3 posts tagged “javascript”

...I added Mark’s blog as a favorite. Primarily because it’s awesome, and also because of his tagline about bacon. <3

To favorite someone’s blog, you’ll need Firefox with Firebug installed.

1. Open someone’s blog and view source.
2. Look for an HTML comment like this:
   <!-- Asset Stream 6a00b8ea0717f31bc000b8ea0717f51bc0 -->
3. Copy the asset ID (the 6aNNNN... number).
4. Run this command from the Firebug console (with the above ID):
   app.c.addAssetAsFavorite( { asset_id: "6aNNNN..." } )
5. Check your favorites. It should now be listed!

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 );
    }
} );

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.