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`class B extends A` calls `A.extended(B)` when `A.extended` is a function #516

sstephenson opened this Issue Jul 17, 2010 · 7 comments


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6 participants

sstephenson commented Jul 17, 2010

It'd be useful if CoffeeScript could invoke a callback on a class after it's subclassed with the extends keyword.

With such a callback, you could implement a base class that propagates its class methods to subclasses:

class Base
  @extended: (subclass) ->
    for key, value of @
      subclass[key]: value

class Element extends Base
  @fromHTML: (html) ->
    node: # ...
    new @(node)

class MyElement extends Element
  # ...


You could also implement a test case class that collects its subclasses and runs them automatically:

class TestCase
  @subclasses: []
  @extended: (subclass) ->
    subclass.extended: TestCase.extended
    TestCase.subclasses.push subclass

class BaseTest extends TestCase
  # ...

class ElementTest extends TestCase
  # ...

for testCase in TestCase.subclasses
  TestRunner.run testCase

jashkenas commented Jul 17, 2010

Mind pasting an example of the JavaScript you'd like to generate for this? Something along these lines?

if (Base.extended) Base.extended(Element);

Or do you think there needs to be a check to ensure that it's a function?


michaelficarra commented Jul 18, 2010

For what it's worth, I am strongly in favor of this callback. It would be similar to Ruby's Class::inherited method (http://ruby-doc.org/core/classes/Class.html#M002785), and provide a very useful metaprogramming capability that is not otherwise possible to implement.


TrevorBurnham commented Jul 18, 2010

+1 to this. In general, I'd like to see CoffeeScript classes become more powerful constructs. And the test case example is awesome.


sstephenson commented Jul 18, 2010

Here's my proposal. We'd only call extended if it's a function. And we'd move the __superClass__ assignment after the call to extended, so class method propagation can be implemented without having to skip over the __superClass__ property.


jashkenas commented Jul 18, 2010

sstephenson's patch is now merged to master. Closing the ticket.

very cool!


ghost commented Oct 10, 2011

I've just been pointed to this ticket from: jashkenas#1762 ...what do you guy's think this?

This issue was closed.

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