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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; | |
}()); |
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
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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
.