You Can Create Private Properties In JS (accessor pattern)

It's very common for JavaScript developers to claim that it's impossible to create truly encapsulated properties and methods on an object and use them on its prototype.

In ES6, there are a few ways of easily achieving private properties without memory leaks. For example, you could use a ES6 Symbol:

// Closure
var SomeClass = function() {
    var priv_prop = Symbol();
    var SomeClass = function() {
        this[priv_prop] = 200;
    };
    SomeClass.prototype.test = function() {
        console.log(this[priv_prop]);
    };
    return SomeClass;
}();
var instance = new SomeClass();
instance.test(); // `200` logged

Alternatively, you could use a WeakMap:

// Closure
var SomeClass = function() {
    var priv_prop1 = new WeakMap();
    var priv_prop2 = new WeakMap();
    var SomeClass = function() {
        priv_prop1.set(this, 100);
        priv_prop2.set(this, 200);
    };
    SomeClass.prototype.test = function() {
        console.log(priv_prop1.get(this));
        console.log(priv_prop2.get(this));
    };
    return SomeClass;
}();
var instance = new SomeClass();
instance.test(); // `100` and `200` logged

The problem with the Symbol method is that you can still access those properties using Object.getOwnPropertySymbols. In either case, you would likely have to include bulky polyfills in production code.

Prior to ES6, there was no obvious way to create private properties usable on prototype. The claim is that you either have to abandon the idea or use a memory leaky Map (alternatively, you could use 2 arrays). But what if I told you that there is actually a way to do this that is cross-browser, needs no polyfills or ES6, and doesn't produce memory leaks?

The idea

I haven't seen this method used by anyone (EDIT: it was pointed out to me that I wasn't the first one to come up with this method. Read more here), so I'd like to call it an accessor pattern. The idea is to create a closure, create a key inside the closure and create a storage for private properties that can only be accessed if the correct key is provided. Here's how you would implement it:

/* Here's how you can create truly private
   properties in JS and use them on prototype */

// Closure
var SomeClass = function() {
    var key = {};

    var private = function() {
        var obj = {};
        return function(testkey) {
            if(key === testkey) return obj;
            // If the user of the class tries to access private
            // properties, they won't have the access to the `key`
            console.error('Cannot access private properties');
            return undefined;
        };
    };

    var SomeClass = function() {
        this._ = private(); // Creates a private object
        this._(key).priv_prop = 200; // this._(key) will return the private object
    };

    SomeClass.prototype.test = function() {
        console.log(this._(key).priv_prop); // Using property from prototype
    };

    return SomeClass;
}();

var instance = new SomeClass();
instance.test(); // `200` logged

var wrong_key = {};
instance._(wrong_key); // undefined; error logged

Pretty simple, huh? private function creates the private storage and returns a private access function that will only return the storage if the correct key is provided. Then, in constructor, we assign this private access function to this._ which can be easily used on the prototype, provided that the prototype properties also have access to the key. Basically, there is no way to access the private storage without having the correct key. Hence, if the user tries to call this._ with any argument, maybe with a wrong_key, then the attempt will fail, error will be logged, and all the user will get is undefined.

Advantages of this method:

• It's quite simple. All you need is to create a private storage, and you can access all private properties inline within a constructor / prototype.
• It lets you create truly private properties that can be accessed from the prototype. The user of the class will not be able to access them.

Disadvantage of this method:

• It slightly pollutes the namespace, because you have to assign the private storage to this._ or alike, but there's probably no other way to do this.

A problem

A minor problem with this method is that, in case of prototypal inheritance, if both child and parent use the same property name for the private access function (in this example, this._), then the parent's private properties cannot be accessed within parent's prototype, because this._ will refer to child's private access function. Here's what I mean,

// Note: this Gist is to show a problem with accessor pattern
// and inheritance. Do not use!

var private = function(key) {
    var obj = {};
    return function(testkey) {
        if(key === testkey) return obj;
        console.error('Cannot access private properties');
        return undefined;
    };
};

var ParentClass = function() {
    var key = {};
    var ParentClass = function() {
        this._ = private(key);
        this._(key).priv_prop = 100;
    };
    ParentClass.prototype.parent_test = function() {
        console.log(this._(key).priv_prop);
    };
    return ParentClass;
}();

var ChildClass = function() {
    var key = {};
    var ChildClass = function() {
        ParentClass.call(this);
        this._ = private(key);
        this._(key).priv_prop = 200;
    };
    ChildClass.prototype = Object.create(
        ParentClass.prototype
    );
    ChildClass.prototype.test = function() {
        console.log(this._(key).priv_prop);
    };
    return ChildClass;
}();

var instance = new ChildClass();
instance.test(); // `200` is correctly logged
instance.parent_test(); // ERROR! (expected result: `100`)

When instance.parent_test is called, this._ inside it will refer to the child's private access function, hence, the key will mismatch and the error will be logged. However, this problem can be quite easily solved.

The final solution

The best solution is to namespace and make sure that parent and child have different property names for their private access functions. Here's the final solution:

/* Here's how you can create truly private
   properties in JS and use them on prototype */

// Creates private storage, secures with a key, and
// returns a private access function
var private = function(key) {
    var obj = {};
    return function(testkey) {
        if(key === testkey) return obj;
        console.error('Cannot access private properties');
        return undefined;
    };
};

// Create closure
var ParentClass = function() {
    var priv = '_ParentClass' + Math.random(); // Namespace
    var key = {}; // Create key withing closure
    var ParentClass = function() {
        this[priv] = private(key); // Create private storage
        this[priv](key).priv_prop = 100; // Modify any private data
    };
    ParentClass.prototype.parent_test = function() {
        console.log(this[priv](key).priv_prop); // Access private data
    };
    return ParentClass;
}();

var ChildClass = function() {
    var priv = '_ChildClass' + Math.random();
    var key = {};
    var ChildClass = function() {
        ParentClass.call(this);
        this[priv] = private(key);
        this[priv](key).priv_prop = 200;
    };
    ChildClass.prototype = Object.create(
        ParentClass.prototype
    );
    ChildClass.prototype.test = function() {
        console.log(this[priv](key).priv_prop);
    };
    return ChildClass;
}();

var instance = new ChildClass();
instance.test(); // `200` logged, as expected
instance.parent_test(); // `100` logged, as expected

// Yet, there's no way to access the property from outside of the closure

Pretty much the only difference from the previous code snippet is that we replaced this._ for both child and parent classes with this[priv], where priv is namespaced and randomly generated to ensure that private access function is stored under a different property name for child and parent.

Another recommendation I can make is that you should probably secure this[priv] by making it non-configurable, non-enumerable and read-only:

    Object.defineProperty(this, priv, {
        value: private(key)
    })

Instead of just

    this[priv] = private(key)

This will make sure that user will not be able to remove or modify this[priv], which is crucial for correct private storage functioning.

Conclusion

Go ahead and use accessor pattern! It allows you to create truly encapsulated properties and use them on a prototype. Let others know about this method so we don't continue the misconception that privacy is impossible to achieve in JavaScript. Sharing this article will also help 😊

Comments

[#benaryorg]

I haven't extensively worked with JavaScript, so out of curiosity:
JavaScript is readable by anyone with access to the source code, including the user in most cases.
Why would I go through all of this?
I don't see a reason to restrict access to any property of an application, besides the obvious "Don't touch it" which I usually solve using some sort of PEP 8 naming scheme.
I can imagine that maybe there are security critical applications (think of maybe E2E encrypted messaging) that want to prevent encryption keys to be accessible by injected JavaScript, but that's about it. (EDIT: that however seems to be more of a security through obsurity kind of thing)

Am I overlooking something here?

[Kirill Shestakov]

You're right that the source code is available to everyone and that people can just step through your code and gain access to any object or a variable they encounter. But there's still importance in making properties private as a means to distinguish between a property people can change, play around with and a property that is exposed to the public yet people shouldn't touch. Example: suppose you're creating a Class and publishing it as a library. Users of the library can use your Class to create instances that will have some properties. You might want to have some properties per each Class instance that you don't want people to mess with, in which case it makes sense to make them private. I don't think this solution can help with security though.

[#benaryorg]

Wouldn't a convention/readme saying "don't touch references starting with an underscore" suffice for that?
Building wrappers and doing language-magic seems a bit overkill.

[Kirill Shestakov]

If it suffices for you to just mention it in Readme, good. It's probably not enough for me.

Also, the method I described doesn't require you to do wrappers. It requires you to create a closure, but you should probably have it anyway (module pattern).

[Justin Stone]

Prototypes can be simply avoided 99.99999999% of the time. The vast majority of prototypes I see in code could have been done compositionally instead.

[Kirill Shestakov]

When you're creating Web Components, I don't think you can avoid using prototypes.

That's true in general though, and many people just use factory functions instead as a practice. That's cool, if you can afford it. I still think it's beneficial for people who use prototypes to know this method.

[Justin Stone]

"When you're creating Web Components, I don't think you can avoid using prototypes."

Why not? Genuinely curious...

[Kirill Shestakov]

When you're creating Web Components, you have to inherit native HTML element prototype such as HTMLElement or HTMLInputElement, etc. So you'd have to do something like this:

customElements.define('my-element', class extends HTMLElement {
    constructor() {
        super();
        console.log('test');
    }
});

There's really no way to avoid it here with a factory function. You can probably just define all the properties and methods inside the constructor instead of defining it on the prototype (inside class), but there's no benefit to it performance-wise and you can't make the constructor a factory function since you can't return a new object.

[Justin Stone]

Out of curiosity, why not just do something like:

function myCustomElement() {
  // happy closures go here

  return {
    someTypeOfMetaData,
    someUtilityFunction,
    HTMLElement,
  };
}

[Kirill Shestakov]

This won't help you create a custom element, nor register it. Read more about Web Components.