bastante molesto. Y peor aún, hay cosas que están detrás de flags en V8 y por ende no están en Node. Otras están detrás de flags en node como node --harmony-proxies.

Owner

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;

}());
Owner

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.

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