Sunday, April 13, 2008

"Super" was wrong

My implementation of "super" was wrong. I did an implementation in this blog post. It was wrong for several reasons. The number one reason was because it was too complex. The first hint that it was too complex was when I was setting the "constructor" property manually. This is done for you and only needs to change if you set the constructor's "prototype" property with "{}" or "new Object()". I was guilty of the this sin. Turns out if I play by the rules, I don't need the "subclassFrom" function nor do I need to add the special "super" properties. It's actually less verbose and it's playing by the Javascript rules. When in Rome, do as the Romans.

My second mistake was that I thought each object had an immutable property "prototype". This is why I was getting "undefined" when I asked for it. I should have known that I was wrong and not the implementation. Anyway, my belief was WAY WRONG. I don't know where I got this piece of mythology from. I think I pulled it from the sky. The truth is that the "prototype" property is only on Function objects. It's set on the objects it constructs and it is not available. Mozilla and Adobe implementations have a property "__proto__" that stores it, but it is not in the standard and thus, not to be used. All objects by the standard should get their prototype from their "constructor" property. Learn something new everyday.

For the new implementation, I got rid of the "subclassFrom". It's not needed now and I'm only left with the "methods" function from before. It now simply moves properties from one object to another. It's only for syntactic shorthand now. I did add something new: function "proto". It's "super" basically. Here's the implementation:

Function.prototype.methods=function(funcs) {
for (each in funcs) if (funcs.hasOwnProperty(each)) {
this.prototype[each]=funcs[each];
}
}
Object.prototype.proto=function() {
return this.constructor.prototype;
}

So, how do you do super by playing by the rules? Here's how:
Function.prototype.methods=function(funcs) {
for (each in funcs) if (funcs.hasOwnProperty(each)) {
this.prototype[each]=funcs[each];
}
}

function Base() {
print("base constructor called");
}
Base.prototype.toString=function() { return "aBase"; }

function Sub() {
this.proto().constructor.call(this);
print("sub constructor called");
}
Sub.prototype=new Base();
Sub.prototype.toString=function() { return this.proto().toString.call(this) + " and aSub"; }

Output from the above:
base constructor called
********
base constructor called
aBase
********
base constructor called
sub constructor called
aBase and aSub

Notice the "this.proto().constructor.call" in the Sub constructor function, it expands to "this.constructor.prototype.constructor.call". This is calling the "super" constructor. We could also do "arguments.callee.prototype.constructor.call" instead, but really it's a lot to type in isn't it? We also couldn't put that into a function as easily without calling "arguments.callee".

I got a better implementation and something that's more like Javascript. I like keeping my Javascript close to what a Javascript developer would expect. Prototype-based programming is powerful and it's worth exploring things without classes. But, that's a small digression. All I wanted was super and some shorthands for grouping my methods. I have that now.

No comments:

Amazon