Question

I’d like to changing the binding of the callback function in a child class so that the following code indicates 20, rather than 10:

class A 
  @in = 10
  @b: =>
    alert(@in)

class B extends A
  @in = 20

  @w: ->
    window.setTimeout(@b,500)

B.w()

If I ‘thin-arrow’ the definition of A.b, then the binding is to the timeout calling function so @in is undefined. When I fat-arrow it, it binds to the parent class A. I would like it to bind to the child class B, without redefining the method in the child class.

Thanks

Answer 1

I think the best you can do is force the appropriate binding manually when you set up your setTimeout call. Drop the => when defining @b in A:

class A 
  #...
  @b: -> alert(@in)

And then set the binding in B when you call setTimeout:

class B extends A
  #...
  @w: ->
    f = => @b()
    window.setTimeout(f, 500)

I think that’s as close as you can get using CoffeeScript’s pseudo-class-methods.

Demo: http://jsfiddle.net/ambiguous/Y6S8D/