Friday, May 1, 2015

Class-free object-oriented programming in Javascript

Douglas Crockford has given another talk on the future of Javascript, with particular attention to what he now considers the Right Way to do object-oriented programming, which is quite a departure from classical thought. Years ago, he stopped using new explicitly, and came up with Object.create to use instead, for more straightforward use of Javascript's prototypal inheritance design.

Now, he has also stopped using this, which means he has effectively abandoned Javascript's notion of inheritance altogether. He calls his new style "class-free object-oriented programming", the basic feature of which is that every object actually contains its own methods, rather than sharing them in a prototype or parent object. This is "inefficient" by some measure, but if you think about it, the sharing of methods is simply an optimization, and in most cases, optimizing for space in JS is not going to make a critical difference.

It happens that this was the style I adopted when I got into programming JS, because I was not trained up in classical object-oriented languages. Ironically, more recently, I did get into the typical use of new and prototypes, but I'm going to reconsider that for a while.

Crockford's basic boilerplate for a constructor is:
function constructor(spec) {
  let {member} = spec,
      {other}  = other_constructor(spec),
      method   = function () {
        // accesses member, other, method, spec
      };

  return Object.freeze({
      method,
      other,
  });
}
Which is to say that there is no built-in inheritance; objects that are composed-in are simply member sub-objects. You can, of course, make pass-through member calls.

Danny Fritz blogged about the concept of class-free OO programming, with some helpful illustrative examples, but his techniques differ somewhat from Crockford's, notably in the use of this. Predictably, I have a take that is somewhat different from each of them, but which I think marries their best features.

Crockford uses constructors for everything, which has the code smell of boilerplate in the form of the return Object.freeze portion. Fritz has constructors and extenders, with a copy-in extend function. I have always hated such extenders, and I don't see a reason to have a distinction between constructors and extenders.

Instead, I propose that the proper separation of duties is to have a universal constructor and everything else is an initializer. This gives you freedom to inherit and allows you to avoid the security hazards of this.

The universal constructor looks like this:
Function.prototype.construct = function (spec) {
    var obj = {};
    this(obj, spec);
    return Object.freeze(obj);
}     
Almost too simple to bother with, but it takes care of the boilerplate issue. Note: I formerly had defined this as Object.construct, but this definition makes every function be a potential constructor, which seemed more appropriate.

Now, for any object type (or extension type), all you have to concern yourself with is the initialization. I will illustrate using Fritz's examples of alligator, duck, and goat objects that are defined in terms of extensions Animal, Walking, Swimming, Flying. This is a straightforward port of his alligator:
function alligator (self, spec) {
    self.name = 'alligator';
    self.word = 'grrr';
    animal(self);
    walking(self);
    swimming(self);
}
All of the extensions are just initializers called on the self object (which, being passed in, avoids the security problems of this), and the alligator function is, itself, just an initializer. spec is not used in this case, but I have kept it to show how Crockford's model fits. Most objects do have inputs to their initialization. Similarly, animal et. al. would normally take a second argument.

To create an instance:
var myAlligator = alligator.construct(spec);
It should be clear from this how duck and goat would similarly be created, but let's take a look at the extensions. Again, a straightforward port of animal:
function animal(self) {
    self.name = 'name';
    self.word = 'word';
    self.talk = function () {
        console.log(self.name + ' says ' + self.word);
    }
};
It becomes clear at this point that all the things that inherit from animal have a name and a word. Initialization of those should be done with a spec, rather than having placeholder defaults and each derived object having to know about them:
function animal(self, spec) {
    self.name = spec.name;
    self.word = spec.word;
    self.talk = function () {
        console.log(self.name + ' says ' + self.word);
    }
};
Now we can rewrite alligator:
function alligator (self, spec) {
    animal(self, spec);
    walking(self);
    swimming(self);
}
alligator.construct({name: 'alligator', word: 'grrr'});
So our alligator doesn't merely contain animal, walking, and swimming objects (as would be the case in the Crockford model), but it has been initialized to have those traits directly. This requires that all the traits play nicely with each other. If that can't be guaranteed, Crockford's model offers more security, and can obviously be implemented with pass-through methods like so:
function alligator (self, spec) {
    animal(self, spec);
    var walkTrait = walking.construct();    var swimTrait = swimming.construct();    self.walk = walkTrait.walk;
    self.swim = swimTrait.swim;
}
alligator.construct({name: 'alligator', word: 'grrr'});
Note that I have initialized the alligator self as an animal, then added the other features as traits. Whether you initialize on the object itself or create sub-objects is up to you.

3 comments:

dannyfritz said...

I'll definitely have to try this out and get a feel for it. Thanks for the post!

Bas said...

At first your method seemed much more flexible, but I'm concerned about the following:

function animal(self) {
self.talk = function () {
console.log(self.name + ' says ' + self.word);
}
};

Danny Fritz copies functions by reference on object construction. It seems to me that your method causes the function to be created and stored in memory separately for every object instance. I've once had a project in which a lot of small Vector class instances were created. I first did this your way, attaching the functions on every constructor call, but noticed a 90% speedup when using prototypes.

ido aizenbud said...

How would i use this with typescript, or with flowjs?
I like this attitude, but i would like to add types to my javascript.