CoffeeScript: Extend super method like in Rickshaw Prototype example

Question

I'm playing with the fantastic Rickshaw library which uses Prototype's class-based OOP system. This includes the extend method which allows a sub-class to extend a method in a parent class like this:

Parent class:

defaults: function() {
  return {
    tension: 0.8,
    // etc...
  };
}

Sub class:

defaults: function($super) {
  return Rickshaw.extend( $super(), {
    fill: true,
    // etc...
  } );
}

I'm trying to find the syntax to do the same in CoffeeScript, so that CoffeeScript handles the Class inheritance rather than Prototype. Can you help?

EDIT

Thanks to @tomahaug's answer I got this working, and I have now realised there is a better way. While $super is a Prototype feature, extend is actually defined both in Prototype and the Rickshaw top-level object, like this:

extend: (destination, source) ->
  for property of source
    if (source.hasOwnProperty(property))
      destination[property] = source[property]
  destination

So making use of this method, the solution looks like this:

defaults: () ->
  Rickshaw.extend super(), 
    fill: true,
    // etc...

So the only way this could be improved another step is if it turns out that this is built into CoffeeScript in some way.

Answer 1

If you want to call the parent method and then decorate the result with more information before it's returned, you can do it like this with pure CoffeeScript:

class Parent
  defaults: () ->
    return {
       tension: 0.8
    }

class Sub extends Parent
  defaults: () ->
    r = super
    r.fill = true

    return r

sub = new Sub 

sub.tension # 0.8
sub.fill # true

Using a library like http://underscorejs.org/ would probably make it a little prettier as you could do:

  defaults: () ->
    _.extend super, {
      fill: true
    }

Answer 2

The CoffeeScript syntax for those two would be:

Parent class:

defaults: () ->
    return {
        tension: 0.8,
        # etc ...
    }

Sub class:

defaults: ($super) ->
    return Rickshaw.extend $super(), {
        fill: true,
        # etc...
    }

Notice that this is just one out of many ways to write the code in CoffeeScript.

Also, remember that there's implicit return in CoffeeScript, so one can remove the return in both cases, given that the methods does not contain any following code.