Refining Superclass Method Calls in JavaScript

nobody@vox.com(ydnar) — Fri, 09 Feb 2007 21:57:07 -0800

Last week I was revisiting the always fun problem of implementing “classical” inheritance in JavaScript. I’d taken a few stabs at it, and had gotten it to a reasonably good state that borrowed some good ideas from Doug Crockford, Sam Stephenson, and Dean Edwards. Joshua Gertzen wrote a good post about various methods on his blog.

I’ve never been terribly thrilled with the form Class.superClass.method.apply( this, arguments ). It was redundant: replicating both the class and method names. Copy & paste of code could lead to subtle errors, and it’s annoying to type that much. But the alternatives were worse: Recompiling the function to generate a “magic” lexical for the superclass or wrapper methods. So the Class object basically sat untouched for a year and a half.

Back to last week…It occurred to me that in all the JavaScript we’d built for Vox, we almost never shared a method between two objects, except via inheritance. There were a couple exceptions, but they could be rewritten (it turned out to be a good idea anyway). Second, functions are objects like everything else, and can have arbitrary properties. Third, arguments.callee is available in every function call in JavaScript. I realized then that storing the superclass was not as useful as just storing the supermethod.

For any given method in a class, store its supermethod as a property of the method: method.__super. Instead of the unwieldy construct above, any method could simply use arguments.callee.__super.apply( this, arguments ).

The Class constructor from Core.js:

Class = function( sc ) {
    var c = function( s ) {
        this.constructor = arguments.callee;
        if( s === __SUBCLASS__ )
            return;
        this.init.apply( this, arguments );
    };

    c.override( Class );
    sc = sc || Object;
    c.override( sc );
    c.__super = sc;
    c.superClass = sc.prototype;

    c.prototype = sc === Object ? new sc() : new sc( __SUBCLASS__ );
    c.prototype.extend( Class.prototype );
    var a = arguments;
    for( var i = 1; i < a.length; i++ )
        c.prototype.override( a[ i ] );

    for( var p in c.prototype ) {
        var m = c.prototype[ p ];
        if( typeof m != "function" || defined( m.__super ) )
            continue;
        m.__super = null;
        var pr = sc.prototype;
        while( pr ) {
            if( defined( pr[ p ] ) ) {
                m.__super = pr[ p ];
                break;
            }
            if( pr === pr.constructor.prototype )
                break;
            pr = pr.constructor.prototype;
        }
    }

    return c;
}

arguments.callee was useful in the constructor too: Instead of creating a circular reference by overriding the constructor like this: constructor.prototype.constructor = constructor, the constructor itself can just set it on the this object when the constructor is called: this.constructor = arguments.callee.

Calling a supermethod can be simplified further, to arguments.callee.applySuper( this, arguments ) via a little sugar:

Function.prototype.extend( {
    applySuper: function( o, args ) {
        return this.__super.apply( o, args );
    },

    callSuper: function( o ) {
        var args = [];
        for( var i = 1; i < arguments.length; i++ )
            args.push( arguments[ i ] );
        return this.__super.apply( o, args );
    }
} );

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Adobe Makes an Open Source Play

nobody@vox.com(ydnar) — Tue, 07 Nov 2006 07:22:59 -0800

Adobe has given the Flash ActionScript (ECMAScript or JavaScript 2.0) interpreter to the Mozilla foundation as the new open source core of SpiderMonkey, the JavaScript engine for Firefox. It's called Tamarin. Brilliant.

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Graphics

nobody@vox.com(ydnar) — Wed, 23 Aug 2006 00:21:33 -0700

Basis (Sun)

Negative Lights + Masking Gels

Model Cloud

Recently dug up some old photos, er screenshots from the days when I was a graphics geek. I'm supposed to give a presentation about same one of these Thursdays.

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Holy War

nobody@vox.com(ydnar) — Mon, 03 Jul 2006 14:27:34 -0700

This is a brilliant solution to the age-old problem of tabs versus spaces in source code. Instead of treating tabs as indent of a fixed size, treat the source code as tabular data, where the tab character jumps to the next logical column, where the width of said column is determined by the widest point in the document.

(via Slashdot)

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