Class factories in Coffeescript

code better • have fun

There’s a neat trick for dynamically constructing heterogeneous classes using a single factory function in CoffeeScript.(This is probably a common pattern among Coffeescript cognoscenti, but I hadn’t bumped into it, so I thought it was worth jotting down.)

I’m in the middle of writing a CoffeeScript application which handles a variety of related but different user interface widgets. I’d normally treat each as an independent class, and use mixins for the common functionality. But in this particular app, it turns out to be incredibly convenient to have a single factory method that takes a DOM element and uses a data-type attribute to generate an object of the correct class. That is, given

<article class="widget" data-type="Wibble"></article>

I’d want an object of class Wibble, and given

<article class="widget" data-type="Wobble"></article>

I’d want a Wobble object.

The initialization code to create these objects will look something like

$(".widget").map(function (_,el) { return Widget.for_element($(el)) })

Here’s what I came up with for the for_element factory method.

class Widget
    @for_element: (el) ->
        type = el.data("type")
        new @[type](el)

    constructor: (@el) ->

class Widget.Wibble extends Widget
    constructor: (el) ->
        # subclass-specific stuff
        superclass Widget.Wobble extends Widget
    constructor: (el) ->
        # subclass-specific stuff
        super

The fun part of this is that by namespacing the individual widget subclasses in the parent Widget class, I can use new @[type](el) to look up the subclass in the factory, and then instantiate the object.