collect.js

// Me circa April 2009
Array.implement({
    "collect": function collect(fn, bind) {
        // fn.calls that throw an exception are
        // excluded and the index is not incremented
        // so that it's like they were never there.
        // can be used instead of a.map( ... ).filter( ... )
        // --------------------------------------------------
        // Throwing is quite slow though, so you'd only really
        // want to use it when fn is already throwing errors IMO
        var results = [];
        var j = 0;

        for (var i = 0, l = this.length; i < l; i++ ){
            try {
                results[j] = fn.call(bind, this[i], j, this);
                j++;
            } catch (err) {}
        }
        return results;
    }
});

// Me today (Apr 2012)
Array.implement({
    collect: function collect(fn, ctx) {
        // The bastard child of `map` and `filter`
        // `fn` is called with `fn.call(ctx, item, index, array, SKIP)`
        //
        //   item: the current item of from the source array
        //   index: the current index in the results 
        //   array: the source array (AKA `this`)
        //   SKIP: if fn returns `SKIP`, nothing is added to the results
        var results = [],
            SKIP = {},
            result, i, index, max;

        for (i = 0, max = this.length; i < max; i++) {
            index = results.length;
            result = fn.call(ctx, this[i], index, this, SKIP);

            if (result !== SKIP) results[index] = result;
        }

        return results;
    }
});