Javascript inheritance idea (part 2)

Question

Okay, my first attempt at trying to explain what I was doing failed miserably. I'm basically copying Crockford's Object.create(), except with private variables.

If you look at the accepted answer here http://stackoverflow.com/questions/2107556/how-to-inherit-from-a-class-in-javascript, you will see Object.create as the last pattern, which I think better fits the prototypal nature of Javascript (objects beget objects) instead of emulating classical inheritance (classes beget objects).

If you look at Wikipedia's article on prototype based programming (http://en.wikipedia.org/wiki/Prototype-based_programming), you can see more of what I mean.

The drawback with Object.create() though is that there is no support for private members. This is what I propose:

Function.prototype.from = function(obj) {
    function F() {this.parent = Object(obj);}
    F.prototype = obj;
    var out = new F();
    this.apply(out);
    return out;
};

Then, you create objects as thus:

// Create an object
var a = function() {
    var private_property = 'blue';
    this.public_property = 7;

    this.public_method = function() {
        alert(this.public_property + ' ' + private_property);
    }
}.from(null); // .from() works too, but .from(null) is more revealing


// Create a new object using 'a' as the prototype
var b = function() {
    var private_property = 'red';
    this.public_property = 8;
}.from(a);


// See the results
a.public_method(); // Alerts '7 blue'
b.public_method(); // Alerts '8 blue' - Parent methods use parent private variables

a.public_method = function() { alert('rabbit'); };

a.public_method(); // Alerts 'rabbit'
b.public_method(); // Alerts 'rabbit'

b.public_method = function() { alert('dog'); };

a.public_method(); // Alerts 'rabbit'
b.public_method(); // Alerts 'dog' - Parent method is overwritten

The way I made the "from" function is such that when a parent object changes its methods, if you want to prevent the change in a child instance, you can specify:

this.public_method = this.parent.public_method;

in the child instance.

Note also that objects created ex nihilo do not inherit from Object (hasOwnProperty, etc..). You must explicitly specify this as .from(Object).

Benefits of this pattern:

1. Memory is not wasted for each new instance
2. It adheres to a true prototypal inheritance pattern
3. You have access to the parent object using this.parent (this._proto_ is browser specific)
4. Private variables now exist

There is one major drawback of this method that I can think of: the 'function()' syntax may confuse people into thinking a function is assigned to the variable instead of an object.

My question is, are there other drawbacks that I am missing? (Don't include drawbacks of the prototypal pattern--that's subjective--but only of my implementation).

Answer 1

First, as already mentioned, the Function.prototype approach is really a pain. Why not implement the same thing like this:

Object.createChild = function(obj, constructor) {
    function F() { this.parent = Object(obj); }
    F.prototype = obj;
    var out = new F();
    if (typeof constructor == 'function') {
        constructor.apply(out);
    }
    return out;
};

Then use

var a = Object.createChild(null, function () { /*...*/ });
var b = Object.createChild(a, function () { /*...*/ });

with the same results as above. Bonus: You can omit the constructor argument, like this:

var c = Object.createChild(anything);

---

Second, I don't know if there's any use for true prototypal inheritance, as you call it. In real life, I'm pretty sure the constructor function is particularly tailored to the object that's about to be extended (a). Thus, you're gonna end up calling

var x = f.from(a);
var y = f.from(a);

with the very same f-a combination over and over again. And yes, you save some bytes of memory as compared to a class-driven approach, but honestly, who cares?

Still, the whole thing is a really good idea in theory.